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WO2024158552A1 - Procédé de fabrication de cpc - Google Patents

Procédé de fabrication de cpc Download PDF

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Publication number
WO2024158552A1
WO2024158552A1 PCT/US2024/010786 US2024010786W WO2024158552A1 WO 2024158552 A1 WO2024158552 A1 WO 2024158552A1 US 2024010786 W US2024010786 W US 2024010786W WO 2024158552 A1 WO2024158552 A1 WO 2024158552A1
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Prior art keywords
clc
cell
low
field
cpc
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Iam-Choon Khoo
Chun-Wei Chen
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Penn State Research Foundation
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Penn State Research Foundation
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
    • G02B1/002—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of materials engineered to provide properties not available in nature, e.g. metamaterials
    • G02B1/005—Optical elements characterised by the material of which they are made; Optical coatings for optical elements made of materials engineered to provide properties not available in nature, e.g. metamaterials made of photonic crystals or photonic band gap materials

Definitions

  • the innovation relates to methods and apparatuses for the fabrication of very thick ( ⁇ 1 mm or more than 1 mm thick) chiral photonic crystals [CPC], which can be utilized to form optics, screens, and other devices.
  • the fabricated CPC can be very thick (e.g., up to 2 mm or 2.2 mm thick, etc.).
  • Photonic crystals are disclosed in U.S. Patent No. 7,660,029, 8,163,575, and 9,513,405 Photonic crystals in their various forms and dimensions are utilized in optical/photonic devices and applications.
  • CPCs Newly emergent chiral photonic crystals
  • Liquid crystals-based display screens are used in devices such as cell phone, televisions, computer monitors, screens, as well as advanced optical information and communication systems to control, manipulate and process light over a very wide spectrum.
  • cholesteric liquid crystals with their helical crystalline axis form and function efficiently as one-dimensional (1-D) CPCs that are highly tunable.
  • CLC cholesteric liquid crystals
  • CLC fabricated using conventional fabrication techniques typically rely on surface alignment enforced selfassembly, which can at most achieve cell thickness of a few to a few ten microns, corresponding to N ⁇ 10 to at most 100 or so.
  • Figure 1 shows a typical texture of a 300-micron thick CLC fabricated using a conventional technique based on molecular self-assembly with tightly bundled defects lines that can be formed by non-standing CLC helices, which can take on the thick oily streaks (TOS) appearance.
  • the defect lines shown in Figure 1 e.g., the lying cholesteric helices
  • FASA Field Assisted Self-Assembly
  • Figure 3 illustrates a photograph and micrograph that provide examples of such leakage and bubble formation.
  • figure 3 is a photograph of a surface-aligned 0.3-mm sample with an applied field of ⁇ 4 V/um in the isotropic liquid phase; bubbles tend to form due to leakage from the edges.
  • the left dashed circle: in Figure 3 identifies bubble formation and the right dashed circle identifies material leakage from a sealant.
  • EHD electrohydrodynamic
  • instabilities e.g., turbulence
  • USH transparent uniform standing helices
  • Embodiments of the process can employ two different-frequency electric fields on a standard CLC mixture with a nematic liquid crystal constituent that possesses negative dielectric anisotropy (As ⁇ 0).
  • Embodiments of this process can also be referred to as a Dual Frequency Fields Assembly (DFFA) technique.
  • DFFA Dual Frequency Fields Assembly
  • Embodiments of the process can start by applying a direct current (DC) or a very low frequency (e.g., a few Hz) alternating current (AC) electric field on the CLC mixture to completely randomize the cholesteric helices of the CLC mixture. This can then be followed by a High-Frequency (>100 Hz, typical range: ⁇ 100-to-10,000 Hz) AC electric field which exerts a reorienting torque on the nematic constituent to reorder these random helices into uniform standing helices — the characteristic of a well-aligned CLC which functions as a 1-D chiral photonic crystals.
  • Embodiments of the process can be configured as a non-thermal process that can bypass the thermal heating and cooling steps of the FASA technique and the hard-to-control isotropic-liquid-crystalline mixed phase state and circumvent all the associated drawbacks and adverse effects.
  • some embodiments of our process for fabrication of a chiral photonic crystal CPC can include applying a low frequency (Low-F) field to a CLC cell for a pre-selected Low-F time period to randomize cholesteric helices of the CLC cell and applying a high frequency (High- F) field to the CLC cell for a pre-selected High-F time period to reorder the randomized cholesteric helices into uniform standing helices.
  • a low frequency (Low-F) field to a CLC cell for a pre-selected Low-F time period to randomize cholesteric helices of the CLC cell
  • High- F high frequency
  • the pre-selected Low-F time period can be a time period that is less than the pre-selected High-F time period.
  • the pre-selected Low-F time period can be greater than 0 seconds and less than 2 minutes, greater than 0 seconds and less than 1 minute, greater than 0 seconds and less than 30 seconds, greater than 0 seconds and less than or equal to 15 seconds, or greater than 0 seconds and less than 1 second.
  • the pre-selected High-F time period can be greater than 1 minute and less than 24 hours, greater than 1 minute and less than 20 hours, greater than2 minutes and less than 10 hours, greater than 2 minutes and less than or equal to 8 hours, or greater than 3 minutes and less than 5 hours.
