Polarization insensitive optical switch device based on metasurface and implementation method

Abstract

The invention discloses a polarization insensitive optical switch device based on a metasurface and an implementation method, and relates to the technical field of micro-nano optical devices. The liquid crystal display sequentially comprises a bottom dielectric substrate, a metal reflecting layer, a spacing layer, a nano-column structure, a first dielectric material layer, a second dielectric material layer, nematic liquid crystal, a liquid crystal orientation layer, an ITO transparent electrode and a top dielectric substrate from bottom to top, the second dielectric material and the liquid crystal are alternately arranged. The metasurface and the liquid crystal are integrated together, the polarization conversion capability of the metasurface and the dynamic regulation and control capability of the liquid crystal are fully utilized, the dependence of a traditional amplitude type LCoS display system on incident light polarization is overcome, high-contrast polarization insensitive light switching is achieved, the use of polarization elements is avoided, the display system can be greatly simplified, and the display effect is improved. The liquid crystal thickness can be further reduced, and the driving voltage is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-nano optical devices, and more specifically, relates to a polarization-insensitive optical switch device based on a metasurface and a realization method thereof. Background Art

[0002] With the continuous innovation of technology and the growth of market demand, projection display technology is developing towards miniaturization, high resolution, high contrast, and low energy consumption. LCoS (Liquid crystal on silicon) devices can provide precise and reconfigurable optical wavefront shaping, and occupy an important position in the projection display field by virtue of the advantages of high resolution, small pixel size, and low power consumption. Polarization sensitivity is a typical characteristic of LCoS microdisplays. On the one hand, polarization selection is required to achieve amplitude modulation, but on the other hand, polarization correlation reduces the energy utilization rate and increases system complexity. Currently, there are two main strategies to achieve polarization-insensitive LCoS devices. One is to use polarization-insensitive liquid crystals, which is an effective method but requires complex liquid crystal packaging technology. The other is to integrate thin-film polarization devices in LCoS, which will lead to an increase in driving voltage.

[0003] The emergence of metasurfaces provides a new idea for overcoming the above limitations. A metasurface is a new type of planar optical modulation element based on the generalized Snell's law in recent years, which is composed of scatterers arranged in a two-dimensional plane with sub-wavelength sizes and intervals. By carefully designing parameters such as the shape, size, and arrangement of the scatterers, arbitrary modulation of optical parameters (such as phase, amplitude, polarization, and frequency, etc.) can be achieved. The flexible structure design and novel mechanism of metasurfaces provide a more integrated and more powerful alternative to traditional bulk optical elements. Currently, functions such as beam deflection, planar lenses, and holographic displays have been realized, and it has broad application prospects. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a polarization-insensitive optical switch device based on a metasurface and a realization method thereof. The switch device integrates the metasurface and liquid crystal together, makes full use of the polarization conversion ability of the metasurface and the dynamic regulation ability of the liquid crystal, overcomes the dependence of the traditional amplitude-type LCoS display system on the polarization of incident light, realizes a polarization-insensitive optical switch with high contrast, avoids the use of polarization elements, can greatly simplify the display system, and can further reduce the liquid crystal thickness and reduce the driving voltage.

[0005] To solve the above technical problems, the technical solution adopted by the present invention is: a polarization-insensitive optical switch device based on a metasurface, including a bottom dielectric substrate, a metal reflection layer, a spacer layer, a nano-column structure, a first dielectric material layer, a second dielectric material layer, a nematic liquid crystal, a liquid crystal alignment layer, an ITO transparent electrode, and a top dielectric substrate. The bottom dielectric substrate is covered with a metal reflection layer, the metal reflection layer is covered with a spacer layer, and a number of sub-wavelength-sized nano-column structures are arranged in an array on the spacer layer. The nano-column structures are arranged in a periodic pattern at 45 degrees or 135 degrees with respect to the X-axis. The first dielectric material layer coats the outside of the nano-column structures and fills the gaps between the nano-column structures to prevent the upper liquid crystal from infiltrating into the gaps between the nano-column structures and affecting the phase response of the nano-column structures. The second dielectric material layer is arranged at intervals on the first dielectric material layer, and the nematic liquid crystal and the second dielectric material layer are arranged alternately. The nematic liquid crystal is covered with a liquid crystal alignment layer, the liquid crystal alignment layer is covered with an ITO transparent electrode, and the ITO transparent electrode is covered with a top dielectric substrate.

