WO2024224472A1 - Optical element, variable focus element, and head-mounted display - Google Patents

Abstract

The present invention provides an optical element capable of highly efficiently switching between polarization modulation and polarization non-modulation over a wide band and at a wide viewing angle, a variable focus element provided with the optical element, and a head-mounted display provided with the variable focus element. An optical element according to the present invention comprises: a first liquid crystal cell having first liquid crystal molecules and a first electrode; a second liquid crystal cell having second liquid crystal molecules and a second electrode, and can switch between a first state and a second state. The azimuth angles in the orientation direction of the second liquid crystal molecules on a third substrate side and a fourth substrate side in the first state are respectively angles obtained by turning the azimuth angles in the orientation direction of the first liquid crystal molecules on a first substrate side and a second substrate side in the second state one quarter of a turn in the same direction. The optical element comprises a first quarter wavelength film and a second quarter wavelength film that are disposed on the opposite side of the first liquid crystal cell from the second liquid crystal cell or on the opposite side of the second liquid crystal cell from the first liquid crystal cell.

Description

Optical element, variable focus element and head mounted display

The following disclosure relates to an optical element, a variable focus element including the optical element, and a head-mounted display including the variable focus element.

In recent years, variable-focus optical systems have been proposed for head-mounted displays and other applications that combine Pancharatnam Berry (PB) lenses with optical elements such as switchable half-wave plates (sHWPs). sHWPs are devices that can switch the polarization state of left and right circularly polarized light, and are realized using liquid crystal.

As a technology relating to a variable focus optical system, for example, Patent Document 1 discloses a display device including a waveguide and a broadband adaptive lens assembly, the waveguide being configured to guide light in a lateral direction parallel to an output surface of the waveguide, and further configured to externally couple the guided light through the output surface, and the broadband adaptive lens assembly being configured to internally couple and diffract the externally coupled light from the waveguide therethrough.

Patent document 2 discloses a variable focus block that includes an sHWP and multiple liquid crystal lenses.

Patent document 3 discloses an achromatic polarization switch that converts linearly polarized light of an initial polarization orientation, the achromatic polarization switch comprising a first liquid crystal (LC) cell having a first orientation axis with respect to the initial polarization orientation, and a second LC cell having a second orientation axis with respect to the first orientation axis.

Patent document 4 discloses an optical element comprising a first laminated birefringent layer and a second laminated birefringent layer, the respective local optical axes of which are rotated at respective twist angles through the respective thicknesses of the first layer and the second layer and aligned along the interface between the first layer and the second layer.

Patent document 5 proposes an optical element made of a laminated liquid crystal structure that rotates the polarization of incident circularly polarized light over a wide range of wavelengths and angles of incidence, for use in head-mounted displays.

Special Publication No. 2021-501361 U.S. Pat. No. 1,037,9419 Special Publication No. 2009-524106 Special table 2014-528597 publication U.S. Pat. No. 1,067,8057

The above-mentioned Patent Documents 1 to 5 have an issue in that it is difficult to realize a device structure that can switch between polarization modulation, which converts the polarization state of left-right circularly polarized light, and polarization non-modulation, which does not convert the polarization state of left-right circularly polarized light, with high efficiency over a wide bandwidth and a wide viewing angle.

The present invention has been made in consideration of the above-mentioned current situation, and aims to provide an optical element that can switch between polarization modulation and polarization non-modulation with high efficiency over a wide bandwidth and a wide viewing angle, a variable focus element that includes the optical element, and a head-mounted display that includes the variable focus element.

(1) One embodiment of the present invention comprises, in order, a first substrate, a first liquid crystal layer containing first liquid crystal molecules, a second substrate, a third substrate, a second liquid crystal layer containing second liquid crystal molecules, and a fourth substrate, the first substrate, the first liquid crystal layer, and the second substrate constituting a first liquid crystal cell, the third substrate, the second liquid crystal layer, and the fourth substrate constituting a second liquid crystal cell, the first liquid crystal cell having a first electrode for applying a voltage to the first liquid crystal layer on at least one of the first substrate and the second substrate, the second liquid crystal cell having a second electrode for applying a voltage to the second liquid crystal layer on at least one of the third substrate and the fourth substrate, the first electrode and the second electrode being arranged to provide a first state in which the second liquid crystal molecules are twisted and the first liquid crystal molecules are vertically aligned, and a second state in which the first liquid crystal molecules are twisted and The second liquid crystal molecules are arranged to be switchable between a first state in which the second liquid crystal molecules are vertically aligned and a second state in which the second liquid crystal molecules are vertically aligned, and the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and the azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state are angles obtained by rotating the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state and the azimuth angle of the alignment direction of the first liquid crystal molecules on the second substrate side in the second state in the same direction by a quarter, and further comprising a first ¼ wavelength film and a second ¼ wavelength film arranged on the opposite side of the first liquid crystal cell to the second liquid crystal cell or the opposite side of the second liquid crystal cell to the first liquid crystal cell, and the first ¼ wavelength film is arranged between the second ¼ wavelength film and the first liquid crystal cell and the second liquid crystal cell.

(2) In addition to the configuration of (1) above, one embodiment of the present invention is an optical element in which the retardation Re of the first quarter-wave film at a wavelength of 550 nm is 72 nm or more and 210 nm or less.

(3) In addition to the configuration of (1) or (2), one embodiment of the present invention is an optical element in which the azimuth angle of the slow axis of the first quarter-wave film is 52° or more and 66° or less.

(4) In addition to the configuration of (1), (2), or (3), one embodiment of the present invention is an optical element in which the retardation Re of the second quarter-wave film at a wavelength of 550 nm is 112 nm or more and 162 nm or less.

(5) In addition, one embodiment of the present invention is an optical element having the configuration of (1), (2), (3), or (4) above, in which the azimuth angle of the slow axis of the second quarter-wave film is 4° or more and 23° or less.

(6) In addition to the configuration of (1), (2), (3), (4) or (5), an embodiment of the present invention is an optical element in which at least one of the first quarter-wave film and the second quarter-wave film has reverse wavelength dispersion characteristics.

(7) In addition, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), or (6), at least one of the first ¼ wavelength film and the second ¼ wavelength film has flat wavelength dispersion characteristics.

(8) In addition, one embodiment of the present invention is an optical element having the configuration of (1), (2), (3), (4), (5), (6), or (7) above, in which the retardation Re of the first liquid crystal layer in the second state at a wavelength of 550 nm is 196 nm or more and 280 nm or less.

(9) In addition, one embodiment of the present invention is an optical element having the configuration of (1), (2), (3), (4), (5), (6), (7), or (8) above, in which the retardation Re of the second liquid crystal layer in the first state at a wavelength of 550 nm is 200 nm or more and 280 nm or less.

(10) Furthermore, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), (6), (7), (8), or (9), the first liquid crystal molecules in the second state are twisted and aligned with a twist angle of 58° or more and 78° or less.

(11) Furthermore, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), (6), (7), (8), (9), or (10), the second liquid crystal molecules in the first state are twisted and aligned with a twist angle of 57° or more and 76° or less.

(12) In addition, an embodiment of the present invention is an optical element having the configuration of (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), or (11), in which the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state is -12° or more and 10° or less.

(13) In addition, an embodiment of the present invention is an optical element having the configuration of (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11) or (12) above, in which the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state is 79° or more and 98° or less.

(14) In addition, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12) or (13), an optical element further comprises a first positive C plate arranged between the first 1/4 wavelength film and the second 1/4 wavelength film, a second positive C plate arranged on the side of the second liquid crystal cell opposite the first liquid crystal cell, and a third positive C plate arranged on the side of the first liquid crystal cell opposite the second liquid crystal cell.

(15) In addition to the configuration of (14), one embodiment of the present invention is an optical element in which the retardation Rth in the thickness direction of the first positive C plate is 0 nm or more and 320 nm or less.

(16) In addition to the configuration of (14) or (15), an embodiment of the present invention is an optical element in which the retardation Rth in the thickness direction of the second positive C plate is 0 nm or more and 252 nm or less.

(17) In addition, in one embodiment of the present invention, in addition to the configuration of (14), (15), or (16), the retardation Rth in the thickness direction of the third positive C plate is 0 nm or more and 290 nm or less.

(18) In addition, in one embodiment of the present invention, in addition to the configuration of (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12), (13), (14), (15), (16) or (17), the optical element further comprises a negative C plate between the first liquid crystal cell and the second liquid crystal cell.

(19) In addition to the configuration of (18), one embodiment of the present invention is an optical element in which the retardation Rth in the thickness direction of the negative C plate is -410 nm or more and 0 nm or less.

(20) Another embodiment of the present invention is a variable focus element comprising an optical element as described in (1), (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12), (13), (14), (15), (16), (17), (18) or (19) above, and a Pancharatnam Berry lens.

(21) In addition to the configuration of (20), one embodiment of the present invention is a variable focus element, in which the Pancharatnam Berry lens is disposed within the optical element.

(22) Another embodiment of the present invention is a head-mounted display comprising the variable focus element described in (20) or (21) above.

The present invention provides an optical element capable of switching between polarization modulation and polarization non-modulation with high efficiency over a wide bandwidth and a wide viewing angle, a variable focus element including the optical element, and a head-mounted display including the variable focus element.

1 is a schematic cross-sectional view of an optical element according to a first embodiment. 2 is a schematic cross-sectional view of a first liquid crystal cell and a second liquid crystal cell included in the optical element according to the first embodiment. FIG. 3A to 3C are schematic diagrams illustrating the alignment of liquid crystal molecules in a first state and a second state of the optical element according to the first embodiment. FIG. 1 is a schematic cross-sectional view of an optical element according to Comparative Example 1. FIG. 11 is a schematic cross-sectional view of an optical element according to Comparative Example 2. 13 is a graph showing the relationship between the Stokes parameter S3 during modulation and the wavelength of emitted light for the optical elements according to Comparative Example 1 and Comparative Example 2. 1 is a schematic cross-sectional view illustrating a first state of an optical element according to a first embodiment. FIG. 5 is a schematic cross-sectional view illustrating a second state of the optical element according to the first embodiment. FIG. FIG. 2 is a diagram illustrating a polarization state. FIG. 2 is a schematic cross-sectional view of an optical element according to a first modified example of the first embodiment. FIG. 11 is a schematic cross-sectional view of an optical element according to a second embodiment. 11 is a schematic cross-sectional view of a first liquid crystal cell and a second liquid crystal cell included in an optical element according to Embodiment 3. FIG. 11A to 11C are schematic diagrams illustrating the alignment of liquid crystal molecules in a first state and a second state of an optical element according to a third embodiment. 11 is a schematic cross-sectional view illustrating a first state of an optical element according to a third embodiment. FIG. 11 is a schematic cross-sectional view illustrating a second state of the optical element according to the third embodiment. FIG. 11 is a schematic cross-sectional view of a first liquid crystal cell and a second liquid crystal cell included in an optical element according to Embodiment 4. FIG. 10A to 10C are schematic diagrams illustrating the alignment of liquid crystal molecules in a first state and a second state of an optical element according to a fourth embodiment. 11 is a schematic cross-sectional view illustrating a first state of an optical element according to embodiment 4. FIG. 13 is a schematic cross-sectional view illustrating a second state of the optical element according to the fourth embodiment. FIG. FIG. 13 is a schematic cross-sectional view of a variable focus element according to a fifth embodiment. 13 is an example of a schematic cross-sectional view of a PB lens included in a variable focus element according to embodiment 5. FIG. 23 is a schematic cross-sectional view of a variable-focus element according to a first modified example of the fifth embodiment. FIG. 23 is an enlarged schematic cross-sectional view of a variable-focus element according to a first modified example of the fifth embodiment. 13A to 13C are schematic diagrams illustrating the alignment of liquid crystal molecules in a first state and a second state of an optical element according to Modification 1 of Embodiment 5. 13 is a plan view schematic diagram showing the orientation pattern of a PB lens included in a variable focal length element according to Modification 1 of Embodiment 5. FIG. 13 is a schematic cross-sectional view illustrating a detailed configuration of a variable-focus element according to a first modified example of the fifth embodiment. FIG. 13A and 13B are diagrams illustrating the polarization state of a variable-focus element according to Modification 1 of the fifth embodiment in the F-2.5 mode. FIG. 13 is a schematic cross-sectional view of a head mounted display according to a sixth embodiment. FIG. 13 is a schematic perspective view showing an example of the appearance of a head mounted display according to a sixth embodiment. 11 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to the first embodiment in a non-modulated state when the incident angle is set to 30°. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle during modulation of the optical element according to Example 1, when the incident angle is set to 30°. 11 is a graph showing the Stokes parameter (worst S3) versus retardation Re of the second quarter-wave film included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the azimuth angle of the slow axis of the second quarter-wave film included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the retardation Rth in the thickness direction of the first positive C plate included in the optical element of Example 1. 1 is a graph showing the Stokes parameter (worst S3) versus retardation Re of a first quarter-wave film included in the optical element of Example 1. 1 is a graph showing the Stokes parameter (worst S3) versus the azimuth angle of the slow axis of a first quarter-wave film included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the retardation Rth in the thickness direction of the second positive C plate included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus retardation Re in a first state at a wavelength of 550 nm in the second liquid crystal layer included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the twist angle in a first state of second liquid crystal molecules of the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state of the optical element of Example 1. 1 is a graph showing the Stokes parameter (worst S3) versus the retardation Rth in the thickness direction of the negative C plate included in the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus retardation Re in a second state at a wavelength of 550 nm in the first liquid crystal layer included in the optical element of Example 1. 13 is a graph showing the Stokes parameter (worst S3) versus the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state of the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the twist angle in the second state of the first liquid crystal molecule of the optical element of Example 1. 11 is a graph showing the Stokes parameter (worst S3) versus the retardation Rth in the thickness direction of the third positive C plate included in the optical element of Example 1. FIG. 4 is a schematic cross-sectional view of an optical element according to Comparative Example 1. FIG. 11 is a schematic cross-sectional view of an optical element according to Comparative Example 2. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle in the non-modulated state of the optical element according to Comparative Example 1, when the incident angle is set to 30°. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle during modulation of the optical element according to Comparative Example 1, when the incident angle is set to 30°. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle in the non-modulated state of the optical element according to Comparative Example 2, when the incident angle is set to 30°. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle during modulation of the optical element according to Comparative Example 2, when the incident angle is set to 30°. FIG. 11 is a schematic cross-sectional view of an optical element according to a second embodiment. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle in the non-modulated state of the optical element according to Example 2, when the incident angle is set to 30°. 13 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle during modulation of the optical element according to Example 2, when the incident angle is set to 30°.

