CN102169205A - Low-loss medium loaded surface plasmon excimer optical waveguide - Google Patents

Abstract

The invention discloses a medium-loaded surface plasmon optical waveguide with low transmission loss and strong optical field confinement capability. The cross-section of the waveguide structure includes a metal base layer (1), a high The medium area of high refractive index (3), the medium area of low refractive index (2) surrounded by the medium area of high refractive index and the metal base layer, and the cladding layer (4). The high refractive index medium region on the metal base layer can significantly reduce the light field distribution range of the waveguide structure, and realize the two-dimensional sub-wavelength confinement of the transmitted light field; at the same time, the existence of the low refractive index medium region makes the waveguide still maintain Lower transmission loss. The optical waveguide structure overcomes the contradiction between the optical field confinement capability and transmission loss of the existing medium-loaded surface plasmon optical waveguide, and provides the possibility for the realization of an ultra-highly integrated optical waveguide chip.

Description

A low-loss dielectric-loaded surface plasmon optical waveguide

technical field

The invention relates to the technical field of optical waveguides, in particular to a low-loss dielectric-loaded surface plasmon optical waveguide.

Background technique

A surface plasmon is a pattern of electromagnetic waves caused by the interaction of light and free electrons on the surface of a metal. This mode exists near the metal-dielectric interface, and its field strength reaches its maximum at the interface, and decays exponentially on both sides of the interface along the direction perpendicular to the interface. Surface plasmons have strong field confinement properties, which can confine the field energy to a region whose spatial size is much smaller than its free-space transmission wavelength, and its properties can change with the change of the metal surface structure. In the surface plasmon optical waveguide structure composed of appropriate metal and dielectric, the transverse optical field distribution can be limited to a range of tens of nanometers or even smaller, which can exceed the limit of the diffraction limit. Surface plasmons have shown great application potential in the field of nanophotonics, and it is possible to realize highly integrated nanophotonic chips.

Mode field confinement capability and transmission loss are two important parameters to characterize the characteristics of surface plasmon optical waveguide modes. Traditional surface plasmon waveguides mainly include metal/dielectric/metal type and dielectric/metal/dielectric type structures. Among them, the dielectric/metal/dielectric optical waveguide has low transmission loss, but its poor mode field confinement ability restricts its application in high-integration optical circuits; on the other hand, the metal/dielectric/metal optical waveguide has strong However, its transmission loss is too large, making it impossible to transmit long-distance optical signals. Aiming at the contradiction between the mode field confinement capability and transmission loss of the traditional surface plasmon optical waveguide, the researchers proposed a dielectric-loaded surface plasmon optical waveguide. The cross-section of the waveguide consists of a metal base and a finite-sized dielectric region above it. Compared with other types of surface plasmon optical waveguides, this dielectric-loaded surface plasmon optical waveguide can not only provide sub-wavelength size confinement in the lateral direction, but also have relatively small transmission loss. In addition, the fabrication The simplicity also makes this type of waveguide have good application potential in integrated optics. At present, many foreign research groups have carried out systematic theoretical research on dielectric-loaded surface plasmon optical waveguides and reported the experimental progress of micro-nano devices based on related waveguides.

Traditional dielectric-loaded surface plasmon waveguides usually use low-refractive-index polymer materials with a refractive index of about 1.535. Such waveguides enable low-loss optical signal transmission but tend to be relatively large in size. Usually, in order to ensure single-mode conditions and maintain a long transmission distance, the length and width of the polymer cross-section are often around 600 nanometers, and the corresponding mode field size has reached the order of nearly microns, which is not conducive to the integration of waveguides and devices. . However, using a material with a high refractive index (such as a semiconductor material) as a dielectric layer can reduce the overall size of the waveguide and improve the mode field confinement capability, but the resulting transmission loss will increase significantly.