  • Other time ranges for the pre-selected Low-F time period and the pre-selected High-F time period can also be utilized.
  • the low-F field can be less than 100 Hz and greater than 0 Hz and the High-F field can be greater than or equal to 100 Hz (e g. greater than 100 Hz and less than 1 MHz, between 100 Hz and 10 MHz, etc.).
  • the applying of the High-F field can occur after the Low-F field was applied to the CLC cell as well.
  • Embodiments of the process can also include other steps or features.
  • the process can also include pretreating cell walls and positioning a CLC mixture between the cell walls so the cell walls enclose the CLC mixture to form the CLC cell.
  • the pretreating can include applying a transparent electrode coating to the cell walls.
  • the process can include coating the cell walls, baking the cell walls for a pre-selected first baking time period at a first pre-selected first bake temperature, and baking the cell walls for a second pre-selected second bake temperature that is greater than the first preselected first bake temperature for a second pre-selected second bake time period.
  • the cell walls can be coated via spin-coating at a pre-selected spin coating rotational speed for a pre-selected polymer coating application time period.
  • the process can also include rubbing the cell walls unidirectionally at a pre-selected rubbing speed and pre-selected rubbing pressure for a pre-selected rubbing time period or applying polarized light to apply surface alignment to the cell walls.
  • the process can include filling in space between cell walls of the CLC cell with a pre-selected CLC starting material from a formed structure before the applying of the Low-F field to the CLC cell.
  • Embodiments of an apparatus for fabrication of a CPC can be configured to implement an embodiment of our process.
  • Embodiments of the apparatus include a low frequency (Low-F) treatment device configured to apply a Low-F field to at least one CLC cell for a pre-selected Low- F time period to randomize cholesteric helices of the CLC cell and a high frequency (High-F) treatment device configured to receive the CLC cell with the randomized cholesteric helices and apply a High-F field to the CLC cell for a pre-selected High-F time period to reorder the randomized cholesteric helices into uniform standing helices.
  • Low-F low frequency
  • High-F high frequency
  • Embodiments of the apparatus can be configured so that the low-F field can be less than 100 Hz and greater than 0 Hz and the High-F field can be greater than or equal to 100 Hz (e.g. greater than 100 Hz and less than 1 MHz, between 100 Hz and 10 MHz, etc.).
  • the apparatus can be arranged and configured so that the application of the High-F field via the High-F treatment device can occur after the Low-F field was applied to the CLC cell as well.
  • the apparatus can also include other features or elements.
  • embodiments of the apparatus can include a pretreatment mechanism to pretreat cell walls that encapsulate a CLC of the CLC cell.
  • Embodiments of devices are also provided.
  • a device that includes the CLC cell having the USH formed from an exemplary embodiment of our process and/or apparatus can be provided.
  • the device can be configured as a display, an optical device, a liquid crystal display, or an optical system in some embodiments.
  • a high-periodic number (N) chiral photonic crystal is also provided, where N is greater than 1000.
  • the HN-CPC can include the CLC cell formed via an embodiment of our process and/or apparatus to have the USH.
  • the CLC cell can be 1 millimeter (mm) thick to 2 mm thick or can be 1 mm to 2.2 mm thick (e.g. can range from 1 mm thick to 2.2 mm thick, can be between 1 mm and 2 mm thick, etc.).
  • N can be greater than 5000 (e.g. between 5000 and 10000, etc.).
  • a HN-CPC can be considered a super high periodic number chiral photonic crystal (SN-CPC) as well.
  • a device including the HN-CPC is also provided.
  • Embodiments of the device can be a display, an optical device, a liquid crystal display, or an optical system, for example.
  • the apparatus can include a treatment apparatus configured to apply a low frequency (Low-F) treatment to at least one CLC cell and subsequently apply a high frequency (High-F) treatment to the CLC cell.
  • the Low-F treatment can apply a Low-F field to the CLC cell for a pre-selected Low-F time period to randomize cholesteric helices of the CLC cell and the High-F treatment can include applying a High-F field to the CLC cell for a pre-selected High-F time period to reorder the randomized cholesteric helices into uniform standing helices.
  • the pre-selected Low-F time period can be a time period that is less than the pre-selected High-F time period.
  • the pre-selected Low-F time period can be greater than 0 seconds and less than 2 minutes, greater than 0 seconds and less than 1 minute, greater than 0 seconds and less than 30 seconds, greater than 0 seconds and less than or equal to 15 seconds, or greater than 0 seconds and less than 1 second.
  • the pre-selected High-F time period can be greater than 1 minute and less than 24 hours, greater than 1 minute and less than 20 hours, greater than2 minutes and less than 10 hours, greater than 2 minutes and less than or equal to 8 hours, or greater than 3 minutes and less than 5 hours.
  • Other time ranges for the pre-selected Low-F time period and the pre-selected High-F time period can also be utilized.
  • the low-F field can be less than 100 Hz and greater than 0 Hz and the High-F field can be greater than or equal to 100 Hz (e.g. greater than 100 Hz and less than 1 MHz, between 100 Hz and 10 MHz, etc.).
  • a field strength of the Low-F field can be 0.84 V/pm and a field strength of the High-F field can be 0.84 V/pm.
  • Other embodiments may utilize other field strengths.