The following describes an embodiment of the present invention. The present invention is not limited to the contents described in the following embodiment, and appropriate design changes can be made within the scope of the configuration of the present invention. In the following description, the same reference numerals are used in different drawings as appropriate for the same parts or parts having similar functions, and repeated explanations are omitted as appropriate. The various aspects of the present invention may be combined as appropriate within the scope of the gist of the present invention.

(Definition of terms)
In this specification, the azimuth refers to the direction of interest when projected onto the substrate surface on the emission side of the optical element, and is expressed as the angle (azimuth angle) between the target direction and the reference azimuth. Here, the reference azimuth (0°) is set to the horizontal right direction of the screen of the liquid crystal panel when the optical element is viewed from the emission side. The azimuth angle is positive in the counterclockwise direction and negative in the clockwise direction. Both the counterclockwise and clockwise directions represent the rotation direction when the optical element is viewed from the emission side. The azimuth angle represents a value measured when the optical element is viewed in plan from the emission side.

In this specification, when two straight lines (including axes, directions, and orientations) are perpendicular to each other, it means that they are perpendicular when the optical element is viewed in a planar view from the emission side. Furthermore, when one of the two straight lines is obliquely arranged with respect to the other straight line, it means that one straight line is obliquely arranged with respect to the other straight line when the optical element is viewed in a planar view from the emission side. Furthermore, the angle between two straight lines means the angle between one straight line and the other straight line when the optical element is viewed in a planar view from the emission side.

In this specification, when two straight lines (including axes, directions, and orientations) are perpendicular, this means that the angle between them is 90°±5°, preferably 90°±3°, more preferably 90°±1°, and particularly preferably 90° (completely perpendicular). When two straight lines are parallel, this means that the angle between them is 0°±5°, preferably 0°±3°, more preferably 0°±1°, and particularly preferably 0° (completely parallel).

In this specification, the retardation in the in-plane direction (in-plane phase difference, in-plane retardation) Re is defined as Re = (ns - nf) d. The retardation in the thickness direction Rth is defined as Rth = (nz - (nx + ny) / 2) d. ns refers to the larger of nx and ny, and nf refers to the smaller one. Furthermore, nx and ny indicate the principal refractive index in the in-plane direction of the birefringent layer (including the retardation film and the liquid crystal layer), nz indicates the principal refractive index in the out-of-plane direction, i.e., the direction perpendicular to the surface of the birefringent layer, and d indicates the thickness of the birefringent layer.

In this specification, the measurement wavelength for optical parameters such as the principal refractive index and phase difference is 550 nm unless otherwise specified. In addition, the retardation Re in the in-plane direction is also simply called "retardation Re."

The following describes an embodiment of the present invention. The present invention is not limited to the contents described in the following embodiment, and appropriate design changes can be made within the scope of the configuration of the present invention.

(Embodiment 1)
Fig. 1 is a cross-sectional schematic diagram of an optical element according to embodiment 1. Fig. 2 is a cross-sectional schematic diagram of a first liquid crystal cell and a second liquid crystal cell included in the optical element according to embodiment 1. Fig. 3 is a schematic diagram for explaining the orientation of liquid crystal molecules in a first state and a second state of the optical element according to embodiment 1.

1 to 3, the optical element 10 of this embodiment includes a first substrate 100, a first liquid crystal layer 500 containing first liquid crystal molecules 510, a second substrate 200, a third substrate 300, a second liquid crystal layer 600 containing second liquid crystal molecules 610, and a fourth substrate 400, in that order. The first substrate 100, the first liquid crystal layer 500, and the second substrate 200 constitute a first liquid crystal cell 11A, and the third substrate 300, the second liquid crystal layer 600, and the fourth substrate 400 constitute a second liquid crystal cell 11B. The first liquid crystal cell 11A has a first solid electrode 120 and a second solid electrode 220 as the first electrodes for applying a voltage to the first liquid crystal layer 500 on at least one of the first substrate 100 and the second substrate 200. The second liquid crystal cell 11B has a third solid electrode 320 and a fourth solid electrode 420 as the second electrode for applying a voltage to the second liquid crystal layer 600, on at least one of the third substrate 300 and the fourth substrate 400. The first electrode and the second electrode are arranged to be switchable between a first state in which the second liquid crystal molecules 610 are twistedly aligned and the first liquid crystal molecules 510 are vertically aligned, and a second state in which the first liquid crystal molecules 510 are twistedly aligned and the second liquid crystal molecules 610 are vertically aligned. The azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state and the azimuth angle of the alignment direction 612A of the second liquid crystal molecules 612 on the fourth substrate 400 side in the first state are angles obtained by rotating the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state and the azimuth angle of the alignment direction 512A of the first liquid crystal molecules 512 on the second substrate 200 side in the second state by 1/4 in the same direction, respectively. By adopting such an embodiment, it becomes possible to drive the first state and the second state in the same way except that the entire system is rotated by 1/4, and it becomes possible to realize polarization modulation in one of the first state and the second state and polarization non-modulation in the other state in a wide band and a wide viewing angle. In other words, it is possible to realize an optical element that can switch between polarization modulation and non-modulation over a wide bandwidth and a wide viewing angle, more specifically, a switchable half-wave plate (sHWP) element.

Furthermore, the optical element 10 of this embodiment includes a first 1/4 wavelength film 13 and a second 1/4 wavelength film 14 arranged on the side of the first liquid crystal cell 11A opposite the second liquid crystal cell 11B, or on the side of the second liquid crystal cell 11B opposite the first liquid crystal cell 11A, and the first 1/4 wavelength film 13 is arranged between the second 1/4 wavelength film 14 and the first liquid crystal cell 11A and the second liquid crystal cell 11B. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with high efficiency.

Here, if one tries to realize sHWP with a single liquid crystal layer, the optical element 10R1 of comparative form 1 can be configured as shown in FIG. 4, using a liquid crystal cell 11R1 with a 90° twisted TN liquid crystal layer 500R1. More specifically, the optical element 10R1 of comparative form 1 includes, in order, a quarter-wave film 15R with a slow axis azimuth angle of 75°, a half-wave film 16R with a slow axis azimuth angle of 15°, a liquid crystal cell 11R1, a half-wave film 17R with a slow axis azimuth angle of -75°, and a quarter-wave film 18R with a slow axis azimuth angle of -15°. FIG. 4 is a schematic cross-sectional view of the optical element of comparative form 1.

In addition, when trying to realize sHWP with two liquid crystal layers, a configuration of optical element 10R2 of comparative embodiment 2 in which TN liquid crystal layer 500R2 with a 70° twist and TN liquid crystal layer 500R3 with a -70° twist are stacked as shown in Figure 5 is conceivable. Figure 5 is a schematic cross-sectional view of the optical element of comparative embodiment 2.

Figure 6 is a graph showing the relationship between the Stokes parameter S3 during modulation and the wavelength of the emitted light for the optical elements of Comparative Example 1 and Comparative Example 2. Figure 6 shows the wavelength dependency of the polarization state of the emitted light when right-handed circularly polarized light (Stokes parameter S3 = +1) is incident. The closer to S3 = -1, the more the light is converted to left-handed circularly polarized light. The closer to -1 over a wide wavelength range, the wider the bandwidth during modulation.

The optical element 10R1 of comparative form 1 is easy to design, but due to the influence of wavelength dispersion of the 90° twisted TN liquid crystal layer 500R1, it is difficult to achieve a wide bandwidth as shown in FIG. 6. In addition, the optical element 10R2 of comparative form 2 can achieve a wide bandwidth by stacking liquid crystal layers twisted by about 70 degrees, but it is difficult to achieve a wide viewing angle. On the other hand, the optical element 10 of this embodiment can switch between polarization modulation and polarization non-modulation over a wide bandwidth and a wide viewing angle.

The above-mentioned Patent Document 1 does not disclose any polarization modulation characteristics. Patent Document 1 discloses a single-layer TN liquid crystal layer configuration, but with this configuration, during polarization modulation (when inactive, when the voltage is OFF in Patent Document 1), the polarization conversion is only performed appropriately at specific wavelengths, and it is not possible to achieve polarization conversion over a wide band.

More specifically, in the single-layer configuration disclosed in Patent Document 1, the liquid crystal molecules are twisted 90° during polarization modulation, and are vertically oriented when a vertical electric field is applied during polarization non-modulation. Because the liquid crystal molecules are twisted 90° during polarization modulation, there is wavelength dependence, and polarization modulation cannot be achieved over a wide bandwidth. Even if it were possible to achieve polarization modulation over a wide bandwidth by adjusting the twist angle of the liquid crystal molecules or the cell thickness of the liquid crystal layer, it would be impossible to achieve polarization non-modulation over a wide bandwidth due to the influence of residual retardation by the liquid crystal molecules near the substrate during polarization non-modulation. In other words, it is not possible to achieve both polarization modulation and polarization non-modulation over a wide bandwidth.

The above-mentioned Patent Document 5 does not disclose any modulation characteristics. In addition, the specific physical properties of the retardation film and the like are not described. Furthermore, since one of the stacked liquid crystal cells in Patent Document 5 is used as a backup, it is thought to have the same cell design as the other liquid crystal cell.

This embodiment is described in detail below.

The alignment direction of the first liquid crystal molecules on the first substrate side is the alignment direction of the first liquid crystal molecules that are horizontally aligned near the first substrate. More specifically, when the alignment film provided on the first liquid crystal layer side of the first substrate is a horizontal alignment film, the alignment direction of the first liquid crystal molecules on the first substrate side refers to the alignment direction of the first liquid crystal molecules located at the interface of the first liquid crystal layer on the first substrate side. When the alignment film provided on the first liquid crystal layer side of the first substrate is a vertical alignment film, the liquid crystal molecules located at the interface of the first liquid crystal layer on the first substrate side are vertically aligned, so the alignment direction of the first liquid crystal molecules on the first substrate side refers to the alignment direction of the first liquid crystal molecules in a horizontal alignment state that are located inside the first liquid crystal layer from the interface on the first substrate side.

Similarly, the alignment direction of the first liquid crystal molecules on the second substrate side is the alignment direction of the first liquid crystal molecules that are horizontally aligned near the second substrate. More specifically, when the alignment film provided on the first liquid crystal layer side of the second substrate is a horizontal alignment film, the alignment direction of the first liquid crystal molecules on the second substrate side refers to the alignment direction of the first liquid crystal molecules located at the interface of the first liquid crystal layer on the second substrate side. When the alignment film provided on the first liquid crystal layer side of the second substrate is a vertical alignment film, the liquid crystal molecules located at the interface of the first liquid crystal layer on the second substrate side are vertically aligned, so the alignment direction of the first liquid crystal molecules on the second substrate side refers to the alignment direction of the first liquid crystal molecules in a horizontal alignment state that are located inside the first liquid crystal layer from the interface on the second substrate side.

Similarly, the alignment direction of the second liquid crystal molecules on the third substrate side is the alignment direction of the second liquid crystal molecules that are horizontally aligned near the third substrate. More specifically, when the alignment film provided on the second liquid crystal layer side of the third substrate is a horizontal alignment film, the alignment direction of the second liquid crystal molecules on the third substrate side refers to the alignment direction of the second liquid crystal molecules located at the interface of the second liquid crystal layer on the third substrate side. When the alignment film provided on the second liquid crystal layer side of the third substrate is a vertical alignment film, the liquid crystal molecules located at the interface of the second liquid crystal layer on the third substrate side are vertically aligned, so the alignment direction of the second liquid crystal molecules on the third substrate side refers to the alignment direction of the second liquid crystal molecules in a horizontal alignment state that are located inside the second liquid crystal layer from the interface on the third substrate side.