To solve this problem, the present invention improves the original structure of the above-mentioned high-refractive-index medium-loaded surface plasmon optical waveguide. By introducing a composite structure composed of high and low refractive index media, the new surface plasmon optical waveguide obtained has both low transmission loss and strong mode field confinement ability. Since the low-refractive-index medium region can be filled with air or other gases, the transmission loss of the waveguide can be significantly reduced, and on the other hand, the field enhancement effect is further enhanced. In addition, since the high-refractive-index dielectric layer of the proposed waveguide can be made of semiconductor materials, the two-dimensional structure can be matched with the semiconductor planar chip processing technology, and can be easily applied to highly integrated optical waveguide chips, which is very important for realizing large-scale integrated optical circuits. Significance.

Contents of the invention

The purpose of the present invention is to overcome the defect of large field transmission loss of the medium-loaded surface plasmon optical waveguide based on high-refractive index materials, and propose a medium-loaded surface plasmon with low transmission loss and strong field confinement ability Optical waveguide structure.

The present invention provides a medium-loaded surface plasmon optical waveguide structure with both low transmission loss and strong field confinement capability. The high-refractive-index medium region and the low-refractive-index medium region surrounded by the metal base layer, and the cladding; wherein, the width of the high-refractive index medium region is 0.06-0.4 times the wavelength of the transmitted optical signal, and the height range is 0.06-0.4 times the wavelength of the transmitted optical signal. 0.06-0.4 times the wavelength of the optical signal, the low-refractive index medium area is in contact with the metal base layer, and the width of the low-refractive index medium area is 0.01-0.39 times the wavelength of the transmitted optical signal, and the height range is 0.01-0.39 times the wavelength of the transmitted optical signal. 0.01-0.3 times the wavelength of the optical signal; the material refractive index of the high-refractive index medium is higher than that of the low-refractive-index medium and the material of the cladding, and the materials of the low-refractive index medium and the cladding can be the same material or different materials, low The ratio of the maximum value of the material refractive index of the refractive index medium and the cladding layer to the material refractive index of the high refractive index medium is less than 0.75.

The material of the metal layer in the medium-loaded surface plasmon optical waveguide structure is any one of gold, silver, aluminum, copper, titanium, nickel, chromium that can generate surface plasmons, or their respective alloys, Or composite materials of different metal layers.

In the medium-loaded surface plasmon optical waveguide structure, the outer contour of the cross-section of the area formed by the high-refractive-index medium region and the low-refractive-index medium region is any one of square, rectangle, or trapezoid.

The shape of the section of the low refractive index medium region in the medium-loaded surface plasmon optical waveguide structure is any one of square, rectangle, circle, ellipse or trapezoid.

The dielectric-loaded surface plasmon optical waveguide of the present invention has the following advantages:

1. The material of the low-refractive-index dielectric region of the proposed medium-loaded surface plasmon waveguide can be made of low-refractive-index materials such as silicon dioxide or other low-refractive-index polymer materials, or can be filled with air or other gases. The transmission loss can be significantly reduced, and on the other hand, the field enhancement effect can be further enhanced, which cannot be achieved by traditional dielectric-loaded optical waveguides.

2. Compared with the existing medium-loaded surface plasmon optical waveguide based on low refractive index, the proposed dielectric-loaded surface plasmon optical waveguide has significantly reduced size, improved integration, and kept low transmission loss. Compared with high-refractive index-based dielectric-loaded surface plasmon optical waveguides, its transmission loss is greatly reduced while maintaining the subwavelength mode field confinement capability.

3. Since the high-refractive index dielectric layer of the proposed medium-loaded surface plasmon waveguide can be made of semiconductor material, the two-dimensional structure can be matched with the semiconductor planar chip processing technology, and can be easily applied to highly integrated optical waveguide chips.

Description of drawings

Fig. 1 is a schematic structural diagram of a dielectric-loaded surface plasmon optical waveguide. Area 1 is the metal base layer, area 2 is the low-refractive index medium area, its width is W _l , and its height is h _l ; area 3 is the high-refractive index medium area, its width is W _h , and its height is h _h ; Area 4 is layers.