  • other embodiments can be adapted so that the Low-F field has a different field strength than the High-F field that is utilized (e.g. the Low-F field strength can be greater than the High-F field strength or the Low-F field strength can be less than the High-F field strength).
  • Figure 1 is a photograph that illustrates a typical texture of a 300-micron thick CLC fabricated with conventional technique based on molecular self-assembly.
  • Figure 2 is a flow chart illustrating a process for fabricating a well aligned 300-micron thick CLC as disclosed in our paper, Chun-Wei Chen and I. C. Khoo, “Optical vector field rotation and switching with near-unity transmission by fully developed chiral photonic crystals,” Proceedings of the National Academy of Sciences (PNAS) April 20,2021 118 (16). http s : //www. pnas . org/content/ 1 18/ 16/e2021304118 ) .
  • Figure 3 is a photograph of a surface-aligned 0.3-mm sample with an applied field of ⁇ 4 V/pm. Bubbles tend to form due to leakage from the edges.
  • the left dashed circle in Figure 3 identifies an example of bubble formation and the right dashed circle in Figure 3 identifies an example of material leakage from sealant.
  • FIG 4 is a flow chart of an exemplary embodiment of a process for fabrication of a CPC. This process can be implemented to fabricate at least one CPC (e.g. at least one CPC, a plurality of CPCs, etc.).
  • This process can be implemented to fabricate at least one CPC (e.g. at least one CPC, a plurality of CPCs, etc.).
  • Figure 5 illustrates photographs of a non-surface treated 0.3 mm sample captured (i) at the initial state, (ii) immediately after a 15 -Hz AC low frequency field (Low-F) treatment was applied for a Low- F time period and (iii) immediately after 1000 Hz alternating current high frequency treatment (High-F) treatment was applied.
  • Low-F AC low frequency field
  • High-F alternating current high frequency treatment
  • Figure 6 illustrates micrographs of the non-surface -treated 0.3-mm sample captured (i) at the initial state, (ii) immediately after a 15-Hz DC current Low-F treatment was applied for a Low-F time period and (iii) thirty minutes after 1000 Hz alternating current High-F treatment was applied.
  • Figure 7 is a graph illustrating an unpolarized transmission spectrum of a 300-micron thick CLC sample made using an exemplary embodiment of our process.
  • Figure 8 is a graph illustrating an unpolarized transmission spectrum of a CLC formed using a conventional fabrication process.
  • Figure 9 is a graph illustrating the unpolarized transmission spectra of two samples fabricated after an exemplary embodiment of a Dual Frequency Field Assembly (DFFA) process was applied to the sample.
  • One sample was formed without a surface treatment (w/o surface treatment) and the other sample was formed with a surface treatment (w/ surface treatment).
  • Figure 10 is a photograph and micrograph of an exemplary embodiment of a CLC based CPC fabricated using an exemplary embodiment of our process to have a CPC with a thickness (d) ranging up to 900 microns (e.g. 0.9 mm), an area size of 6.3 *9.0 mm 2 and having a number of periods (N) of about 3300.
  • a photograph of this sample is the “(a)” image included in Figure 10.
  • the micrograph of the portion of the sample labeled as “PSU x NSYSU” shown in Figure 10 that was captured several days after fabrication is the “(b)” image included in Figure 10.
  • Figure 11 is a graph illustrating the unpolarized transmission spectrum of the exemplary embodiment shown in Figure 10.
  • Figure 12 is a graph illustrating an optical rotation angle of the sample shown in Figure 10 as a function of the wavelength.
  • FIG 13 is a block diagram illustrating an exemplary embodiment of an apparatus for fabrication of a CPC.
  • This embodiment of our apparatus can utilize an embodiment of our process for fabrication of a CPC (e.g. at least one CPC, a plurality of CPCs, etc.).
  • Figure 14 is a flow chart similar to Figure 4 of an exemplary embodiment of a process for fabrication of a CPC. This process can be implemented to fabricate at least one CPC (e.g. at least one CPC, a plurality of CPCs, etc.). Figure 14 includes an enlargement to help better illustrate the change from completely disordered helices (CDH) to uniform standing helices (USH) that can be provided via the reordering that occurs as a result of the application of a High-f field.
  • CDH completely disordered helices
  • USH uniform standing helices
  • Figure 15 is a photograph and micrograph of an exemplary embodiment of a CLC based CPC fabricated using an exemplary embodiment of our process to have a CPC with a thickness (L) ranging up to 1.1 mm and area size of 6.3 *9.0 mm 2 and having a number of periods (N) of about 3400.
  • a photograph ofthis sample is the “(a)” image included in Figure 15.
  • the micrograph of the portion of the sample labeled as “PSU x NSYSU” shown in Figure 15 that was captured several days after fabrication is the “(b)” image included in Figure 15.
  • Figure 16 is a graph illustrating the transmission spectra of the exemplary embodiment shown in Figure 15 after it was formed (“As formed”) and six months after it was formed and stored at room temperature (“After 6 months”).
  • Figure 17 is a graph illustrating the spectrally resolved polarization rotation angle obtained with the sample shown in Figure 15. This graph shows a giant optical rotation of at least 2TI and up to several thousand degrees for this sample.