Similarly, the alignment direction of the second liquid crystal molecules on the fourth substrate side is the alignment direction of the second liquid crystal molecules that are horizontally aligned near the fourth substrate. More specifically, when the alignment film provided on the second liquid crystal layer side of the fourth substrate is a horizontal alignment film, the alignment direction of the second liquid crystal molecules on the fourth substrate side refers to the alignment direction of the second liquid crystal molecules located at the interface of the second liquid crystal layer on the fourth substrate side. When the alignment film provided on the second liquid crystal layer side of the fourth substrate is a vertical alignment film, the liquid crystal molecules located at the interface of the second liquid crystal layer on the fourth substrate side are vertically aligned, so the alignment direction of the second liquid crystal molecules on the fourth substrate side refers to the alignment direction of the second liquid crystal molecules in a horizontal alignment state that are located inside the second liquid crystal layer from the interface on the fourth substrate side.

The azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and the azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state are respectively an angle rotated 1/4 in the same direction of the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state and the azimuth angle of the alignment direction of the first liquid crystal molecules on the second substrate side in the second state, respectively, means that the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and the azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state are respectively an angle rotated 1/4 in the same direction of the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and the azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state. The azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the first state and the azimuth angle of the alignment direction of the first liquid crystal molecules on the second substrate side in the second state are rotated 1/4 in the positive direction, or the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and the azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state are rotated 1/4 in the negative direction, respectively.

Here, 1/4 rotation means 80° or more and 100° or less, preferably 85° or more and 95° or less, and more preferably 87° or more and 93° or less.

The first liquid crystal cell 11A comprises, in order from the entrance side to the exit side, a first substrate 100, a first liquid crystal layer 500 containing first liquid crystal molecules 510, and a second substrate 200. The first substrate 100 comprises a first support substrate 110 and a first solid electrode 120, and the second substrate 200 comprises a second support substrate 210 and a second solid electrode 220.

The second liquid crystal cell 11B comprises, in order from the entrance side to the exit side, a third substrate 300, a second liquid crystal layer 600 containing second liquid crystal molecules 610, and a fourth substrate 400. The third substrate 300 comprises a third support substrate 310 and a third solid electrode 320, and the fourth substrate 400 comprises a fourth support substrate 410 and a fourth solid electrode 420.

The first support substrate 110, the second support substrate 210, the third support substrate 310, and the fourth support substrate 410 may be, for example, an insulating substrate such as a glass substrate or a plastic substrate. Examples of materials for the glass substrate include glass such as float glass and soda glass. Examples of materials for the plastic substrate include plastics such as polyethylene terephthalate, polybutylene terephthalate, polyethersulfone, polycarbonate, polyimide, and alicyclic polyolefin.

The first solid electrode 120, the second solid electrode 220, the third solid electrode 320 and the fourth solid electrode 420 can be formed by forming a single layer or multiple layers of a transparent conductive material such as indium tin oxide (ITO), indium zinc oxide (IZO), zinc oxide (ZnO) or tin oxide (SnO) or an alloy thereof by a sputtering method or the like, and then patterning the layer using a photolithography method. In this specification, a solid electrode refers to an electrode that does not have a slit or opening at least in the area that overlaps with the optical opening of the pixel when viewed in a plan view.

One of the first solid electrode 120 and the second solid electrode 220 is a pixel electrode, and the other is a common electrode. One of the third solid electrode 320 and the fourth solid electrode 420 is a pixel electrode, and the other is a common electrode.

The first liquid crystal layer 500 contains a liquid crystal material, and by applying a voltage to the first liquid crystal layer 500 and changing the orientation state of the first liquid crystal molecules 510 in the liquid crystal material in response to the applied voltage, the polarization state of light passing through the first liquid crystal layer 500 can be changed.

The second liquid crystal layer 600 contains a liquid crystal material, and by applying a voltage to the second liquid crystal layer 600 and changing the orientation state of the second liquid crystal molecules 610 in the liquid crystal material in response to the applied voltage, the polarization state of light passing through the second liquid crystal layer 600 can be changed.

The first liquid crystal molecule 510 and the second liquid crystal molecule 610 may be positive type liquid crystal molecules having a positive dielectric anisotropy (Δε) defined by the following formula (L), or may be negative type liquid crystal molecules having a negative dielectric anisotropy. In addition, one of the first liquid crystal molecule 510 and the second liquid crystal molecule 610 may be a positive type liquid crystal molecule, and the other may be a negative type liquid crystal molecule. In this embodiment, a case where the first liquid crystal molecule 510 and the second liquid crystal molecule 610 are positive type liquid crystal molecules will be described as an example. The long axis direction of the liquid crystal molecule is the direction of the slow axis.
Δε=(dielectric constant of liquid crystal molecules in the long axis direction)−(dielectric constant of liquid crystal molecules in the short axis direction) (L)

The first liquid crystal layer 500 contains first liquid crystal molecules 510 that are twisted between the first substrate 100 and the second substrate 200. In the second state, the first liquid crystal molecules 510 are twisted from the first substrate 100 side to the second substrate 200 side.

The second liquid crystal layer 600 contains second liquid crystal molecules 610 that are twisted between the third substrate 300 and the fourth substrate 400. In the first state, the second liquid crystal molecules 610 are twisted from the third substrate 300 side to the fourth substrate 400 side.

The twisted orientation of the first liquid crystal molecules 510 and the second liquid crystal molecules 610 can be achieved, for example, by adding a chiral agent to the liquid crystal material. There are no particular limitations on the chiral agent, and any conventionally known agent can be used. For example, S-811 (manufactured by Merck) can be used as the chiral agent.

The first liquid crystal molecules 510 and the second liquid crystal molecules 610 of this embodiment are positive type liquid crystal molecules with a twisted alignment. Therefore, when the first liquid crystal layer 500 is in a voltage applied state and the second liquid crystal layer 600 is in a voltage not applied state, a first state can be realized in which the first liquid crystal molecules 510 are vertically aligned and the second liquid crystal molecules 610 are twisted aligned. Also, when the first liquid crystal layer 500 is in a voltage not applied state and the second liquid crystal layer 600 is in a voltage applied state, a second state can be realized in which the first liquid crystal molecules 510 are twisted aligned and the second liquid crystal molecules 610 are vertically aligned. In this embodiment, polarization non-modulation can be realized in the first state, and polarization modulation can be realized in the second state.

The retardation Re of the first liquid crystal layer 500 in the second state at a wavelength of 550 nm is preferably 196 nm or more and 280 nm or less, and more preferably 210 nm or more and 265 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency. In this specification, the voltage applied state in which a voltage equal to or greater than the threshold value is applied to the liquid crystal layer is also simply referred to as the "voltage applied state" or "when voltage is applied," and the no-voltage applied state in which a voltage less than the threshold value (including no voltage application) is applied to the liquid crystal layer is also simply referred to as the "no-voltage applied state" or "when no voltage is applied."

The retardation Re of the second liquid crystal layer 600 in the first state at a wavelength of 550 nm is preferably 200 nm or more and 280 nm or less, and more preferably 220 nm or more and 264 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

The first liquid crystal molecules 510 in the second state are preferably twisted at a twist angle of 58° or more and 78° or less, and more preferably at a twist angle of 62° or more and 75° or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency. The twist angle of the liquid crystal molecules can be determined by measuring the Mueller matrix after emission from the liquid crystal layer using Axoscan (manufactured by Optoscience).

The second liquid crystal molecules 610 in the first state are preferably twisted at a twist angle of 57° or more and 76° or less, and more preferably at a twist angle of 62° or more and 73° or less. By adopting such an embodiment, it is possible to more efficiently switch between polarization modulation and polarization non-modulation.

The twist angle of the first liquid crystal molecule 510 in the second state refers to the angle between the azimuth angle of the alignment direction 511A of the first liquid crystal molecule 511 on the first substrate 100 side in the second state and the azimuth angle of the alignment direction 512A of the first liquid crystal molecule 512 on the second substrate 200 side. The twist angle of the second liquid crystal molecule 610 in the first state refers to the angle between the azimuth angle of the alignment direction 611A of the second liquid crystal molecule 611 on the third substrate 300 side in the first state and the azimuth angle of the alignment direction 612A of the second liquid crystal molecule 612 on the fourth substrate 400 side.

The azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state is preferably -12° or more and 10° or less, and more preferably -7.5° or more and 7.2° or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

In the first state, the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side is preferably 79° or more and 98° or less, and more preferably 83° or more and 95° or less. By adopting such an embodiment, it is possible to more efficiently switch between polarization modulation and polarization non-modulation.

For example, the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state can be set to 0°, and the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state can be set to 90°.

The first liquid crystal cell 11A preferably has a first alignment film 41 on the first liquid crystal layer 500 side of the first substrate 100, and a second alignment film 42 on the first liquid crystal layer 500 side of the second substrate 200. The second liquid crystal cell 11B preferably has a third alignment film 43 on the second liquid crystal layer 600 side of the third substrate 300, and a fourth alignment film 44 on the second liquid crystal layer 600 side of the fourth substrate 400.

The first alignment film 41 and the second alignment film 42 have the function of controlling the alignment of the first liquid crystal molecules 510 in the first liquid crystal layer 500, and when the first liquid crystal layer 500 is in a state where no voltage is applied, the alignment of the first liquid crystal molecules 510 in the first liquid crystal layer 500 is controlled mainly by the action of the first alignment film 41 and the second alignment film 42.

The third alignment film 43 and the fourth alignment film 44 have the function of controlling the alignment of the second liquid crystal molecules 610 in the second liquid crystal layer 600, and when the second liquid crystal layer 600 is in a state where no voltage is applied, the alignment of the second liquid crystal molecules 610 in the second liquid crystal layer 600 is controlled mainly by the action of the third alignment film 43 and the fourth alignment film 44. Hereinafter, the first alignment film 41, the second alignment film 42, the third alignment film 43, and the fourth alignment film 44 are also simply referred to as alignment films.

As the material for the alignment film, materials commonly used in the field of liquid crystal display panels, such as polymers having polyimide in the main chain, polymers having polyamic acid in the main chain, and polymers having polysiloxane in the main chain, can be used. The alignment film can be formed by applying an alignment film material, and the application method is not particularly limited, and for example, flexographic printing, inkjet application, etc. can be used.

The alignment film may be a horizontal alignment film that aligns the liquid crystal molecules approximately horizontally to the film surface, or a vertical alignment film that aligns the liquid crystal molecules approximately perpendicularly to the film surface. In this embodiment, the first alignment film 41, the second alignment film 42, the third alignment film 43, and the fourth alignment film 44 are horizontal alignment films.

The horizontal alignment film has the function of aligning the liquid crystal molecules in the liquid crystal layer in the pixel region in a horizontal direction relative to the surface of the horizontal alignment film when no voltage is applied to the liquid crystal layer. Here, the liquid crystal molecules being aligned in a horizontal direction relative to the surface of the horizontal alignment film means that the pretilt angle of the liquid crystal molecules is 0° to 5° relative to the surface of the horizontal alignment film, preferably 0° to 2°, and more preferably 0° to 1°. The pretilt angle of the liquid crystal molecules means the angle at which the long axis of the liquid crystal molecules is inclined relative to the main surface of each substrate when no voltage is applied to the liquid crystal layer.

The vertical alignment film has the function of aligning the liquid crystal molecules in the liquid crystal layer in the pixel region in a direction perpendicular to the surface of the vertical alignment film when no voltage is applied to the liquid crystal layer. Here, the liquid crystal molecules being aligned in a direction perpendicular to the surface of the vertical alignment film means that the pretilt angle of the liquid crystal molecules is 86° to 90° with respect to the surface of the vertical alignment film, preferably 87° to 89°, and more preferably 87.5° to 89°.

The alignment film may be a photo-alignment film that has photofunctional groups and has been subjected to a photo-alignment treatment as an alignment treatment, or a rubbed alignment film that has been subjected to a rubbing treatment as an alignment treatment. By performing the alignment treatment, a pretilt can be imparted to the liquid crystal molecules.

Since the orientation direction of the liquid crystal molecules is the direction of the orientation main axis (the direction in which the molecular long axes are aligned on average in nematic liquid crystal), the azimuth angle of the orientation direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side coincides with the azimuth angle of the orientation treatment direction of the orientation film (first orientation film 41) provided on the first liquid crystal layer 500 side of the first substrate 100. The azimuth angle of the orientation direction 512A of the first liquid crystal molecules 512 on the second substrate 200 side coincides with the azimuth angle of the orientation treatment direction of the orientation film (second orientation film 42) provided on the first liquid crystal layer 500 side of the second substrate 200. The azimuth angle of the orientation direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side coincides with the azimuth angle of the orientation treatment direction of the orientation film (third orientation film 43) provided on the second liquid crystal layer 600 side of the third substrate 300. The azimuth angle of the alignment direction 612A of the second liquid crystal molecules 612 on the fourth substrate 400 side coincides with the azimuth angle of the alignment treatment direction of the alignment film (fourth alignment film 44) provided on the second liquid crystal layer 600 side of the fourth substrate 400.

FIG. 7 is a schematic cross-sectional view illustrating a first state of the optical element according to embodiment 1. FIG. 8 is a schematic cross-sectional view illustrating a second state of the optical element according to embodiment 1. The optical element 10 of this embodiment preferably includes a negative C plate 12 between the first liquid crystal cell 11A and the second liquid crystal cell 11B. By adopting such an embodiment, as shown in FIG. 7, in the first state, the phase difference at the time of oblique incidence of the first liquid crystal cell 11A can be canceled by the negative C plate 12. Also, as shown in FIG. 8, in the second state, the phase difference at the time of oblique incidence of the second liquid crystal cell 11B can be canceled by the negative C plate 12. As a result, it is possible to activate only the liquid crystal layer that is not driven, and it is possible to switch between polarization modulation and polarization non-modulation in a wider band and at a wider viewing angle.