[0006] Preferably, the nano-column structure is a columnar structure, and the cross-sectional shape of the nano-column structure has anisotropy, and can be a rectangle, an ellipse, a cross, an L shape, a crescent shape, or a combined shape.

[0007] Preferably, the material of the nano-column structure can be a metal such as gold, silver, or aluminum, or a dielectric material such as titanium dioxide, silicon nitride, hafnium dioxide, silicon, zirconium dioxide, gallium nitride, gallium arsenide, zinc sulfide, or aluminum nitride.

[0008] Preferably, the bottom dielectric substrate and the top dielectric substrate can be a quartz substrate, a silicon substrate, or a silicon oxide substrate.

[0009] Preferably, the metal reflection layer can be made of a metal material such as gold, silver, or aluminum.

[0010] Preferably, the spacer layer can be a silicon dioxide or silicon nitride dielectric material.

[0011] Preferably, the first dielectric material layer can be hydrogen silsesquioxane or spin-on glass dielectric material.

[0012] Preferably, the second dielectric material layer can be polymethyl methacrylate, silicon dioxide, titanium dioxide, silicon nitride, hafnium dioxide, or silicon dielectric material.

[0013] Preferably, it includes the following steps:

[0014] S1: Obtain the phase response and polarization conversion efficiency of nanocolumn structures with different structural dimensions at the designed wavelength through numerical simulation, and find out the nanocolumn structure that can achieve the ideal polarization conversion function; optimize and obtain the thickness of the spacer layer and the thickness of the first dielectric material layer through numerical simulation.

[0015] S2: The bottom dielectric substrate uses silicon material, and the spacer layer uses silicon dioxide material. Expose and develop on the bottom dielectric substrate through laser direct writing technology, and prepare pixelated aluminum electrodes and alignment marks through thermal evaporation of aluminum and lift-off process, and sputter the spacer layer of silicon dioxide.

[0016] S3: Spin-coat PMMA photoresist on the spacer layer, use electron beam lithography technology for overlay exposure and then develop to produce a hole structure complementary to the nanocolumn structure array.

[0017] S4: Evaporate aluminum using thermal evaporation technology to fill the holes.

[0018] S5: Obtain the nanocolumn structure array using the lift-off technology, and the nanocolumn structures are arranged periodically at 45 degrees or 135 degrees with respect to the X-axis.

[0019] S6: The first dielectric material layer uses HSQ, and spin-coat HSQ on the nanocolumn structure as a protective layer to coat the structure.

[0020] S7: The second dielectric material layer uses PMMA, then spin-coat PMMA on the nanocolumn structure, and use electron beam lithography technology for overlay exposure and then develop.

[0021] S8: Select a transparent dielectric substrate with a layer of ITO electrode film as the top dielectric substrate 10, coat a liquid crystal alignment layer on the ITO electrode film and align it, place the two substrates opposite to each other, and finally pour liquid crystal between the two substrates and glue them into a cell to complete the device packaging.

[0022] The beneficial effects produced by adopting the above technical solutions are as follows:

[0023] 1. In the present invention, the nanocolumn structures arranged in an array form the first metasurface, and the second dielectric material layers arranged at intervals form the second metasurface. At the same time, nematic liquid crystals are arranged in the second dielectric material layers. Through the combined regulation of the first metasurface and the liquid crystals, whether the incident light is X-polarized light or Y-polarized light incident on the device, after the polarization conversion by the nanocolumn structures, the light reflected from the device can obtain the same modulation effect, realizing polarization-insensitive phase modulation, and off-axis extraordinary reflected light can be generated; the second dielectric material layers and the nematic liquid crystals are arranged alternately, and the principle of destructive interference can be used to realize the switching of the extraordinary reflected light. The present invention ingeniously integrates the metasurface and the liquid crystals, making full use of the polarization conversion ability of the metasurface and the dynamic regulation ability of the liquid crystals, overcoming the dependence of the traditional amplitude-type LCoS display system on the polarization of the incident light, realizing a high-contrast polarization-insensitive light switch, avoiding the use of polarization elements, and greatly simplifying the display system.

[0024] 2. By adjusting the period of the nanocolumn structures, that is, adjusting the spacing between the nanocolumn structures, the present invention designs the red, green, and blue metasurface pixels to generate extraordinary reflected light at the same angle, and a single-chip polarization-insensitive LCoS chip can be used to realize color projection display without the need for beam splitting and beam combining elements, which can further greatly simplify the LCoS projection system; the present invention conducts dynamic amplitude regulation through the propagation phase difference between the liquid crystals and the second dielectric material layers, has higher robustness in the liquid crystal thickness, can further reduce the liquid crystal thickness, thereby reducing the driving voltage and having low power consumption.