Fig. 2 is a structural view of the dielectric-loaded surface plasmon optical waveguide described in Examples 1 and 2. 201 is the metal base layer, and _nm is its refractive index; 202 is a low refractive index medium area, n ₁ is its refractive index, W _l is its width, h _l is its height; 203 is a high refractive index medium area, n _h is its refractive index, W _h is its width, h _h is Its height; 204 is the cladding layer, n _c is its refractive index.

Fig. 3 is the electric field intensity distribution curve of the surface plasmon mode light field of the dielectric-loaded surface plasmon optical waveguide described in Example 1 when the wavelength of the transmitted optical signal is 1.55 μm. Among them, Fig. 3(a) is the distribution curve of the Y component of the electric field intensity along the X axis direction, and Fig. 3(b) is the distribution curve of the Y component of the electric field intensity along the Y axis direction.

Fig. 4 is a curve of the effective refractive index of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 1 as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm.

Fig. 5 is a curve of the transmission distance of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 1 as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm.

Fig. 6 is the variation curve of the normalized effective mode field area of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 1 with the width W _l when the wavelength of the transmitted optical signal is 1.55 μm

Fig. 7 is the electric field intensity distribution curve of the surface plasmon mode optical field of the dielectric-loaded surface plasmon optical waveguide described in Example 2 when the wavelength of the transmitted optical signal is 1.55 μm. Among them, Fig. 7(a) is the distribution curve of the Y component of the electric field intensity along the X axis direction, and Fig. 7(b) is the distribution curve of the Y component of the electric field intensity along the Y axis direction.

Fig. 8 is a curve of the effective refractive index of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 2 as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm.

FIG. 9 is a curve of the transmission distance of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 2 as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm.

Fig. 10 is the variation curve of the normalized effective mode field area of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in Example 2 with the width W _l when the wavelength of the transmitted optical signal is 1.55 μm

Detailed ways

The mode characteristic of surface plasmon waves is an important index to characterize surface plasmon optical waveguides. The mode characteristic parameters mainly include the real part of the effective refractive index, the transmission distance and the normalized effective mode field area.

The transmission distance L is defined as the distance when the electric field strength on any interface decays to the initial value 1/e, and its expression is:

L=λ/[4π/Im(n _eff )] (1)

Where Im(n _eff ) is the imaginary part of the effective refractive index of the mode, and λ is the wavelength of the transmitted optical signal.

The calculation expression of the effective mode field area is as follows:

A _eff = (∫∫E(x, y)| ² dxdy) ² /∫∫E(x, y)| ⁴ dxdy(2)

Among them, A _eff is the effective mode field area, and E(x, y) is the electric field of the surface plasmon wave. The normalized effective mode field area is the ratio of the effective mode field area calculated by formula (2) to the diffraction-limited aperture area. The area of the diffraction-limited aperture is defined as follows:

A ₀ =λ ² /4 (3)

Among them, A ₀ is the area of the diffraction-limited aperture, and λ is the wavelength of the transmitted optical signal. Therefore, the normalized effective mode field area A is:

A＝A _eff /A ₀ (4)

The size of the normalized effective mode field area represents the mode field confinement ability of the mode, and the case where the value is less than 1 corresponds to the subwavelength size constraint.

Example 1: Optical waveguide structure with large difference in refractive index of materials in high and low refractive index medium regions

FIG. 2 is a structural view of the dielectric-loaded surface plasmon optical waveguide described in Example 1. FIG. 201 is the metal base layer, n _m is its refractive index; 202 is the low refractive index medium area, n ₁ is its refractive index, W _l is its width, h _l is its height; 203 is a high refractive index medium area, n _h is Its refractive index, W _h is its width, h _h is its height; 204 is the cladding, n _c is its refractive index.

In this example, the wavelength of the transmitted optical signal is selected as 1.55 μm, the material of 201 is silver, and the refractive index at the wavelength of 1.55 μm is 0.1453+i*11.3587; the material of 202 is set as air, and its refractive index is 1 ; The material of 203 is silicon, whose refractive index is 3.5; the material of 204 is silicon dioxide, whose refractive index is 1.5.