An example of the negative C plate 12 is a stretched cycloolefin polymer film.

The retardation Rth in the thickness direction of the negative C plate 12 is preferably -410 nm or more and 0 nm or less, and more preferably -339 nm or more and -115 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency. Due to the convenience of production, the retardation Re of the negative C plate 12 may occur on the order of several nanometers, so the retardation Re of the negative C plate 12 is, for example, 0 nm or more and 5 nm or less.

As shown in FIG. 1 etc., the optical element 10 of this embodiment includes a first quarter-wave film 13 and a second quarter-wave film 14 on the side of the first liquid crystal cell 11A opposite the second liquid crystal cell 11B, or on the side of the second liquid crystal cell 11B opposite the first liquid crystal cell 11A, and the first quarter-wave film 13 is disposed between the second quarter-wave film 14 and the first liquid crystal cell 11A and second liquid crystal cell 11B. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with high efficiency over a wide bandwidth.

The quarter-wave films (specifically, the first quarter-wave film 13 and the second quarter-wave film 14) need only provide a retardation Re of 20 nm or more and 240 nm or less for light with a wavelength of at least 550 nm.

Examples of the material for the quarter-wave film include photopolymerizable liquid crystal materials. Examples of the structure of the photopolymerizable liquid crystal material include a structure having a photopolymerizable group, such as an acrylate group or a methacrylate group, at the end of the skeleton of the liquid crystal molecule.

The quarter-wave film can be formed, for example, by the following method. First, a photopolymerizable liquid crystal material is dissolved in an organic solvent such as propylene glycol monomethyl ether acetate (PGMEA). Next, the resulting solution is applied to the surface of a substrate (e.g., a polyethylene terephthalate (PET) film) to form a coating of the solution. After that, the coating of the solution is pre-baked, irradiated with light (e.g., ultraviolet light), and then baked in order to form a quarter-wave film.

In addition, a chiral agent may be added to the photopolymerizable liquid crystal material, and the liquid crystal polymer polymerized in a 68° twisted state may be used as a quarter-wave film.

As the quarter-wave film, for example, a polymer film that has been stretched can also be used. Examples of materials for the polymer film include cycloolefin polymer, polycarbonate, polysulfone, polyethersulfone, polyethylene terephthalate, polyethylene, polyvinyl alcohol, norbornene, triacetyl cellulose, diacetyl cellulose, etc.

The retardation Re of the first quarter-wave film 13 at a wavelength of 550 nm is preferably 72 nm or more and 210 nm or less, and more preferably 110 nm or more and 175 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

The azimuth angle of the slow axis of the first quarter-wave film 13 is preferably 52° or more and 66° or less, and more preferably 55° or more and 64° or less. By adopting such an embodiment, it is possible to more efficiently switch between polarization modulation and polarization non-modulation.

The retardation Re of the second quarter-wave film 14 at a wavelength of 550 nm is preferably 112 nm or more and 162 nm or less, and more preferably 121 nm or more and 152 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

The azimuth angle of the slow axis of the second quarter-wave film 14 is preferably 4° or more and 23° or less, and more preferably 8° or more and 19° or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

It is preferable that at least one of the first quarter-wave film 13 and the second quarter-wave film 14 has an inverse wavelength dispersion characteristic. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

It is preferable that at least one of the first quarter-wave film 13 and the second quarter-wave film 14 has flat wavelength dispersion characteristics. A quarter-wave film with flat wavelength dispersion characteristics is less expensive than a quarter-wave film with reverse wavelength dispersion characteristics, so by having at least one of the first quarter-wave film 13 and the second quarter-wave film 14 have flat wavelength dispersion characteristics, costs can be reduced.

It is preferable that one of the first quarter-wave film 13 and the second quarter-wave film 14 has flat wavelength dispersion characteristics, and the other has reverse wavelength dispersion characteristics. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency while suppressing costs.

It is also preferable that the first quarter-wave film 13 and the second quarter-wave film 14 have flat wavelength dispersion characteristics. By adopting such an embodiment, costs can be further reduced.

In this specification, "wavelength dispersion of a retardation film" refers to the correlation between the absolute value of the retardation imparted by the retardation film and the wavelength of the incident light. In the visible light range, the property where the absolute value of the retardation imparted by the retardation film does not change even if the wavelength of the incident light changes is called "flat wavelength dispersion characteristic." In addition, in the visible light range, the property where the absolute value of the retardation imparted by the retardation film decreases as the wavelength of the incident light increases is called "positive wavelength dispersion characteristic," and in the visible light range, the property where the absolute value of the retardation imparted by the retardation film increases as the wavelength of the incident light increases is called "reverse wavelength dispersion characteristic."

Having flat wavelength dispersion characteristics means that the retardation Re at a wavelength of 450 nm is 0.96 times or more and 1.06 times or less relative to the retardation Re at a wavelength of 550 nm, and that the retardation Re at a wavelength of 650 nm is 0.94 times or more and 1.04 times or less relative to the retardation Re at a wavelength of 550 nm.

Having inverse wavelength dispersion characteristics means that the retardation Re at a wavelength of 450 nm is 0.84 times or more and 1.00 times or less relative to the retardation Re at a wavelength of 550 nm, and that the retardation Re at a wavelength of 650 nm is 0.99 times or more and 1.09 times or less relative to the retardation Re at a wavelength of 550 nm.

The light incident on the optical element 10 is preferably circularly polarized light. By adopting such an embodiment, it is possible to realize an optical element 10 that can switch the polarization state of the circularly polarized light.

Figure 9 is a diagram explaining the polarization state. The principle of polarization modulation of the optical element 10 of this embodiment will be explained using the Poincaré sphere in Figure 9. First, right-handed circularly polarized light (S3 = +1) is incident on the first liquid crystal cell 11A (plot indicated by X1 in Figure 9).

The right-handed circularly polarized light then passes through the first liquid crystal layer 500, which is twisted by 68 degrees, and is then converted once into the polarization state of the plot indicated by X2 in FIG. 9. The multiple plots indicated by X2 correspond to different wavelengths within the range of 380 nm to 780 nm. Light with a wavelength of around 550 nm is linearly polarized (on the equator on the Poincaré sphere), but other wavelengths are plotted in the northern hemisphere of the Poincaré sphere and are elliptically polarized.

Then, the polarized light in the state shown by X2 in FIG. 9 passes through the first quarter-wave film 13 and is converted to the polarization state shown by the plot shown by X3 in FIG. 9. Light with a wavelength of around 550 nm is linearly polarized (on the equator on the Poincaré sphere), but other wavelengths are elliptically polarized.

Furthermore, when the polarized light in the state shown by X3 in FIG. 9 passes through the second quarter-wave film 14, almost all wavelengths are emitted as left-handed circularly polarized light (south pole position on the Poincaré sphere) (converted to the polarization state shown by the plot shown by X4 in FIG. 9). In other words, it can be seen that modulation from right-handed circularly polarized light to left-handed circularly polarized light has occurred.

Similarly, when not modulated, right-handed circularly polarized light becomes linearly polarized once after passing through the second liquid crystal layer 600, which is twisted by 68 degrees. However, because the entire orientation of the liquid crystal is rotated by 90 degrees, the linear polarization is approximately 90 degrees different from when modulated. Then, after the polarized light passes through the first 1/4 wavelength film 13 and the second 1/4 wavelength film 14, all wavelengths become right-handed circularly polarized light. In other words, right-handed circularly polarized light can be output as right-handed circularly polarized light, and it becomes unmodulated.

(First Modification of First Embodiment)
In this modification, an aspect will be described in which the optical element 10 of the above-mentioned embodiment 1 further includes three positive C plates. Fig. 10 is a schematic cross-sectional view of an optical element according to modification 1 of embodiment 1. As shown in Fig. 10, the optical element 10 of this modification further includes a first positive C plate 19X disposed between the first quarter-wave film 13 and the second quarter-wave film 14. By adopting such an aspect, it is possible to more efficiently switch between polarization modulation and polarization non-modulation.

In addition to the first positive C plate 19X, the optical element 10 of this modified example further includes a second positive C plate 19A arranged on the side of the second liquid crystal cell 11B opposite the first liquid crystal cell 11A, and a third positive C plate 19B arranged on the side of the first liquid crystal cell 11A opposite the second liquid crystal cell 11B, as shown in FIG. 10. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with even higher efficiency.

As the first positive C plate 19X, the second positive C plate 19A and the third positive C plate 19B, for example, a film containing a material with negative intrinsic birefringence as a component and biaxially stretched in both the longitudinal and transverse directions, or a film coated with a liquid crystal material such as a nematic liquid crystal, can be appropriately used.

The retardation Rth in the thickness direction of the first positive C plate 19X is preferably 0 nm or more and 320 nm or less, and more preferably 0 nm or more and 230 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

As shown in FIG. 10, the second positive C plate 19A is preferably disposed between the second liquid crystal cell 11B and the first quarter-wave film 13. By adopting such an embodiment, it is possible to more efficiently switch between polarization modulation and polarization non-modulation.

The retardation Rth in the thickness direction of the second positive C plate 19A is preferably 0 nm or more and 252 nm or less, and more preferably 0 nm or more and 174 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

The retardation Rth in the thickness direction of the third positive C plate 19B is preferably 0 nm or more and 290 nm or less, and more preferably 0 nm or more and 213 nm or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

The retardation Rth in the thickness direction of the first positive C plate 19X, the second positive C plate 19A, and the third positive C plate 19B may be the same or different from each other. In addition, the retardation Re of the first positive C plate 19X, the second positive C plate 19A, and the third positive C plate 19B may be the same or different from each other.

For production reasons, the first positive C plate 19X, the second positive C plate 19A, and the third positive C plate 19B may have a retardation Re of about several nanometers, so the retardation Re of the first positive C plate 19X is, for example, 0 nm or more and 5 nm or less.

(Embodiment 2)
In this embodiment, the features unique to this embodiment will be mainly described, and the description of the contents overlapping with those of the above-mentioned embodiment 1 and its modified examples will be omitted. This embodiment is substantially the same as embodiment 1, except that the negative C plate 12 is not provided.

Figure 11 is a schematic cross-sectional view of an optical element according to embodiment 2. In embodiment 1 above, the optical element 10 is described as having a negative C plate 12, but as shown in Figure 11, the optical element 10 does not have to have a negative C plate 12. By adopting such an embodiment, the optical element 10 can be manufactured thin and at low cost.

(Embodiment 3)
In this embodiment, the characteristics unique to this embodiment will be mainly described, and descriptions of the contents overlapping with those of the above-mentioned embodiment 1 and its modified examples, and embodiment 2 will be omitted. This embodiment is substantially the same as embodiment 1, except that the configurations of the first liquid crystal cell 11A and the second liquid crystal cell 11B are different.

Figure 12 is a cross-sectional schematic diagram of a first liquid crystal cell and a second liquid crystal cell provided in an optical element according to embodiment 3. Figure 13 is a schematic diagram illustrating the orientation of liquid crystal molecules in a first state and a second state of an optical element according to embodiment 3. Figure 14 is a cross-sectional schematic diagram illustrating a first state of an optical element according to embodiment 3. Figure 15 is a cross-sectional schematic diagram illustrating a second state of an optical element according to embodiment 3.

The first liquid crystal molecules 510 and the second liquid crystal molecules 610 of the optical element 10 of this embodiment shown in Figures 12 to 15 are negative type liquid crystal molecules with a twisted alignment. Therefore, as shown in Figure 14, when the first liquid crystal layer 500 is in a voltage-free state and the second liquid crystal layer 600 is in a voltage-applied state, a first state can be realized in which the first liquid crystal molecules 510 are vertically aligned and the second liquid crystal molecules 610 are twistedly aligned. In the first state, the phase difference of the first liquid crystal cell 11A can be canceled by the negative C plate 12. Also, as shown in Figure 15, when the first liquid crystal layer 500 is in a voltage-applied state and the second liquid crystal layer 600 is in a voltage-applied state, a second state can be realized in which the first liquid crystal molecules 510 are twistedly aligned and the second liquid crystal molecules 610 are vertically aligned. In the second state, the phase difference of the second liquid crystal cell 11B can be canceled by the negative C plate 12.

The first alignment film 41, the second alignment film 42, the third alignment film 43 and the fourth alignment film 44 are preferably vertical alignment films.

(Embodiment 4)
In this embodiment, the characteristics unique to this embodiment will be mainly described, and descriptions of the contents overlapping with the above-mentioned Embodiment 1 and its modifications, and Embodiments 2 to 3 will be omitted. This embodiment is substantially the same as Embodiment 1, except that the configuration of the second liquid crystal cell 11B is different.