[0025] 3. The present invention can be directly fabricated on the existing LCoS backplane, further improving the display resolution, and is compatible with the CMOS (Complementary metal oxide semiconductor) process, having the potential for large-scale production.

[0026] 4. The present invention can adopt the method of electric control tuning, and the tuning process is simple and does not have mechanical moving parts, so mechanical losses can be avoided.

[0027] 5. The metasurface of the present invention can be used within a certain broadband wavelength range. By selecting different materials, it can be designed to be used in the ultraviolet band, visible light band, and infrared band, with a wide range of applications. Brief Description of the Drawings

[0028] Figure 1 is a schematic cross-sectional structure diagram of a polarization-insensitive light switch device;

[0029] Figure 2 is a functional schematic diagram of a polarization-insensitive light switch device;

[0030] Figure 3It is a three-dimensional schematic diagram of the principle for the device to achieve polarization-insensitive modulation;

[0031] Figure 4 It is a process flow chart for the preparation of the device in the embodiment;

[0032] Figure 5 It is a schematic diagram of six cross-sectional patterns of the nanorod structure;

[0033] In the figure: 1. Bottom dielectric substrate, 2. Metal reflective layer, 3. Spacer layer, 4. Nanorod structure, 5. First dielectric material layer, 6. Second dielectric material layer, 7. Nematic liquid crystal, 8. Liquid crystal alignment layer, 9. ITO transparent electrode, 10. Top dielectric substrate, 11. Incident light, 12. Anomalous reflected light, 13. Incident X-polarized light, 14. Y-polarized light emitted after modulation, 15. Incident Y-polarized light, 16. X-polarized light emitted after modulation;

[0034] 01. Polarization-insensitive optical switch device, 02. Arbitrarily polarized incident light, 03. Picture displayed in the far field. Detailed implementation manners

[0035] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present application.

[0036] Such as Figure 1As shown in the figure, in the "off" state, the abnormal reflected light of the device disappears. The polarization-insensitive optical switch device is composed of a bottom dielectric substrate 1, a metal reflection layer 2, a spacer layer 3, a nanorod structure 4, a first dielectric material layer 5, a second dielectric material layer 6, a nematic liquid crystal 7, a liquid crystal alignment layer 8, an ITO (Indium tin oxide) transparent electrode 9, and a top dielectric substrate 10 from bottom to top. The device converts the polarization of the incident light through the metasurface composed of the nanorod structure 4 array at the bottom layer, constructs a phase gradient that enables abnormal reflection of the light beam, and then changes the orientation of the liquid crystal molecules by applying a voltage between the metal reflection layer 2 and the ITO transparent electrode 9, so that the refractive index of the liquid crystal changes, thereby changing the propagation phase of the light beam to control destructive interference to achieve a dynamic optical switch. By carefully designing the metasurface and the thickness of the second dielectric material, the device can achieve a polarization-insensitive dynamic optical switch with high contrast, and through multi-pixel array arrangement or direct integration with an LCoS backplane, high-resolution and high-contrast dynamic projection display can be realized. Compared with the existing amplitude modulation devices, the present invention has higher resolution and contrast, greatly simplifies the structure of the projection display system, can avoid mechanical loss due to the absence of mechanical moving parts, has low power consumption, small volume, is compatible with CMOS technology, and has the potential for large-scale production.

[0037] In this embodiment, the bottom dielectric substrate 1 is made of silicon material, the metal reflection layer 2 is aluminum, the spacer layer 3 is silicon dioxide, the nanorod structure 4 is aluminum, the first dielectric material layer 5 is HSQ (Hydrogen Silsesquioxane), the second dielectric material layer 6 is PMMA (Polymethyl Methacrylate), and the top dielectric substrate 10 is a transparent glass substrate.