In this example, the height h _l of 202 = 50 nm; the width W _h of 203 = 200 nm, and the height h _h = 200 nm; the value range of the width W _l of 202 is 30-150 nm.

The above-mentioned waveguide structure in this embodiment is simulated using the full vector finite element method, and the mode field distribution and mode characteristics of the surface plasmon mode at a wavelength of 1.55 μm are calculated.

Fig. 3 is the electric field intensity distribution curve of the surface plasmon mode optical field of the dielectric-loaded surface plasmon optical waveguide described in the example when the wavelength of the transmitted optical signal is 1.55 μm, where the width of 202 W _l =100nm. Among them, Fig. 3(a) is the distribution curve of the Y component of the electric field intensity along the X axis direction, and Fig. 3(b) is the distribution curve of the Y component of the electric field intensity along the Y axis direction. It can be seen from FIG. 3 that the electric field intensity curve of the medium-loaded surface plasmon optical waveguide light field has an obvious field enhancement effect in the low refractive index medium region.

Fig. 4 is a curve of the effective refractive index of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from FIG. 4 that the effective refractive index of the surface plasmon mode of the dielectric-loaded optical waveguide decreases as the width W _l increases.

Fig. 5 is a curve of the transmission distance of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from FIG. 5 that the transmission distance of the surface plasmon mode of the dielectric-loaded optical waveguide is between 21 and 60 microns, and increases with the increase of the width W ₁ . Under the same conditions, replace the low-refractive-index medium with a high-refractive-index medium (corresponding to W _h =200nm, h _h =200nm, W _l =h _l =0, and keep other parameters unchanged), and the obtained traditional high-refractive index medium-loaded surface The transmission distance of the plasmonic optical waveguide mode is 17 microns. It can be seen that the dielectric-loaded optical waveguide has lower transmission loss.

Fig. 6 is a curve of the normalized effective mode field area of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from Fig. 6 that the mode field area of the surface plasmon mode of the dielectric-loaded optical waveguide increases with the increase of the width W _l . It can be seen that the increase of the transmission distance of the surface plasmon mode is at the expense of the mode field at the expense of limited capacity. At the same time, it can be seen from the figure that the normalized effective mode field area is still very small, and far less than 1, indicating that the dielectric-loaded optical waveguide has a sub-wavelength mode field confinement capability.

Example 2: Optical waveguide structure with small difference in refractive index of materials in high and low refractive index medium regions

The structural diagram of the medium-loaded surface plasmon optical waveguide described in Example 2 is shown in Figure 2. 201 is the metal base layer, n _m is its refractive index; 202 is the low refractive index medium region, n ₁ is its refractive index, W _l is the width, h _l is the height; 203 is the high reflectivity medium area, n _h is the refractive index, W _h is the width, h _h is the height; 204 is the cladding, n _c is the refractive index.

In this example, the wavelength of the transmitted optical signal is selected as 1.55 μm, the material of 201 is silver, and the refractive index at the wavelength of 1.55 μm is 0.1453+i*11.3587; the material of 202 is silicon nitride, and its refractive index 2; the material of 203 is silicon, whose refractive index is 3.5; the material of 204 is silicon dioxide, whose refractive index is 1.5.

In this example, the height h _l of 202 = 50 nm; the width W _h of 203 = 200 nm, and the height h _h = 200 nm; the value range of the width W _l of 202 is 30-150 nm.

The above-mentioned waveguide structure in this embodiment is simulated using the full vector finite element method, and the mode field distribution and mode characteristics of the surface plasmon mode at a wavelength of 1.55 μm are calculated.

7 is the electric field intensity distribution curve of the surface plasmon mode optical field of the dielectric-loaded surface plasmon optical waveguide described in the example when the wavelength of the transmitted optical signal is 1.55 μm, where the width of 202 W _l =100 nm. Among them, Fig. 7(a) is the distribution curve of the Y component of the electric field intensity along the X axis direction, and Fig. 7(b) is the distribution curve of the Y component of the electric field intensity along the Y axis direction. It can be seen from FIG. 7 that the electric field intensity curve of the medium-loaded surface plasmon optical waveguide light field has an obvious field enhancement effect in the low refractive index medium region.