Figure 16 is a cross-sectional schematic diagram of a first liquid crystal cell and a second liquid crystal cell provided in an optical element according to embodiment 4. Figure 17 is a schematic diagram illustrating the orientation of liquid crystal molecules in a first state and a second state of an optical element according to embodiment 4. Figure 18 is a cross-sectional schematic diagram illustrating a first state of an optical element according to embodiment 4. Figure 19 is a cross-sectional schematic diagram illustrating a second state of an optical element according to embodiment 4.

The first liquid crystal molecules 510 of the optical element 10 of this embodiment shown in Figures 16 to 19 are positive type liquid crystal molecules with a twisted alignment, and the second liquid crystal molecules 610 are negative type liquid crystal molecules with a twisted alignment. Therefore, as shown in Figure 18, when the first liquid crystal layer 500 and the second liquid crystal layer 600 are both in a voltage applied state, a first state in which the first liquid crystal molecules 510 are vertically aligned and the second liquid crystal molecules 610 are twisted aligned can be realized. In the first state, the phase difference of the first liquid crystal cell 11A can be canceled by the negative C plate 12. Also, as shown in Figure 19, when the first liquid crystal layer 500 and the second liquid crystal layer 600 are both in a voltage non-applied state, a second state in which the first liquid crystal molecules 510 are twisted aligned and the second liquid crystal molecules 610 are vertically aligned can be realized. In the second state, the phase difference of the second liquid crystal cell 11B can be canceled by the negative C plate 12.

It is preferable that the first alignment film 41 and the second alignment film 42 are horizontal alignment films, and the third alignment film 43 and the fourth alignment film 44 are vertical alignment films.

(Embodiment 5)
In this embodiment, features unique to this embodiment will be mainly described, and descriptions of contents overlapping with the above-mentioned embodiment 1 and its modified examples, and embodiments 2 to 4 will be omitted. In this embodiment, a variable-focus element including the optical element (sHWP) of the above-mentioned embodiments 1 to 4 will be described. FIG. 20 is a cross-sectional schematic diagram of a variable-focus element according to embodiment 5. The variable-focus element 30 of this embodiment shown in FIG. 20 includes an optical element 10 and a Pancharatnam Berry (PB) lens 20.

As described above, the optical element 10 of embodiments 1 to 4 can modulate circularly polarized light. In addition, the PB lens 20 has different focal lengths for right-handed and left-handed circularly polarized light, so that a variable-focus element 30 can be realized by combining the optical element 10 and the PB lens 20.

The PB lens 20 has the function of focusing and diverging circularly polarized light. The PB lens 20 can be produced, for example, by the method described in International Publication No. WO 2019/189818.

Figure 21 is an example of a cross-sectional schematic diagram of a PB lens provided in a variable focus element according to embodiment 5. As shown in Figure 21, the PB lens 20 has an optically anisotropic layer 700. As an example, the PB lens 20 refracts and transmits incident light in a predetermined direction, targeting circularly polarized light. Note that in Figure 21, the incident light is left-handed circularly polarized light.

In the portion shown in FIG. 21, the optically anisotropic layer 700 has three regions R0, R1, and R2 from the left side in FIG. 21, and the length Λ of one period is different in each region. Specifically, the length Λ of one period is shorter in the order of regions R0, R1, and R2. Furthermore, regions R1 and R2 have a structure in which the optical axis is twisted and rotated in the thickness direction of the optically anisotropic layer (hereinafter also referred to as a twisted structure). The twist angle in the thickness direction of region R1 is smaller than the twist angle in the thickness direction of region R2. Note that region R0 is a region that does not have a twisted structure (i.e., the twist angle is 0°). Note that the twist angle is the twist angle in the entire thickness direction.

In the optical element 10, when left-handed circularly polarized light LC1 enters region R1 in the plane of the optically anisotropic layer 700, it is refracted at a predetermined angle in the direction of arrow X with respect to the incident direction, i.e., in one direction in which the orientation of the optical axis of the liquid crystal molecules 710 changes while continuously rotating, and is then transmitted. Similarly, when left-handed circularly polarized light LC2 enters region R2 in the plane of the optically anisotropic layer 700, it is refracted at a predetermined angle in the direction of arrow X with respect to the incident direction and is then transmitted. Similarly, when left-handed circularly polarized light LC0 enters region R0 in the plane of the optically anisotropic layer 700, it is refracted at a predetermined angle in the direction of arrow X with respect to the incident direction and is then transmitted.

Here, since one period Λ _R2 of the liquid crystal alignment pattern in region R2 is shorter than one period Λ _R1 of the liquid crystal alignment pattern in region R1, the angle of refraction of incident light by the optically anisotropic layer 700 is larger for the angle θ _R2 of the transmitted light in region R2 than for the angle θ _R1 of the transmitted light in region R1, as shown in Fig. 21. Also, since one period Λ _R0 of the liquid crystal alignment pattern in region R0 is longer than one period Λ _R1 of the liquid crystal alignment pattern in region R1, the angle of refraction of incident light is smaller for the angle θ _R0 of the transmitted light in region R0 than for the angle θ _R1 of the transmitted light in region R1, as shown in Fig. 21.

Here, in the diffraction of light by an optically anisotropic layer having a liquid crystal orientation pattern in which the orientation of the optical axis of the liquid crystal molecules changes while rotating continuously within the plane, there is a problem that the diffraction efficiency decreases as the diffraction angle increases, i.e., the intensity of the diffracted light weakens. Therefore, if the optically anisotropic layer is configured to have regions with different lengths of one period in which the orientation of the optical axis of the liquid crystal molecules rotates 180° within the plane, the diffraction angle differs depending on the position of incidence of the light, and therefore the amount of diffracted light differs depending on the position of incidence within the plane. In other words, depending on the position of incidence within the plane, there are regions where the transmitted and diffracted light becomes dark.

In contrast, the PB lens 20 of this embodiment has a region where the optically anisotropic layer twists and rotates in the thickness direction, and has regions with different magnitudes of twist angles in the thickness direction. In the example shown in Fig. 21, the twist angle _φR2 in the thickness direction of region R2 of the optically anisotropic layer 700 is larger than the twist angle _φR1 in the thickness direction of region R1. Moreover, region R0 does not have a twist structure in the thickness direction. This makes it possible to suppress a decrease in the diffraction efficiency of refracted light.

In the example shown in FIG. 21, by providing a twisted structure to regions R1 and R2, which have a larger diffraction angle than region R0, it is possible to suppress a decrease in the amount of light refracted in regions R1 and R2. In addition, by making the twisted angle of the twisted structure in region R2, which has a larger diffraction angle than region R1, larger than that of region R1, it is possible to suppress a decrease in the amount of light refracted in region R2. This makes it possible to make the amount of transmitted light uniform depending on the incident position within the plane.

Thus, in the PB lens 20 of this embodiment, in areas in the plane where refraction by the optically anisotropic layer is large, incident light passes through a layer with a large twist angle in the thickness direction and is refracted. In contrast, in areas in the plane where refraction by the optically anisotropic layer is small, incident light passes through a layer with a small twist angle in the thickness direction and is refracted. That is, in the PB lens 20, the twist angle in the thickness direction in the plane can be set according to the magnitude of refraction by the optically anisotropic layer, thereby making it possible to brighten the transmitted light relative to the incident light. Therefore, the PB lens 20 can reduce the refraction angle dependency of the amount of transmitted light in the plane.

The angle of light refraction in the plane of the optically anisotropic layer 700 is larger as the period Λ of the liquid crystal orientation pattern is shorter. In addition, the twist angle in the thickness direction in the plane of the optically anisotropic layer 700 is larger in a region with a short period Λ in which the direction of the optical axis rotates 180° along the arrow X direction in the liquid crystal orientation pattern than in a region with a large period Λ. In the PB lens 20, as shown in FIG. 21 as an example, one period Λ _R2 of the liquid crystal orientation pattern in the region R2 of the optically anisotropic layer 700 is shorter than one period Λ _R1 of the liquid crystal orientation pattern in the region R1, and the twist angle φ _R2 in the thickness direction is larger. That is, the region R2 of the optically anisotropic layer 700 on the light incident side refracts light more.

Therefore, by setting the in-plane twist angle φ in the thickness direction for one period Λ of the target liquid crystal orientation pattern, it is possible to advantageously brighten the transmitted light that is refracted at different angles in different regions in the plane.

As described above, in the PB lens 20, the shorter the period Λ of the liquid crystal orientation pattern, the larger the angle of refraction, so it is possible to brighten the transmitted light by making the twist angle in the thickness direction larger in areas where the period Λ of the liquid crystal orientation pattern is shorter. Therefore, in the PB lens 20, it is preferable to have areas where the permutation of the length of one period and the permutation of the magnitude of the twist angle in the thickness direction are different in areas where the length of one period of the liquid crystal orientation pattern is different.

From the above, the PB lens 20 has an optically anisotropic layer 700 formed using a liquid crystal composition containing liquid crystal molecules 710, and the optically anisotropic layer 700 has a liquid crystal orientation pattern in which the direction of the optical axis derived from the liquid crystal molecules changes while rotating continuously along at least one direction in the plane, and has a region in which the optical axis rotates twistedly in the thickness direction of the optically anisotropic layer 700, and preferably has a region with a different magnitude of twist angle in the thickness direction.

When the length of the optical axis direction originating from the liquid crystal molecule 710 rotating 180° in the plane is taken as one period, it is preferable that the PB lens 20 has regions in which the length of one period in the liquid crystal orientation pattern varies.

In the optically anisotropic layer 700, the multiple regions having different lengths of one period in the liquid crystal orientation pattern are arranged in the order of the length of the one period, and the multiple regions having different magnitudes of twist angles in the thickness direction are arranged in the order of the magnitudes of the twist angles in the thickness direction, and it is preferable that the optically anisotropic layer 700 has a region in which the direction of the permutation of the lengths of one period differs from the direction of the permutation of the magnitudes of the twist angles in the thickness direction.

It is preferable that the optically anisotropic layer 700 has a region in which the twist angle in the thickness direction is 10° to 360°.

It is preferable that the optically anisotropic layer 700 has one period of the liquid crystal orientation pattern that gradually becomes shorter toward the one direction in which the direction of the optical axis originating from the liquid crystal molecules 710 in the liquid crystal orientation pattern changes while continuously rotating.

The liquid crystal orientation pattern of the optically anisotropic layer 700 is preferably a concentric pattern extending from the inside to the outside in the one direction in which the orientation of the optical axis derived from the liquid crystal molecules 710 changes while continuously rotating.

The PB lens 20 shown in FIG. 21 is a PB lens whose twist angle changes in-plane, and is an element with high diffraction efficiency even when the diffraction angle is large, but the PB lens 20 may be a PB lens whose twist angle does not change in-plane. Specifically, the PB lens 20 may be a PB lens that has no twist in the thickness direction or has a constant twist angle in-plane, and for example, the polarized diffraction grating described in JP2008-532085A can be used.

The PB lens 20 is a PB lens having multiple optically anisotropic layers 700, and it is preferable that the optically anisotropic layers 700 have different twist angles in the thickness direction of the optically anisotropic layers 700.

The PB lens 20 is a PB lens having multiple optically anisotropic layers 700, and it is preferable that the optically anisotropic layers 700 have different twist angles in the thickness direction of the optically anisotropic layers 700.

The PB lens 20 is a PB lens having multiple optically anisotropic layers 700, and it is preferable that the optically anisotropic layers 700 have a liquid crystal orientation pattern in which the directions of the optical axes derived from the liquid crystal molecules 710 rotate continuously along at least one direction in the plane are the same.

The length of one period in the liquid crystal orientation pattern is preferably 50 μm or less.

The variable focus element 30 may be a binary variable focus element 30A having one laminated body made of an optical element 10 and a PB lens 20, or a multi-stage variable focus element 30B having two or more laminated bodies made of an optical element 10 and a PB lens 20. In this way, by combining multiple sets of optical elements 10 and PB lenses 20, a variable focus element 30B with multi-stage tunability can be realized.

The variable focus element 30 can be produced, for example, by attaching a PB lens 20 produced by the method described in International Publication No. 2019/189818 to the optical element 10.

(Variation 1 of the fifth embodiment)
In this modification, a variable focus element 30 in which the PB lens 20 in the above-mentioned fifth embodiment is disposed within an optical element 10 and in-cell is described. Fig. 22 is a cross-sectional schematic diagram of a variable focus element according to a first modification of the fifth embodiment. Fig. 23 is an enlarged cross-sectional schematic diagram of a variable focus element according to a first modification of the fifth embodiment. Fig. 24 is a schematic diagram for explaining the orientation of liquid crystal molecules in a first state and a second state of an optical element according to a first modification of the fifth embodiment.

The variable focus element 30 of this modified example is a multi-stage variable focus element 30B having two or more sets of laminated bodies each made of an optical element 10 and a PB lens 20, as shown in FIG. 22.

The PB lens 20 provided in the variable focal element 30 of this modified example is disposed within the optical element 10, as shown in FIG. 23. By in-celling the PB lens 20 in this manner, it is not necessary to attach the PB lens 20 externally, and therefore manufacturing costs can be significantly reduced. It is also possible to reduce the thickness of the variable focal element 30. Note that, for the sake of convenience, the optical element 10 and the PB lens 20 are illustrated separately in FIG. 22.