  • Figure 18 is series of graphs illustrating the sharp PBF and out-of-PBG transmittance for the 1.1-mm-thick sample (L of about 1.1 mm) of Figure 15 and a 2.2-mm-thick sample (L of about 2.2 mm) that was fabricated using an exemplary embodiment of our DFFA fabrication method.
  • Figure 19 is schematic diagram of an exemplary embodiment of an apparatus for reversible tuning of optical polarization rotation utilizing an exemplary embodiment of a super high N chiral photonic crystal (SN-CPC) formed via an exemplary embodiment of our DFFA.
  • the SN-CPC of this embodiment is positioned between glass windows, or cell walls, coated with indium tin oxide (ITO).
  • ITO indium tin oxide
  • Figure 20 is a schematic diagram of an exemplary embodiment of an apparatus for ultrafast polarization switching of a probe pulse that utilizes an exemplary embodiment of a super high N chiral photonic crystal (SN-CPC) formed via an exemplary embodiment of our DFFA.
  • SN-CPC super high N chiral photonic crystal
  • Figures 4 and 14 each illustrates an embodiment of DFFA, which can be configured as a non-thermal process that can bypass thermal heating and cooling steps of the FAS A process so that the isotropic-liquid-crystalline mixed phase state can be bypassed in the formation of at least one chiral photonic crystal (CPC).
  • CPC chiral photonic crystal
  • the process can utilize two different-frequency electric fields on a standard cholesteric liquid crystal (CLC) mixture with a nematic liquid crystal constituent that possesses negative dielectric anisotropy.
  • CLC cholesteric liquid crystal
  • a low-frequency field (Low-F) treatment can be applied for a Low-F time period.
  • a DC current or a very low frequency (e.g., a few Hz) alternating current (AC) electric field can first be used to completely randomize the cholesteric helices of the CLC mixture.
  • a high frequency treatment (High-F) can be applied for a pre-selected High-F time period.
  • This High- Frequency (> 100 Hz) treatment can be a high-frequency AC electric field, which can exert a reorienting torque on the nematic constituent to reorder the random CLC helices into uniform standing helices (USH).
  • This can provide a CPC with characteristics of a well-aligned CLC.
  • the x, y, and z labels included in Figure 4 identify the perspective of the flow chart views of the CLC material during the illustrated process, where z is the thickness dimension (axis) of the device, and x and y are the dimensions (axes) in the plane of the cell walls (perpendicular to the thickness dimension).
  • an embodiment of the process can utilize an initial pretreatment to form a desired cell wall structure for being positioned for surface alignment of CLC material that is subsequently fed to a DFFA treatment apparatus for providing a Low-F treatment and a High-F treatment.
  • the treatment apparatus can include a Low-F treatment device that applies a low-F treatment to the material and then the material can be fed to a High-F treatment device to undergo the High-F treatment.
  • Some embodiments of the process can utilize an initial pretreatment to form an initial state of CLC material.
  • embodiments can utilize a standard commercial production practice for fabricating cells filled with the CLC starting materials (e.g., constituents composition determined by the desired photonic crystal properties).
  • the CLC filled cell can be fed to a single device that is able to apply the Low-F treatment for a Low-F treatment time period and then apply the High-F treatment for a High-F treatment time period.
  • a single device that is able to apply the Low-F treatment for a Low-F treatment time period and then apply the High-F treatment for a High-F treatment time period.
  • Such an apparatus can include multiple different frequency generating devices for generating the Low-F and High-F treatments so that the material can undergo both treatments while housed or positioned within the same device.
  • the CLC-filled cell After the CLC-filled cell has undergone the High-F treatment (e.g., is output from the High- F treatment device or is output from the treatment apparatus after having undergone the High-F treatment), it forms the desired thick High-N chiral photonic crystal for incorporation into an optical device, a display, or other equipment.
  • the High-F treatment e.g., is output from the High- F treatment device or is output from the treatment apparatus after having undergone the High-F treatment
  • it forms the desired thick High-N chiral photonic crystal for incorporation into an optical device, a display, or other equipment.
  • an empty cell made with two cell walls allows the CLC starting mixture/material to be inserted between or to be encapsulated within cell walls.
  • the cell walls can also be considered cell windows.
  • the cell walls can first be pre-treated via a coating application device and/or other pretreatment mechanism.
  • the coating application device can also receive a coating material (e.g., a polyamide or other suitable coating material).
  • the coating material can be a type of transparent electrode coating material, for example.
  • the coating material can be applied to the cell walls via the coating application device.
  • the coating application device can be configured to coat the cell walls via spin-coating at a pre-selected spin coating rotational speed (e.g., 5900 rotations per minute (rpm) or other suitable speed) for a pre-selected coating application time period (e.g., ninety seconds or another suitable time).
  • the cell walls can be output from the coating application device and fed to a soft bake device to heat the cell walls at a pre-selected first baking time period for a pre-selected first bake temperature (e.g., the material can be soft baked at 90°C for three minutes or soft baked at another pre-selected first bake temperature for another suitable first pre-selected first bake time period).
  • the soft baked cell walls can be output from the soft bake device and subsequently fed to a hard bake device to hard bake the material.
  • the cell walls can be retained in the hard bake device for a second pre-selected second bake temperature for a second pre-selected second bake time period.
  • An example of such a pre-selected second bake temperature for the second pre-selected second bake time period can be 230°C for an hour or other suitable hard bake temperature and time period.