Fig. 8 is a curve of the effective refractive index of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from FIG. 8 that the effective refractive index of the surface plasmon mode of the dielectric-loaded optical waveguide decreases as the width W ₁ increases.

Fig. 9 is a curve of the transmission distance of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from FIG. 9 that the transmission distance of the surface plasmon mode of the dielectric-loaded optical waveguide is between 20 and 37 microns, and decreases as the width W ₁ increases. Under the same conditions, replace the low-refractive-index medium with a high-refractive-index medium (corresponding to W _h =200nm, h _h =200nm, W _l =h _l =0, and keep other parameters unchanged), and the obtained traditional high-refractive index medium-loaded surface The transmission distance of the plasmonic optical waveguide mode is 17 microns. It can be seen that the dielectric-loaded optical waveguide has lower transmission loss.

Fig. 10 is a curve of the normalized effective mode field area of the surface plasmon mode transmitted in the dielectric-loaded surface plasmon optical waveguide described in the example as a function of the width W _l when the wavelength of the transmitted optical signal is 1.55 μm. It can be seen from Fig. 10 that the mode field area of the surface plasmon mode of the dielectric-loaded optical waveguide increases with the increase of the width W _l . It can be seen that the increase of the transmission distance of the surface plasmon mode is at the expense of the mode field at the expense of limited capacity. At the same time, it can be seen from the figure that the normalized effective mode field area is still very small, and far less than 1, indicating that the dielectric-loaded optical waveguide has a sub-wavelength mode field confinement capability.

The simulation results of Example 1 and Example 2 show that the high and low refractive index medium regions in the waveguide structure of the present invention can be realized by materials with a large difference in refractive index, or can be realized by materials with a small difference in refractive index.

Finally, it should be noted that the embodiments in the above figures are only used to illustrate the surface plasmon optical waveguide structure of the present invention, but are not limiting. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent replacements to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all of them should be included in the scope of the present invention. within the scope of the claims.

Claims (4)

1. one kind possesses the low transmission loss and simultaneously than the medium loaded type surface plasmon optical waveguide structure of high field restriction ability, low refractive index dielectric zone and covering that its xsect comprises metallic substrate layer, be positioned at high refractive index medium zone on the metallic substrate layer, surrounded by high refractive index medium zone and metallic substrate layer; Wherein, the width range in high refractive index medium zone be institute's transmitting optical signal wavelength 0.06-0.4 doubly, altitude range be the light signal that transmitted wavelength 0.06-0.4 doubly, the low refractive index dielectric zone joins with metallic substrate layer, and the width range in low refractive index dielectric zone be institute's transmitting optical signal wavelength 0.01-0.39 doubly, altitude range be the light signal that transmitted wavelength 0.01-0.3 doubly; The material refractive index of high refractive index medium is higher than the material refractive index of low refractive index dielectric and covering, the material of low refractive index dielectric and covering can be same material or different materials, and the ratio of the maximal value of the material refractive index of low refractive index dielectric and covering and the material refractive index of high refractive index medium is less than 0.75.

2. optical waveguide structure according to claim 1, it is characterized in that the material of metal level is any or alloy separately or the compound material of different metal layer in the gold, silver, aluminium, copper, titanium, nickel, chromium that can produce surface plasmons in the described structure.

3. optical waveguide structure according to claim 1 is characterized in that, in the described structure outer contour shape in high refractive index medium zone and the cross section in the common zone that constitutes, low refractive index dielectric zone be square, rectangle or trapezoidal in any.

4. optical waveguide structure according to claim 1 is characterized in that, in the described structure cross section in low refractive index dielectric zone be shaped as square, rectangle, circle, ellipse or trapezoidal in any.

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