As shown in FIG. 23, more specifically, the variable focus element 30 of this modified example comprises, in order from the entrance side to the exit side, a second quarter-wave film 14, a first quarter-wave film 13, a first substrate 100, a first liquid crystal layer 500, a second substrate 200, a third substrate 300, a second liquid crystal layer 600, a PB lens 20, and a fourth substrate 400.

The variable focus element 30 may include a first alignment film 41 between the first substrate 100 and the first liquid crystal layer 500. The variable focus element 30 may also include a second alignment film 42 between the second substrate 200 and the first liquid crystal layer 500. The variable focus element 30 may also include a third alignment film 43 between the third substrate 300 and the second liquid crystal layer 600. The variable focus element 30 may also include a fourth alignment film 44 between the PB lens 20 and the second liquid crystal layer 600.

In this modified example, the first liquid crystal cell 11A and the second liquid crystal cell 11B have the same configuration as in embodiment 1, and the first alignment film 41, the second alignment film 42, the third alignment film 43 and the fourth alignment film 44 are horizontal alignment films.

As shown in FIG. 24, in this modified example, the azimuth angle of the alignment direction 512A of the first liquid crystal molecules 512 on the second substrate 200 side in the second state is preferably -12° or more and 10° or less, and more preferably -7.5° or more and 7.2° or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency. Furthermore, the azimuth angle of the alignment direction 612A of the second liquid crystal molecules 612 on the fourth substrate 400 side in the first state is preferably 79° or more and 98° or less, and more preferably 83° or more and 95° or less. By adopting such an embodiment, it is possible to switch between polarization modulation and polarization non-modulation with higher efficiency.

In this modified example, a negative C plate 12 is not disposed between the first liquid crystal cell 11A and the second liquid crystal cell 11B, but a negative C plate 12 may be disposed between the first liquid crystal cell 11A and the second liquid crystal cell 11B.

The in-cell PB lens 20 (PB lens layer) is, in other words, an in-cell retardation layer patterned so that the slow axis direction rotates in-plane.

The PB lens can be in-cell formed, for example, as follows. A photosensitive material for forming an in-cell PB lens, which contains a polymer represented by the following general formula (PB-1), is applied to the fourth substrate 400, a film for forming the PB lens is formed, and then an orientation process is performed on the film for forming the PB lens, thereby forming the PB lens 20 in-cell.

（上記式中、Ｖはスペーサ基を表し、Ｗは光官能基を有する二価の有機基を表し、Ｒ^５は一価の基を表し、ｐは１以上の整数を表す。）

(In the above formula, V represents a spacer group, W represents a divalent organic group having a photofunctional group, ^R5 represents a monovalent group, and p represents an integer of 1 or more.)

V in the above general formula (PB-1) represents a spacer group. It is preferable that V has an alkylene group having 2 or more carbon atoms and represented by -(CH ₂ ) _n - (where n is an integer of 2 or more). By adopting such an embodiment, it is possible to develop a good retardation. It is preferable that the alkylene group is linear.

W in the above general formula (PB-1) represents a divalent organic group having a photofunctional group. Examples of divalent organic groups having a photofunctional group include divalent organic groups containing a photofunctional group (photoreactive site) that undergoes reactions such as photodimerization, photoisomerization, photo-Fries rearrangement, and photodecomposition. Examples of photofunctional groups capable of photodimerization and photoisomerization include, for example, cinnamate groups, chalcone groups, coumarin groups, and stilbene groups. Examples of photofunctional groups capable of photoisomerization include, for example, azobenzene groups. Examples of photofunctional groups capable of photo-Fries rearrangement include, for example, phenol ester groups. Examples of photofunctional groups capable of photodecomposition include, for example, cyclobutane rings.

In formula (PB-1), ^R5 represents a monovalent group. ^R5 is preferably a hydrogen atom or a monovalent hydrocarbon group, and more preferably a hydrogen atom, a methyl group, or an ethyl group.

The alignment treatment of the PB lens forming film is performed by a plurality of alignment treatments, and the directions of the polarized light irradiated in the plurality of alignment treatments are different from each other. The alignment treatment of the PB lens forming film includes, for example, a first alignment treatment of the PB lens forming film with polarized light having an azimuth angle of 0°, a second alignment treatment of the PB lens forming film with polarized light having an azimuth angle of 45°, a third alignment treatment of the PB lens forming film with polarized light having an azimuth angle of 90°, and a fourth alignment treatment of the PB lens forming film with polarized light having an azimuth angle of 135°.

Fig. 25 is a plan view schematic diagram showing the orientation pattern of a PB lens provided in a variable-focus element according to Modification 1 of Embodiment 5. As shown in Fig. 25, the orientation pattern of the PB lens 20 has an orientation direction that continuously rotates, for example, from the center to the periphery. In addition, in a plan view, the orientation directions of the liquid crystal molecules 710 at the position of radius R are all the same. In other words, there is a predetermined angle distribution according to the distance from the center. The period _P1 of the orientation pattern and the diffraction angle θ are expressed as follows:
P ₁ =2×λ/sinθ
The shorter the period of the orientation pattern, the greater the light diffraction. Therefore, if you want to obtain a lens effect that focuses light, you can achieve this by making the pitch wider (smaller diffraction angle) toward the center of the optical element and shorter (larger diffraction angle) toward the periphery.

PB lenses 20 with different diopters D, as described below, can be manufactured by changing the design of this orientation pattern period. The orientation pattern can also be set based on WO 2020/186123, JP 2008-532085 A, etc.

In this modified example, the alignment process is performed using four exposures, but the more the number of exposure divisions, the more efficient the diffraction efficiency of the variable-focus element 30. Fabrication using a multi-light alignment process that applies a light alignment device is compatible with existing liquid crystal factories, and allows for high productivity. In this embodiment, fabrication of the PB lens 20 using a multi-light alignment process is described, but the alignment pattern may also be created using existing methods such as optical interference and laser direct drawing.

The phase difference of the in-cell PB lens 20 (PB lens layer) is preferably 100 nm or more and 500 nm or less, more preferably 200 nm or more and 350 nm or less, and particularly preferably λ/2 (i.e., 275 nm). The diffraction efficiency is expressed by the following formula (1), and is maximum when Δnd = λ/2.

The variable-focus element 30 of this modified example, i.e., a multi-stage variable-focus element 30 that combines multiple laminates of an optical element 10 and a PB lens 20 in-cell in the optical element 10, has, for example, the following characteristics.

Figure 26 is a schematic cross-sectional view illustrating a detailed configuration of a variable focus element according to Modification 1 of Embodiment 5. As shown in Figure 26, the variable focus element 30 includes, in order from the entrance side to the exit side, an optical element 10, a first PB lens 20A1, an optical element 10, a first PB lens 20A1, an optical element 10, a second PB lens 20A2, an optical element 10, a second PB lens 20A2, an optical element 10, a third PB lens 20A3, an optical element 10, and a third PB lens 20A3.

The first PB lens 20A1 has a diopter D = ±0.25, the second PB lens 20A2 has a diopter D = ±0.5, and the third PB lens 20A3 has a diopter D = ±1 lens characteristic. When right-handed circularly polarized light is incident, it has a + (converging) characteristic, and when left-handed circularly polarized light is incident, it has a - (diverging) characteristic.

The following Table 1 explains the state of the optical element 10 and the PB lenses 20A1, 20A2, and 20A3 in each mode of the variable focal element 30 according to the first modification of the fifth embodiment.

The F0 mode will be explained using Table 1 above. In this mode, all optical elements 10 are in the first state (non-modulated). When right-handed circularly polarized light is incident, it is not modulated by the first optical element 10 and enters the first first PB lens 20A1 as is. Here, it is focused by 0.25D. At that time, the exiting light becomes left-handed circularly polarized light. Here, it is a characteristic of the PB lens 20 that the direction of the circularly polarized light changes even after passing through the PB lens 20. Since the optical element 10 is non-modulated, it passes through the second optical element 10 as left-handed circularly polarized light. The second first PB lens 20A1 causes a divergence of -0.25D. As a result, the incident light passes through the first four lenses from the entrance side (optical element 10, first PB lens 20A1, optical element 10, and first PB lens 20A1) as is. After that, the light passes through the second PB lens 20A2 and the PB lens 20A3 in the same manner, and is emitted as it is at 0D, just like the incident light.

The F1 mode will be explained using Table 1 above. In this mode, only the fourth optical element 10 from the incident side is in the second state. In this state, after passing through the first second PB lens 20A2, the light is left-handed circularly polarized with 0.5D, as in the F0 mode. It is then converted to right-handed circularly polarized light by the optical element in the second state. Next, the second second PB lens 20A2 gives it +0.5D, and it is output as left-handed circularly polarized light with a total of 1D. After that, it is output as 1D left-handed circularly polarized light. Because it has become left-handed circularly polarized light after passing through the second second PB lens 20A2, the sign at the third PB lens 20A3 is opposite to that at F0.

The F-2.5 mode will be explained using Table 1 and FIG. 27 above. FIG. 27 is a diagram for explaining the polarization state in the F-2.5 mode of the variable focal point element according to the first modification of the fifth embodiment. As shown in Table 1 and FIG. 27, in the F-2.5 mode, the first four lenses from the entrance side (optical element 10, first PB lens 20A1, optical element 10, and first PB lens 20A1) give -0.5D to the right circularly polarized light, and the last four lenses on the exit side (optical element 10, third PB lens 20A3, optical element 10, and third PB lens 20A3) give -2D to the light, and it is output as right circularly polarized light with a total of -2.5D.

In addition, by using the same principle, multiple focal lengths can be achieved depending on which optical element 10 is set to the second state of the modulation state. In this modified example, only three conditions are shown.

(Modification 2 of the fifth embodiment)
In the above-mentioned embodiment 5 and the first modification of the embodiment 5, the aspect in which a film-like (in-cell polymer-like) PB lens is provided has been described, but the PB lens may be a liquid crystal layer having fluidity, that is, a liquid crystal layer that can be driven by a voltage. In this modification, a case in which the PB lens is a liquid crystal layer that can be driven by a voltage will be described.

As in the above-mentioned embodiment 5 and the first modification of embodiment 5, the PB lens made of a polymer is called a passive PB lens because it cannot be changed by voltage. On the other hand, the PB lens made of a liquid crystal layer with fluidity is called an active PB lens because it can be driven by voltage.

An active PB lens can be fabricated by the following procedure. First, an alignment treatment of the PB lens pattern is performed on the alignment film of one of the pair of substrates. The alignment film of the other substrate is a weak anchoring alignment film (slippery interface). Both substrates are provided with transparent electrodes. When this pair of substrates is bonded together with a liquid crystal layer sandwiched between them, the liquid crystal molecules are aligned along the alignment pattern, and the liquid crystal layer also adopts the alignment of the PB lens pattern. This allows the active PB lens to be realized. More preferably, a PSA (Polymer Sustained Alignment) treatment is then performed to stabilize the alignment of the interface of the liquid crystal molecules, resulting in an active PB lens with higher alignment stability and reliability.

When the voltage is OFF, the active PB lens has a PB lens pattern, so it focuses or diverges light depending on the incident polarization state. When the voltage is ON, the liquid crystal molecules are vertically aligned, so the light passes through as is without focusing or diverging.

In a variable-focus element that combines an sHWP and a passive PB lens as in the above-mentioned embodiment 5 and modification 1 of embodiment 5, the switching is between two values, focusing and divergence, whereas in a variable-focus element that combines an sHWP and an active PB lens as in this modification, the switching can be between three values, focusing, divergence, and transmission. As a result, smoother focus control can be achieved. Alternatively, the number of stacked voltage-driven elements can be reduced to achieve the same number of focal lengths.

(Embodiment 6)
In this embodiment, features unique to this embodiment will be mainly described, and descriptions of contents overlapping with those of the above-mentioned embodiment 1 and its modifications, and embodiments 2 to 5 and their modifications will be omitted. In this embodiment, a head mounted display including a variable focal element 30 will be described. Fig. 28 is a cross-sectional schematic diagram of a head mounted display according to embodiment 6. Fig. 29 is a perspective schematic diagram showing an example of the appearance of the head mounted display according to embodiment 6.

As shown in Figures 28 and 29, the head mounted display 1 of this embodiment includes a display panel 1P that displays an image, a retardation plate 40, and a variable focus element 30. By using the head mounted display 1, light emitted from the display panel 1P, such as a liquid crystal display device or an organic electroluminescence display device, passes through the retardation plate 40 to become circularly polarized light, which then passes through the variable focus element 30 and is visually perceived by the user U.

The effects of the present invention will be explained below using examples and comparative examples, but the present invention is not limited to these examples.

Example 1
A simulation was performed on the optical element 10 of Example 1 having the same configuration as Modification 1 of the above-mentioned Embodiment 1, and the switching performance of polarization modulation and polarization non-modulation was evaluated. The optical element 10 of this example had a first liquid crystal cell 11A and a second liquid crystal cell 11B, and had a negative C plate 12 between them with a retardation Rth of -213 nm in the thickness direction. The first liquid crystal molecules 510 and the second liquid crystal molecules 610 were positive type liquid crystal molecules, and had a refractive index anisotropy Δn = 0.07. The first alignment film 41, the second alignment film 42, the third alignment film 43, and the fourth alignment film 44 were horizontal alignment films.