  • the cell walls can be fed to a unidirectional rubbing device so that they can be rubbed with rubbing material at a pre-selected rubbing speed and pre-selected rubbing pressure for a pre-selected rubbing time period.
  • the cell walls can incorporate photo-sensitive materials (e.g., azo dyes) and be exposed to polarized light to apply surface alignment to the cell walls.
  • photo-sensitive materials e.g., azo dyes
  • no rubbing or other surface alignment processing may be utilized or a different type of surface alignment process for the cell walls can be utilized.
  • the formed structure (FS) having the CLCs can be a planar structure having a pre-determined shape (e.g., circular, rectangular, square, polygonal, oval, etc.).
  • the formed structure containing the CLCs between the cell walls can be fed to a Low-F treatment device or be fed to a treatment device that can subsequently provide the Low-F treatment before it then applies the High-F treatment.
  • the Low-F treatment device can apply a low frequency field to the formed structure containing CLCs for a Low-F time period.
  • a DC electric field or a very low frequency e.g., a few Hz, greater than 0 Hz and less than 10 Hz, greater than 0 Hz and less than 30 Hz or less than 100 Hz
  • AC electric field can be applied for providing the Low-F treatment to completely randomize the cholesteric helices of the CLCs in the formed structure.
  • the formed structure can be output from the Low-F treatment device and fed to a High-F treatment device or, for embodiments in which a treatment device provides both the Low-F treatment and the High-F treatment, the Low-F treatment can be stopped, and the High-F treatment can then be started to run for a High-F treatment time period. Alternating Low-F and High-F treatments can also be applied (e.g., there may be multiple Low-F treatments at the same or different Low-F treatment time periods and multiple High-F treatments at the same or different High-F treatment time periods).
  • the application of such treatments can occur in different ways, e.g., (i) all the Low-F treatments are applied and then all the High-F treatments are applied, (ii) each Low-F treatment is followed by a High-F treatment and that cycle is repeated multiple times, or (iii) a series of Low - F treatments are applied followed by a series of High-F treatments and this cycle is then repeated one or more additional times.
  • the High-F treatment can apply a high-frequency electric field to the CLCs for a pre-selected High-F time period.
  • This High-Frequency electric field that is applied can be greater than 100 Hz, for example, 1000 Hz.
  • the high-frequency electric field that is applied can be a high frequency AC electric field, for example.
  • the High-Frequency field that is applied for the pre-selected High-F time period for the High-F treatment can be configured to exert a reorienting torque on the nematic constituent (that possesses negative dielectric anisotropy) of the CLCs to reorder the random helices into USH.
  • This sample can then be output from the High-F treatment device to provide a CPC with characteristics of a well-aligned CLC.
  • the High-F time period can be several minutes long to several hours long (e.g. greater than 3 minutes up to 24 hours, greater than 1 minute and less than 20 hours, greater than 2 minutes and less than 16 hours, etc.) while the Low-F time period can be a short as several milliseconds to several seconds (e g. less than 1 minute and greater than 0 seconds, greater than 0 seconds and up to 10 seconds, between greater than 0 seconds and 30 seconds, etc.). Examples of embodiments that may utilize such time ranges can include embodiments designed to fabricate millimeter-thick CPC. Other embodiments can utilize different pre-selected time periods suitable to meet a desired set of design and fabrication criteria.
  • the frequency of the applied field can be low enough to trigger a so-called conductive dielectric electrohydrodynamic (EHD) instability, which can lead to strong turbulence in the CLC (note: the threshold field strength for conductive EHD instability can be much lower than the threshold for the purely dielectric EHD instability).
  • EHD conductive dielectric electrohydrodynamic
  • Such turbulence can stem from the field-driven flows of space charges in accordance, which can exert destabilizing torques on the cholesteric helices regardless of their initial orientations and essentially mash any/all thick oily streaks (TOS) into thin ones.
  • TOS thick oily streaks
  • the EHD turbulence stops immediately, leaving the CLC cell in a state of CDHs (see e.g., Low-f field, disordering step of Fig. 14 or upon Low-F treatment step of Fig. 4).
  • This state can serve as a starting point for the High-F treatment, where the frequency for dielectric alignment can be set to several 100s to 1000s of Hz.
  • conductive EHD instability can be absent because the redistribution of space charges cannot keep up with the electric field oscillation.
  • the field strength needed e.g. 0.84 V/pm, or 250V in a 300-pm-thick cell
  • the threshold field strength e.g.
  • Eth ⁇ 2 V/pm Eth ⁇ 2 V/pm
  • dielectric EHD instability which is detrimental to helical alignment
  • only the aligning torque by the high frequency field can be in action as a result of the application of the High-F treatment that follows the Low-F treatment.
  • a High-F treatment that is also below the threshold field strength that can trigger dielectric EHD instability
  • the time period of High-F treatment that is needed can be between a few seconds to a number of hours depending on the thickness of the sample being processed and the field strength being utilized.
  • a well-aligned CLC can be obtained after the application of the High-F treatment.
  • liquid crystal orientations at the cell walls may not be uniform in the first place, and during the formation of uniform standing helices via field assembly, the dislocation and deformation can be extended from the surface to the bulk.
  • alignment- treated cell walls having surface anchoring can impart boundary order, which can help effectively prevent the formation of transversely misoriented grains of USHs and associated grain boundaries.