In the second state, the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side was 0°, and the azimuth angle of the alignment direction 512A of the first liquid crystal molecules 512 on the second substrate 200 side in the second state was 68°. In addition, the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state was 90°, and the azimuth angle of the alignment direction 612A of the second liquid crystal molecules 612 on the fourth substrate 400 side in the first state was 158°. In addition, the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state, the alignment direction 512A of the first liquid crystal molecules 512 on the second substrate 200 side in the second state, the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state, and the alignment direction 612A of the second liquid crystal molecules 612 on the fourth substrate 400 side in the first state correspond to the alignment treatment directions of the first alignment film 41, the second alignment film 42, the third alignment film 43, and the fourth alignment film 44, respectively, so the alignment directions 511A, 512A, 611A, and 612A can be obtained from the alignment treatment directions of the first alignment film 41, the second alignment film 42, the third alignment film 43, and the fourth alignment film 44, respectively.

The first quarter-wavelength film 13 had a reverse wavelength dispersion characteristic, and the second quarter-wavelength film 14 had a flat wavelength dispersion characteristic. Specifically, the quarter-wavelength film with reverse wavelength dispersion characteristic used in the examples and comparative examples had a retardation Re of 450 nm that was 1.01 times that of 550 nm, and a retardation Re of 650 nm that was 0.99 times that of 550 nm. Specifically, the quarter-wavelength film with flat wavelength dispersion characteristic used in the examples and comparative examples had a retardation Re of 450 nm that was 0.89 times that of 550 nm, and a retardation Re of 650 nm that was 1.04 times that of 550 nm.

The retardation Re of the first quarter-wave film 13 and the second quarter-wave film 14 for light with a wavelength of 550 nm was both 140 nm. The azimuth angle of the slow axis 13A of the first quarter-wave film 13 was 58.3°, and the azimuth angle of the slow axis 14A of the second quarter-wave film 14 was 13.9°. The azimuth angles of the slow axis 13A of the first quarter-wave film 13 and the slow axis 14A of the second quarter-wave film 14 were set so that the worst value of S3 emitted from the front of the optical element of Example 1 was optimal.

The optical element 10 of this embodiment further included a first positive C plate 19X between the first quarter-wave film 13 and the second quarter-wave film 14. The retardation Rth in the thickness direction of the first positive C plate 19X was 108 nm.

The optical element 10 of this embodiment further includes a second positive C plate 19A on the side of the second liquid crystal cell 11B opposite the first liquid crystal cell 11A, and a third positive C plate 19B on the side of the first liquid crystal cell 11A opposite the second liquid crystal cell 11B. The retardation Rth in the thickness direction of the second positive C plate 19A was 79 nm. The retardation Rth in the thickness direction of the third positive C plate 19B was 98 nm.

In this embodiment, three positive C plates are arranged to achieve a wide viewing angle: a first positive C plate 19X, a second positive C plate 19A, and a third positive C plate 19B. Note that although ideal C plates are assumed in the embodiment, these C plates may have retardation Re due to manufacturing variations, etc.

The first state is driven by applying a voltage to the first liquid crystal layer 500. In the first state, it is preferable to apply as high a voltage as possible to the first liquid crystal layer 500, and in this embodiment, 20 V was applied. Since the negative C plate 12 is designed so that the phase difference between the driven liquid crystal layer (first liquid crystal layer 500) and the negative C plate 12 is cancelled, only the non-driven liquid crystal layer (second liquid crystal layer 600) is effective. Therefore, a wide viewing angle and wideband sHWP can be realized.

In the second state, the opposite of the first state, a voltage is applied to the second liquid crystal layer 600 to drive it, and the liquid crystal layer (first liquid crystal layer 500) that is rotated 90 degrees from the liquid crystal layer (second liquid crystal layer 600) that was active in the first state becomes active, so that the light that has passed through the two quarter-wave films (first quarter-wave film 13 and second quarter-wave film 14) becomes circularly polarized light having a polarization state opposite to that of the light that entered the optical element 10.

(Evaluation of Example 1)
The Stokes parameter S3 of the emitted light was evaluated when right-handed circularly polarized light (S3=+1) and left-handed circularly polarized light (S3=-1) were incident on the optical element (sHWP) of Example 1. The evaluation of the Stokes parameter S3 was performed by optical calculation using an LCD-MASTER 1D manufactured by Shintech Co., Ltd.

Figure 30 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Example 1 when it is not modulated, when the incident angle is set to 30°. Figure 31 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Example 1 when it is modulated, when the incident angle is set to 30°. Figures 30 and 31 show the evaluation results at wavelengths of 450 nm, 550 nm, and 650 nm.

In Example 1, in the case of right-handed circularly polarized light (S3 = +1) and left-handed circularly polarized light (S3 = -1) shown in Figures 30 and 31, |S3| ≧ 0.98 was achieved in all directions in the range of 450 nm to 650 nm in both modulated and non-modulated states. In other words, the optical element of this example was able to switch between polarization modulation and polarization non-modulation with high efficiency over a wide bandwidth and wide viewing angle. In this example, the results of right-handed circularly polarized light (S3 = +1), which gave worse results than left-handed circularly polarized light (S3 = -1), are shown in Figures 30 and 31.

In order to study the design of a suitable liquid crystal cell, optical calculations were performed on the optical element 10 of Example 1 using an LCD-MASTER 1D manufactured by Shintech Co., Ltd. Specifically, similarly to the above, the Stokes parameter S3 of the emitted light was evaluated when right-handed circularly polarized light (S3=+1) and left-handed circularly polarized light (S3=-1) were incident. In this example, the simulation was performed under the following <conditions>.
<Conditions>
Wavelength: 450nm, 550nm, 650nm (3 wavelengths)
Direction: 0° to 360° in all directions
Polar angle: 30° (fixed)
State: Modulated, unmodulated (2 states)
Incident polarization: Right circular polarization (S3 = +1), left circular polarization (S3 = -1) (2 states)

In the following, based on the results of the simulation, the range in which 90% or more modulation and non-modulation can be achieved at an incident angle of 30° and a wavelength range of 450 nm to 650 nm was determined to be the preferable range. In this example, simulations were performed for all combinations of the above <conditions>, but below, for simplicity, only the results for the worst S3 are shown.

In the simulation of this embodiment, the modulated and unmodulated states were simulated for each of the two incident polarization states (S3 = +1 and S3 = -1). In other words, a total of four states were simulated to obtain the optimal design. Here, the incident polarization state or the output polarization state may be fixed by installing a circular polarizer on the incident or output side. In that case, only that polarization state is considered, that is, the optimal design is performed for the two states.

First, in order to consider the preferred range of the retardation Re of the second quarter-wave film 14, the Stokes parameter S3 for the retardation Re of the second quarter-wave film 14 included in the optical element 10 of Example 1 was obtained by simulation. Figure 32 is a graph showing the Stokes parameter (worst S3) for the retardation Re of the second quarter-wave film included in the optical element of Example 1. The area surrounded by the dashed line in Figure 32 is a preferred range in which 90% or more of modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more of modulation and non-modulation can be achieved.

From Figure 32, it can be seen that the retardation Re of the second quarter-wave film 14 at a wavelength of 550 nm is preferably 112 nm or more and 162 nm or less, and more preferably 121 nm or more and 152 nm or less.

In order to consider the preferred range of the azimuth angle of the slow axis 14A of the second quarter-wave film 14, the Stokes parameter S3 for the azimuth angle of the slow axis 14A of the second quarter-wave film 14 provided in the optical element 10 of Example 1 was obtained by simulation. Figure 33 is a graph showing the Stokes parameter (worst S3) for the azimuth angle of the slow axis of the second quarter-wave film provided in the optical element of Example 1. The area surrounded by the dashed line in Figure 33 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 33, it can be seen that the azimuth angle of the slow axis 14A of the second quarter-wave film 14 is preferably 4° or more and 23° or less, and more preferably 8° or more and 19° or less.

In order to consider a suitable range of the retardation Rth in the thickness direction of the first positive C plate 19X, the Stokes parameter S3 for the retardation Rth in the thickness direction of the first positive C plate 19X included in the optical element 10 of Example 1 was obtained by simulation. FIG. 34 is a graph showing the Stokes parameter (worst S3) for the retardation Rth in the thickness direction of the first positive C plate included in the optical element of Example 1. The area surrounded by the dashed line in FIG. 34 is a preferred range in which 90% or more of modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more of modulation and non-modulation can be achieved.

From Figure 34, it can be seen that the retardation Rth in the thickness direction of the first positive C plate 19X is preferably 0 nm or more and 320 nm or less, and more preferably 0 nm or more and 230 nm or less.

In order to consider the preferred range of the retardation Re of the first quarter-wave film 13, the Stokes parameter S3 for the retardation Re of the first quarter-wave film 13 included in the optical element 10 of Example 1 was obtained by simulation. Figure 35 is a graph showing the Stokes parameter (worst S3) for the retardation Re of the first quarter-wave film included in the optical element of Example 1. The area surrounded by the dashed line in Figure 35 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 35, it can be seen that the retardation Re of the first quarter-wave film 13 at a wavelength of 550 nm is preferably 72 nm or more and 210 nm or less, and more preferably 110 nm or more and 175 nm or less.

In order to consider the preferred range of the azimuth angle of the slow axis 13A of the first quarter-wave film 13, the Stokes parameter S3 for the azimuth angle of the slow axis 13A of the first quarter-wave film 13 provided in the optical element 10 of Example 1 was obtained by simulation. Figure 36 is a graph showing the Stokes parameter (worst S3) for the azimuth angle of the slow axis of the first quarter-wave film provided in the optical element of Example 1. The area surrounded by the dashed line in Figure 36 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 36, it can be seen that the azimuth angle of the slow axis 13A of the first 1/4 wavelength film 13 is preferably 52° or more and 66° or less, and more preferably 55° or more and 64° or less.

In order to consider the preferred range of the retardation Rth in the thickness direction of the second positive C plate 19A, the Stokes parameter S3 for the retardation Rth in the thickness direction of the second positive C plate 19A provided in the optical element 10 of Example 1 was obtained by simulation. FIG. 37 is a graph showing the Stokes parameter (worst S3) for the retardation Rth in the thickness direction of the second positive C plate provided in the optical element of Example 1. The area surrounded by the dashed line in FIG. 37 is a preferred range in which 90% or more of modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more of modulation and non-modulation can be achieved.

From Figure 37, it can be seen that the retardation Rth in the thickness direction of the second positive C plate 19A is preferably 0 nm or more and 252 nm or less, and more preferably 0 nm or more and 174 nm or less.

In order to consider the preferred range of retardation Re of the second liquid crystal layer 600 in the first state at a wavelength of 550 nm, the Stokes parameter S3 for retardation Re of the second liquid crystal layer 600 in the first state at a wavelength of 550 nm was obtained by simulation. Figure 38 is a graph showing the Stokes parameter (worst S3) for retardation Re of the second liquid crystal layer in the optical element of Example 1 in the first state at a wavelength of 550 nm. In Figure 38, the area surrounded by the dashed line is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 38, it can be seen that the retardation Re of the second liquid crystal layer 600 in the first state at a wavelength of 550 nm is preferably 200 nm or more and 280 nm or less, and more preferably 220 nm or more and 264 nm or less.

In order to consider the preferred range of the twist angle of the second liquid crystal molecules 610 in the first state, the Stokes parameter S3 for the twist angle in the first state of the second liquid crystal molecules 610 of the optical element 10 of Example 1 was obtained by simulation. Figure 39 is a graph showing the Stokes parameter (worst S3) for the twist angle in the first state of the second liquid crystal molecules of the optical element of Example 1. The region surrounded by the dashed line in Figure 39 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the region surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 39, it can be seen that the second liquid crystal molecules 610 in the first state are preferably twisted at a twist angle of 57° or more and 76° or less, and more preferably at a twist angle of 62° or more and 73° or less.

In order to consider a suitable range of the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state, the Stokes parameter S3 for the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state of the optical element 10 of Example 1 was obtained by simulation. Figure 40 is a graph showing the Stokes parameter (worst S3) for the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state of the optical element of Example 1. In Figure 40, the area surrounded by the dashed line is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 40, it can be seen that the azimuth angle of the alignment direction 611A of the second liquid crystal molecules 611 on the third substrate 300 side in the first state is preferably 79° or more and 98° or less, and more preferably 83° or more and 95° or less.

In order to consider the preferred range of the retardation Rth in the thickness direction of the negative C plate 12, the Stokes parameter S3 for the retardation Rth in the thickness direction of the negative C plate 12 included in the optical element 10 of Example 1 was obtained by simulation. FIG. 41 is a graph showing the Stokes parameter (worst S3) for the retardation Rth in the thickness direction of the negative C plate included in the optical element of Example 1. The area surrounded by the dashed line in FIG. 41 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 41, it can be seen that the retardation Rth in the thickness direction of the negative C plate 12 is preferably -410 nm or more and 0 nm or less, and more preferably -339 nm or more and -115 nm or less.

In order to consider the preferred range of retardation Re of the first liquid crystal layer 500 in the second state at a wavelength of 550 nm, the Stokes parameter S3 for retardation Re of the first liquid crystal layer 500 in the second state at a wavelength of 550 nm was obtained by simulation. Figure 42 is a graph showing the Stokes parameter (worst S3) for retardation Re of the first liquid crystal layer in the optical element of Example 1 in the second state at a wavelength of 550 nm. In Figure 42, the area surrounded by the dashed line is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 42, it can be seen that the retardation Re of the first liquid crystal layer 500 in the second state at a wavelength of 550 nm is preferably 196 nm or more and 280 nm or less, and more preferably 210 nm or more and 265 nm or less.