  • the formed CPCs we made and studied were positioned between glass cells containing the cholesteric mixture (either a first mixture or a second mixture).
  • An empty cell consists of two ITO-coated glass windows and glass or plastic spacers to determine the cell gap, and an epoxy adhesive KS BOND QS-2 was used to bond the window and spacers together.
  • the cell gaps used are 300 pm, 1.1 mm, and 2.2 mm for the thicknesses of the samples.
  • the glass windows used in some of the experiments were pre-coated with a polyimide PI-5291 (Nissan Chemical) and rubbed by a velvet cloth to enforce planar surface alignment of the CLC (e.g. provide a surface treatment).
  • the cell was then filled with a cholesteric mixture at room temperature and finally sealed with the epoxy adhesive.
  • the first cholesteric mixture that was used was composed of 98.2 wt% HNG715500-000 and 1.8 wt% R5011 (both from HCCH), and the second cholesteric mixture that was used for making some of the samples was composed of 99.3 wt% HNG715500-000 and 0.7 wt% R5011.
  • HNG715500-000 is a nematic liquid crystal with a clearing point at 82 °C, n e « 1.55, n 0 ⁇ 1.48, en « 7.4, and e ⁇ « 21.6 (dielectric anisotropy: en - e ⁇ ⁇ -14.2).
  • R5011 is a chiral agent with a helical twisting power of ⁇ 110 pm 1 .
  • the 0.3 mm thick samples were formed via application of Low-F field of 0.84 V/pm at 15 Hz for 5 seconds (s) and a High-F field of 0.84 V/pm at 1000 Hz for 4 hours.
  • the 1.1 mm thick samples were formed via application of Low-F field of 1.67 V/pm at 15 Hz for 5 seconds and a High-F field of 1.67 V/pm at 1000 Hz for 4 hours.
  • the 2.2 mm thick samples were formed via application of Low-F field of 2.5 V/pm at 10 Hz for 10 seconds and a High-F field of 2.5 V/pm at 1000 Hz for 12 hours.
  • other types of Low-F and High-F treatments can be utilized to form other types of embodiments. These parameters were used for the formation of samples provide non-limiting examples. We used a number of the samples we formed for evaluation as discussed below.
  • Figure 5 illustrates photographs of a non-surface treated 0.3 mm sample captured (i) at the initial state, (ii) immediately after a 15-Hz AC Low-F treatment was applied for a Low-F time period and (iii) immediately after 1000 Hz alternating current High-F treatment was applied.
  • the applied field strengths for the 15 Hz Low-F treatment and 1000 Hz High-F treatment were both 0.
  • Figure 6 illustrates micrographs of the non-surface-treated 0.3-mm sample captured (i) at the initial state, (ii) immediately after a 15-Hz DC current Low-F treatment was applied for aLow- F time period and (iii) thirty minutes after 1000 Hz alternating current High-F treatment was applied.
  • the applied field strengths for the 15 Hz Low-F treatment and 1000 Hz High-F treatment were both 0.84 V/pm.
  • Figure 7 illustrates the unpolarized transmission spectrum for this sample captured after a Dual Frequency Field Assembly (DFFA) process was applied to the sample, which occurred after the High-F treatment.
  • Figure 7 identifies a photonic band gap (PBG) for this sample.
  • DFFA Dual Frequency Field Assembly
  • Embodiments of the process can be simple to implement.
  • embodiments of the process can utilize readily available simple electronics and AC/DC voltage supplies and standard components/constituents for making CLC cells.
  • the randomly distributed helices that can be created by the first application of the DC or very Low-Frequency AC field in the Low-F treatment can be energetically favored to assume uniform standing helices by the subsequent application of the High-F treatment (e.g., the High-Frequency AC electric field of greater than 100 Hz, typically 1000 Hz).
  • the High-F treatment e.g., the High-Frequency AC electric field of greater than 100 Hz, typically 1000 Hz.
  • the unpolarized transmission spectrum of a 0.3-mm-thick CLC depicts a well-defined PBG having smooth band edges, and high transmittance outside the PBG. This is in stark contrast to high -?V samples fabricated by traditional techniques (see e.g., Figure 8).
  • the cell walls of the CLCs while in the initial state can be first pretreated with a surface treatment of one or more coatings (e.g., one or more polyimide coatings).
  • a polyamide coating e.g., polyimide SE 5291 from Nissan Chemical
  • a polyamide coating can be utilized to coat the cell walls of the CLCs via spin-coating at 5900 rpm for ninety seconds, and then soft-baked at 90°C for three minutes, followed by hard-baking at 230°C for an hour. After the hard baking the CLCs can be unidirectionally rubbed with a velvet cloth or other rubbing material.
  • This type of pretreatment step can be provided to form the CLCs used in the “initial state” shown in Figure 4, for example.
  • Figure 9 illustrates an unpolarized transmission spectra of two samples fabricated after DFFA — one with surface alignment and darker curve and the other without surface alignment (lighter curve in Fig. 9).
  • the uniformity was improved considerably in the sample with the surface-alignment coating provided via the pretreatment step.
  • the PBG shown in Figure 9 of the surface-aligned sample exhibits a similar width and band-edges but much sharper variation near the band-edges compared to the non-treated counterpart.
  • Figure 10 illustrates a photograph of this sample (the “(a)” image included in Figure 10). This type of CPC is well suited for free-space optical systems.
  • Figure 10 includes a micrograph of the sample shown in Figure 10 that was captured several days after fabrication. This micrograph (e.g., the “(b)” image included in Figure 10) shows the stability and uniformity of the standing-helix alignment in the sample.
  • Figure 15 illustrates a 1.1-mm-thick CLC formed via an embodiment of our DFFA.
  • the CLC exhibited high transparency, uniformity, and stability of the USH alignment.
  • the period number is -3400 for this sample.
  • the available area for the sample is about 6.3 *9.0 mm 2 (and other embodiments of this sample can be made using our DFFA that can be made larger if needed).
  • Long-term stability of this sample can be appreciated from the graph of Figure 16.
  • This graph shows that the unpolarized transmission spectra of the sample captured right after DFFA treatment and 6 months later at room-temperature (—25 °C) storage appear almost identical. Both spectra exhibit a well-defined PBG with sharp band edges in the same wavelength locations and high out- of-bandgap transmittance.
  • Such an extraordinarily thick CPC can rotate the polarization of by 100s to 1000s of degrees while keeping high transmission and high polarization fidelity, which can be appreciated from the graph of Figure 17.
  • a thickness of 2 mm can correspond to having period number N - 6000 for CPCs with a near-infrared PBG ( - 1 pm) or N - 1000 for CPCs with a mid-infrared PBG (optical wavelength X - 6 pm).
  • the ability to fabricate millimeter-thick CPCs can allow the operation wavelength to be extended to -1.0 pm (e.g. for use with Nd:YAG or Yb-doped lasers), or -1.3-1.6 pm (e.g. for use in optical communication equipment), and up to mid-infrared (-3-6 pm) for use in infrared equipment.
  • Embodiments of our process can be of great interest to liquid crystal display industries and research/development laboratories (e.g., commercial, government, and university labs, etc.).
  • embodiments of the high N CPCs formed using an embodiment of our process can be utilized in liquid crystal displays, optical systems, or other applications where thick CPC can perform various device operations that are currently impossible with conventional thin CPC.
  • Embodiments of our tunable CPCs that can be fabricated by an embodiment of our process can also be utilized for other applications.
  • embodiments of our process can be incorporated in standard liquid crystals cell production at a commercial scale.
  • an embodiment of our DFFA and/or apparatus can be utilized to fabricate an SN-CPC that is capable of rotating the polarization at 1550 nm by 2K with high transmittance (e.g. ⁇ 91 %) and a high polarization figure of merit (PFM) (e.g.
  • PFM polarization figure of merit
  • the PFM is defined as (Tinax - min)/( max + / in), where 7 ma x and 7min are the measured maximum polarization transmittance ( /max) and minimum polarization transmittance (Zinin) of the SN-CPC, given that the light source exhibits an extremely high degree of linear polarization (note: the SN-CPC is an example of a type ofHN-CPC).
  • a 1.1-mm-thick CPC formed via an embodiment of our process and apparatus can enable 2 rotation of the input polarization with high transmittance and PFM at an operating wavelength (Xop) of design.
  • Our evaluation work showed that exemplary designs of embodiments of our SN-CPCs made via an embodiment of our DFFA process with thickness fixed at 1.1 mm and N > 500 configured for 2K polarization rotation was able to provide high transmission (e.g. (> 90% of the input power) at each targeted operating wavelength.
  • the RAD can be as high as 100°/nm or more near the band edge and drops to Si l°/nm at -200-400 nm away from the band edge.
  • Having high RAD can enable, for instance, direct generation of a spectral vector beam (e.g., an ultrafast laser beam with wavelengthdependent polarization orientation) for applications in ultrafast spectroscopy or pulse shaping.
  • Spectral vector beams are usually generated using a set of a thick birefringent crystal and conventional waveplates, or a grating-based 4f system comprising a spatial light modulator in the Fourier plane; such setups are rather cumbersome and can need precise alignment of the birefringent components to ensure the polarization evolution in the optical path.
  • an embodiment of our SN-CPC can be utilized as a flat single component generator of spectral vector beams.
  • a spectral vector beam can be generated by simply sending ultrafast laser pulses directly through a SN-CPC operating in the high-RAD regime, where the rotation angle varies substantially with wavelength.
  • the need for precise polarization alignment can also be alleviated because the rotation angle can be independent of the incident polarization orientation.
  • the embodiment of our SN-CPC can function as a polarization rotator of constant rotation angle within a bandwidth of ⁇ 20 nm or wider for such an application.
  • the SN-CPC can be suitable for dynamic polarization control of sub-picosecond laser pulses.
  • FIG. 19 illustrates an exemplary embodiment of an apparatus used to apply a light source to a SN-CPC formed via our DFFA process for reversible electrical tuning of optical polarization rotation of the formed SN-CPC.
  • the white light source was passed through a lens and polarizer for being passed through the SN-CPC before the light was then directed through an analyzer and spectrometer as shown in Figure 19.
  • a DC supply was coupled to the SN-CPC to apply a voltage to the SN-CPC for switching of the output polarization of the light output from the SN-CPC.
  • orthogonal (90°) switching of the output polarization was achieved by applying 9 V to the SN-CPC via the DC supply.