In order to consider a suitable range of the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state, the Stokes parameter S3 for the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state of the optical element 10 of Example 1 was obtained by simulation. Figure 43 is a graph showing the Stokes parameter (worst S3) for the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state of the optical element of Example 1. In Figure 43, the area surrounded by the dashed line is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 43, it can be seen that the azimuth angle of the alignment direction 511A of the first liquid crystal molecules 511 on the first substrate 100 side in the second state is preferably -12° or more and 10° or less, and more preferably -7.5° or more and 7.2° or less.

In order to consider the preferred range of the twist angle of the first liquid crystal molecule 510 in the second state, the Stokes parameter S3 for the twist angle in the second state of the first liquid crystal molecule 510 of the optical element 10 of Example 1 was obtained by simulation. Figure 44 is a graph showing the Stokes parameter (worst S3) for the twist angle in the second state of the first liquid crystal molecule of the optical element of Example 1. The region surrounded by the dashed line in Figure 44 is a preferred range in which 90% or more modulation and non-modulation can be achieved, and the region surrounded by the dashed line is a more preferred range in which 95% or more modulation and non-modulation can be achieved.

From Figure 44, it can be seen that in the second state, the first liquid crystal molecules 510 are preferably twisted at a twist angle of 58° or more and 78° or less, and more preferably at a twist angle of 62° or more and 75° or less.

In order to consider the preferred range of the retardation Rth in the thickness direction of the third positive C plate 19B, the Stokes parameter S3 for the retardation Rth in the thickness direction of the third positive C plate 19B included in the optical element 10 of Example 1 was obtained by simulation. FIG. 45 is a graph showing the Stokes parameter (worst S3) for the retardation Rth in the thickness direction of the third positive C plate included in the optical element of Example 1. The area surrounded by the dashed line in FIG. 45 is a preferred range in which 90% or more of modulation and non-modulation can be achieved, and the area surrounded by the dashed line is a more preferred range in which 95% or more of modulation and non-modulation can be achieved.

From Figure 45, it can be seen that the retardation Rth in the thickness direction of the third positive C plate 19B is preferably 0 nm or more and 290 nm or less, and more preferably 0 nm or more and 213 nm or less.

(Comparative Example 1)
FIG. 46 is a cross-sectional schematic diagram of an optical element according to Comparative Example 1. For the optical element 10R1 of Comparative Example 1 shown in FIG. 46, a simulation was performed in the same manner as in Example 1 to evaluate the switching performance of polarization modulation and polarization non-modulation. The optical element 10R1 of Comparative Example 1 was an optical element corresponding to the optical element of Comparative Example 1. The optical element 10R1 of Comparative Example 1 was provided with, in order from the incident side to the exit side, a quarter-wave film 15R having a slow axis azimuth angle of 75°, a half-wave film 16R having a slow axis azimuth angle of 15°, a liquid crystal cell 11R1 having a TN liquid crystal layer 500R1 twisted by 90°, a half-wave film 17R having a slow axis azimuth angle of -75°, and a quarter-wave film 18R having a slow axis azimuth angle of -15°.

(Comparative Example 2)
Fig. 47 is a cross-sectional schematic diagram of an optical element according to Comparative Example 2. For the optical element 10R2 of Comparative Example 2 shown in Fig. 47, a simulation was performed in the same manner as in Example 1 to evaluate the switching performance of polarization modulation and polarization non-modulation. The optical element 10R2 of Comparative Example 2 was an optical element corresponding to the optical element of Comparative Example 2. The optical element 10R2 of Comparative Example 2 had a structure in which a TN liquid crystal layer 500R2 twisted at 70° and a TN liquid crystal layer 500R3 twisted at -70° were laminated in this order from the incident side to the exit side.

(Evaluation of Comparative Example 1 and Comparative Example 2)
FIG. 48 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Comparative Example 1 when it is not modulated, when the incident angle is set to 30°. FIG. 49 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Comparative Example 1 when it is modulated, when the incident angle is set to 30°. FIG. 50 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Comparative Example 2 when it is not modulated, when the incident angle is set to 30°. FIG. 51 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Comparative Example 2 when it is modulated, when the incident angle is set to 30°. In FIG. 48 to FIG. 51, the evaluation results at wavelengths of 450 nm, 550 nm, and 650 nm are shown.

As shown in Figures 48 and 49, in Comparative Example 1, |S3| ≧ 0.9 could not be achieved both when modulated and when not modulated. As shown in Figures 50 and 51, in Comparative Example 2, |S3| ≧ 0.9 was generally achieved when modulated, but |S3| ≧ 0.9 could not be achieved when not modulated.

Example 2
Fig. 52 is a cross-sectional schematic diagram of an optical element according to Example 2. For the optical element 10 of Example 2 shown in Fig. 52, a simulation was performed in the same manner as in Example 1 to evaluate the switching performance of polarization modulation and polarization non-modulation. The optical element 10 of Example 2 does not include a first positive C plate 19X, the refractive index anisotropy Δn of the first liquid crystal molecules 510 and the second liquid crystal molecules 610 is 0.066, the retardation Rth in the thickness direction of the second positive C plate 19A and the third positive C plate 19B is 70 nm, the retardation Rth in the thickness direction of the negative C plate 12 is -140 nm, the azimuth angle of the slow axis 13A of the first ¼ wavelength film 13 is 57.2 °, and the azimuth angle of the slow axis 14A of the second ¼ wavelength film 14 is 12.2 °. Except for this, the optical element 10 of Example 2 had the same configuration as Example 1. The azimuth angle of the slow axis 13A of the first 1/4 wavelength film 13 and the azimuth angle of the slow axis 14A of the second 1/4 wavelength film 14 were set so as to optimize the worst value of S3 emitted from the front surface of the optical element of Example 2.

Figure 53 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Example 2 when it is not modulated, when the incident angle is set to 30°. Figure 54 is a graph showing the relationship between the Stokes parameter S3 and the azimuth angle of the optical element according to Example 2 when it is modulated, when the incident angle is set to 30°. Figures 53 and 54 show the evaluation results at wavelengths of 450 nm, 550 nm, and 650 nm.

As shown in Figures 53 and 54, Example 2 exhibited better S3 characteristics than Comparative Examples 1 and 2 in the range of 450 nm to 650 nm both during modulation and non-modulation. On the other hand, it was found that Example 1 was able to switch between the modulated and non-modulated states more efficiently than Example 2.

Example 3
In Example 3, the first quarter-wave film 13 of Example 1 having the reverse wavelength dispersion characteristic was changed to a quarter-wave film having the flat wavelength dispersion characteristic, and the configuration was examined. A simulation was performed on the optical element of Example 3 in which the first quarter-wave film 13 has the flat wavelength dispersion characteristic, and the optimal values of the azimuth angle of the slow axis 13A of the first quarter-wave film 13 and the azimuth angle of the slow axis 14A of the second quarter-wave film 14 were obtained so that the worst value of S3 emitted from the front surface of the optical element of Example 3 was the best. The results are shown in Table 2 below together with Examples 1 and 2.

From the results of Example 3 in the table above, it was found that when both the first 1/4 wavelength film 13 and the second 1/4 wavelength film 14 have flat wavelength dispersion characteristics, it is preferable to set the azimuth angle of the slow axis 13A of the first 1/4 wavelength film 13 to 57.9° and the azimuth angle of the slow axis 14A of the second 1/4 wavelength film 14 to 14.3°. As with Example 1, the optical element of this example can switch between polarization modulation and polarization non-modulation with high efficiency over a wide bandwidth and a wide viewing angle.

1: Head-mounted display 1P: Display panel 10, 10R1, 10R2: Optical elements 11A, 11B, 11R1: Liquid crystal cell 12: Negative C plate 13, 14, 15R, 18R: 1/4 wavelength film 13A, 14A: Slow axis 16R, 17R: 1/2 wavelength film 19A, 19B, 19X: Positive C plate 20, 20A1, 20A2, 20A3: Pancharatnam Berry (PB) lens 30, 30A, 30B: Variable focus element 40: Retardation plate 4 1, 42, 43, 44: Alignment films 100, 200, 300, 400: Substrates 110, 210, 310, 410: Support substrates 120, 220, 320, 420: Solid electrodes 500, 600: Liquid crystal layers 500R1, 500R2, 500R3: TN liquid crystal layers 510, 511, 512, 610, 611, 612, 710: Liquid crystal molecules 511A, 512A, 611A, 612A: Alignment direction 700: Optically anisotropic layers LC0, LC1, LC2: Left circularly polarized light R0, R1, R2: Region U: User

Claims (22)

a first substrate, a first liquid crystal layer containing first liquid crystal molecules, a second substrate, a third substrate, a second liquid crystal layer containing second liquid crystal molecules, and a fourth substrate,
the first substrate, the first liquid crystal layer, and the second substrate constitute a first liquid crystal cell;
the third substrate, the second liquid crystal layer, and the fourth substrate constitute a second liquid crystal cell;
the first liquid crystal cell has a first electrode for applying a voltage to the first liquid crystal layer on at least one of the first substrate and the second substrate;
the second liquid crystal cell has a second electrode for applying a voltage to the second liquid crystal layer on at least one of the third substrate and the fourth substrate,
The first electrode and the second electrode are
the second liquid crystal molecules are arranged so as to be switchable between a first state in which the second liquid crystal molecules are twisted and the first liquid crystal molecules are vertically aligned and a second state in which the first liquid crystal molecules are twisted and the second liquid crystal molecules are vertically aligned;
an azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state and an azimuth angle of the alignment direction of the second liquid crystal molecules on the fourth substrate side in the first state are angles obtained by rotating an azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state and an azimuth angle of the alignment direction of the first liquid crystal molecules on the second substrate side in the second state by ¼ in the same direction,
a first quarter-wave film and a second quarter-wave film disposed on a side of the first liquid crystal cell opposite the second liquid crystal cell, or on a side of the second liquid crystal cell opposite the first liquid crystal cell;
An optical element, wherein the first quarter-wave film is disposed between the second quarter-wave film and the first and second liquid crystal cells. The optical element according to claim 1, characterized in that the retardation Re of the first quarter-wave film at a wavelength of 550 nm is 72 nm or more and 210 nm or less. The optical element according to claim 1, characterized in that the azimuth angle of the slow axis of the first quarter-wave film is 52° or more and 66° or less. The optical element according to claim 1, characterized in that the retardation Re of the second quarter-wave film at a wavelength of 550 nm is 112 nm or more and 162 nm or less. The optical element according to claim 1, characterized in that the azimuth angle of the slow axis of the second quarter-wave film is 4° or more and 23° or less. The optical element of claim 1, wherein at least one of the first quarter-wave film and the second quarter-wave film has reverse wavelength dispersion characteristics. The optical element of claim 1, wherein at least one of the first quarter-wave film and the second quarter-wave film has flat wavelength dispersion characteristics. The optical element according to claim 1, characterized in that the retardation Re of the first liquid crystal layer in the second state at a wavelength of 550 nm is 196 nm or more and 280 nm or less. The optical element according to claim 1, characterized in that the retardation Re of the second liquid crystal layer in the first state at a wavelength of 550 nm is 200 nm or more and 280 nm or less. The optical element of claim 1, characterized in that the first liquid crystal molecules in the second state are twisted and aligned with a twist angle of 58° or more and 78° or less. The optical element of claim 1, characterized in that the second liquid crystal molecules in the first state are twisted and aligned with a twist angle of 57° or more and 76° or less. The optical element of claim 1, characterized in that the azimuth angle of the alignment direction of the first liquid crystal molecules on the first substrate side in the second state is -12° or more and 10° or less. The optical element according to claim 1, characterized in that the azimuth angle of the alignment direction of the second liquid crystal molecules on the third substrate side in the first state is 79° or more and 98° or less. a first positive C plate disposed between the first and second quarter wave films;
a second positive C plate disposed on the opposite side of the second liquid crystal cell to the first liquid crystal cell;
2. The optical element according to claim 1, further comprising: a third positive C plate disposed on an opposite side of the first liquid crystal cell to the second liquid crystal cell. The optical element described in claim 14, characterized in that the retardation Rth in the thickness direction of the first positive C plate is 0 nm or more and 320 nm or less. The optical element described in claim 14, characterized in that the retardation Rth in the thickness direction of the second positive C plate is 0 nm or more and 252 nm or less. The optical element described in claim 14, characterized in that the retardation Rth in the thickness direction of the third positive C plate is 0 nm or more and 290 nm or less. The optical element according to claim 1, further comprising a negative C plate between the first liquid crystal cell and the second liquid crystal cell. The optical element described in claim 18, characterized in that the retardation Rth in the thickness direction of the negative C plate is -410 nm or more and 0 nm or less. A variable focus element comprising an optical element according to any one of claims 1 to 19 and a Pancharatnam Berry lens. The variable focus element of claim 20, wherein the Pancharatnam Berry lens is disposed within the optical element. A head mounted display comprising the variable focus element according to claim 20.

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