  • Such electrical tuning of the polarization rotation usually took a few seconds to accomplish and was reversible.
  • Ultrafast (ps-fs) modulation of the refractive indices or birefringence can be mediated by utilizing the electronic nonlinearities of the liquid-crystal molecules in SN-CPCs (the so-called optical Kerr effect, where the induced changes of n e and n o are proportional to the laser intensity by their respective Kerr coefficients, n2e and n2 O ).
  • n e the dominant field-induced change in n e (note: in CLCs, as in most organic molecules, the magnitude of An e is usually much greater than An 0 for a given laser intensity (i.e., n2e » n2o)).
  • embodiment of our SN-CPCs can enable ultrafast switching operations at reasonably low intensities (e.g. ⁇ 0.1 GW/cm 2 to 10 GW/cm 2 ) and with high transmission (in the out-of-PBG regime). This can also be desirable as it is extremely fast, can be activated remotely by sending another ultrashort control pulse or by self-action, and does not involve photoabsorption/heating, mechanical movements, grating period distortion, and other possible complications.
  • Figure 20 illustrates an exemplary embodiment of a polarization switching application for an embodiment of our SN-CPC formed via an embodiment of our DFFA process.
  • a probe can emit a light signal to the SN-CPC, which can be connected to a pump.
  • the SN-CPC can be configured so that the probe’s light is horizontally polarized.
  • the SN-CPC can be configured so that the probe’s light emission is vertically polarized.
  • the embodiment was utilized to evaluate a sample of our SN-CPC to show that the embodiment of the SN-CPC can have an induced change in the birefringence (n e - ri t >) for causing orthogonal polarization switching.
  • the mechanism is not limited to linearly polarized light and that the SN-CPC can be utilized for both spatially uniform vector filed and complex vector fields.
  • the corresponding operation bandwidth was ⁇ 30 nm, within which the polarization rotation ⁇ PR remained almost constant with ⁇ PR(2) - ⁇ ?PR(600 nm) ⁇ 1° both before and after switching in this evaluated embodiment.
  • the particular type of Low-F field or High-F field, the duration of the Low- F treatment, duration of the High-F treatment can be adapted to meet a particular set of design criteria or source material.
  • the cell wall pre-treatment processing e.g., soft and hard bake temperatures and baking time periods, coating material that may be utilized, and coating application process
  • the cell wall pre-treatment processing can be adapted (or not used) to meet a particular set of design criteria.

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  • Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Liquid Crystal (AREA)

Abstract

L'invention concerne une technique pour fabriquer des cristaux photoniques chiraux (CPC) extraordinairement épais (p. ex. supérieur à 1 mm, jusqu'à 2,2 mm, etc.) ayant un nombre de périodes N (N = épaisseur/pas) dépassant plusieurs milliers. La technique peut utiliser deux champs électriques à fréquence différente sur un mélange de cristaux liquides cholestériques (CLC) standard avec un constituant à cristaux liquides nématiques qui possède une anisotropie diélectrique négative. En commençant par le mélange CLC, un courant continu (CC) ou un champ électrique à très basse fréquence peut être utilisé pour randomiser complètement les hélices cholestériques. Ceci peut être suivi d'un champ électrique à courant alternatif (CA) haute fréquence qui peut exercer un couple de réorientation sur le constituant nématique pour réordonner ces hélices aléatoires en hélices stationnaires uniformes (USH) – qui peuvent fournir des caractéristiques d'un CLC bien aligné qui peut fonctionner en tant que cristal photonique chiral 1-D.
PCT/US2024/010786 2023-01-23 2024-01-09 Procédé de fabrication de cpc Ceased WO2024158552A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20050093072A (ko) * 2004-03-18 2005-09-23 인션 인코포레이티드 액정컬러스위치 및 그의 제조방법
US20160033698A1 (en) * 2014-07-31 2016-02-04 North Carolina State University Bragg liquid crystal polarization gratings
KR101825597B1 (ko) * 2017-02-28 2018-02-05 경북대학교 산학협력단 고상 상태의 나선형 광결정 구조체의 제조방법 및 이에 의해 제조되는 광결정 구조체
KR20190089021A (ko) * 2016-12-08 2019-07-29 매직 립, 인코포레이티드 콜레스테릭 액정에 기초한 회절 디바이스들
CN113311625A (zh) * 2021-03-16 2021-08-27 合肥工业大学 一种聚合物稳定胆甾相液晶变色玻璃及其制备方法和应用

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20050093072A (ko) * 2004-03-18 2005-09-23 인션 인코포레이티드 액정컬러스위치 및 그의 제조방법
US20160033698A1 (en) * 2014-07-31 2016-02-04 North Carolina State University Bragg liquid crystal polarization gratings
KR20190089021A (ko) * 2016-12-08 2019-07-29 매직 립, 인코포레이티드 콜레스테릭 액정에 기초한 회절 디바이스들
KR101825597B1 (ko) * 2017-02-28 2018-02-05 경북대학교 산학협력단 고상 상태의 나선형 광결정 구조체의 제조방법 및 이에 의해 제조되는 광결정 구조체
CN113311625A (zh) * 2021-03-16 2021-08-27 合肥工业大学 一种聚合物稳定胆甾相液晶变色玻璃及其制备方法和应用

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