[0038] The manufacturing method of the device is as Figure 4 shown. In this embodiment, the specific steps are as follows:

[0039] S1: Obtain the phase response and polarization conversion efficiency of the nanorod structure 4 with different structural dimensions at the designed wavelength through numerical simulation, find the nanorod structure 4 that can achieve the ideal polarization conversion function, and then arrange the nanorod structure according to the phase response to construct the phase gradient required to generate abnormal reflected light; optimize and obtain the thickness of the spacer layer 3 and the thickness of the first dielectric material layer 5 through numerical simulation;

[0040] S2: Expose and develop on the bottom dielectric substrate 1 through laser direct writing technology, prepare pixelated aluminum electrodes and alignment marks by thermal evaporation of 200 nanometers of aluminum and lift-off process, and sputter 50 nanometers of spacer silicon dioxide;

[0041] S3: Spin-coat 180-nm PMMA photoresist on the spacer layer, use electron beam lithography technology for overlay exposure and then develop it to fabricate a hole structure complementary to the nano-column structure array;

[0042] S4: Evaporate 50-nm aluminum by thermal evaporation technology to fill the holes;

[0043] S5: Use the lift-off technology to obtain the nano-column structure 4 array, and the nano-column structure 4 is arranged periodically at 45 degrees or 135 degrees with respect to the X-axis;

[0044] S6: Spin-coat 70-nm HSQ on the nano-column structure 4 as a protective layer to coat the structure;

[0045] S7: Spin-coat 300-nm PMMA on the nano-column structure 4 again, use electron beam lithography technology for overlay exposure and then develop it;

[0046] S8: Select a transparent dielectric substrate with a layer of ITO electrode film grown on it as the top dielectric substrate 10, coat and align the liquid crystal alignment layer 8 on the ITO electrode film, place the two substrates opposite to each other, and finally pour liquid crystal between the two substrates and bond them into a cell to complete the device packaging.

[0047] The design principle of this device: The metasurface is composed of sub-wavelength-sized scattering structures arranged in a two-dimensional plane. By carefully designing parameters such as the shape, size, and arrangement of the structures, the phase of light can be arbitrarily controlled. In this embodiment, the geometric phase and the plasmon resonance phase of the metasurface are used to jointly control the incident light. The geometric phase is a method of controlling the wavefront phase by the rotation angle of the nanostructure, and the additional phase amount to be controlled is twice the rotation angle of the structure. The plasmon resonance phase refers to the phase change generated due to the strong resonance effect excited by the collective oscillation of free electrons when the surface plasmon resonates at a specific frequency. Through specific structural size design and material selection, the resonance phase of the plasmon can be precisely controlled. When a plane wave is incident on the metasurface with a gradient phase of φ at an angle θ i the direction of the reflected light beam follows the generalized Snell's law:

[0048]

[0049] where, θ r is the reflection angle, n i is the refractive index of the incident and reflected space media, k0 is the wave vector of light in vacuum, and (dφ / dx) is the phase gradient constructed by the metasurface, which can control the angle of extraordinary reflection.

[0050] Liquid crystal is a birefringent molecule with properties of both liquid and crystal. It has the fluidity of liquid and the anisotropy of crystal, and the arrangement of its internal structure is sensitive to the external environment. The change of the applied voltage can affect the refractive index of the liquid crystal material and change the propagation phase of light passing through the liquid crystal. The equivalent refractive index of liquid crystal molecules at different inclination angles is expressed by the following formula:

[0051]

[0052] where n is the equivalent refractive index of the liquid crystal, θ LC is the inclination angle of the long axis of the liquid crystal molecule, n o is the refractive index of the liquid crystal for ordinary light, and n e is the refractive index of the liquid crystal for extraordinary light.

[0053] The resonance phase response and polarization conversion efficiency of nanocolumns with different structural sizes at the designed wavelength can be obtained through numerical simulation. Find two nanocolumn structures with a phase difference of π / 2 and capable of achieving the ideal polarization conversion function, and then rotate these two structures 90 degrees along the X-axis to form a phase difference of π with the previous two structures. Then, arrange the four structures in the order of phase 0, π / 2, π, 3π / 2 along the Y direction to form an increasing phase gradient, as Figure 3 shown. The constructed phase gradient can generate extraordinary reflected light. From formula (1), the extraordinary reflection angle θ = sin -1 λ / 4p can be derived, where λ is the designed wavelength and p is the period of the nanostructure. Arrange four other structures at 90 degrees to the previous column in the X direction to form a phase difference of π. Alternately arrange liquid crystal and the second dielectric material on these two columns. By changing the refractive index of the liquid crystal through voltage, the phase difference between the two columns can be controlled, thereby controlling the interference to achieve the dynamic switching of extraordinary reflected light. The principle of the device to achieve polarization-insensitive modulation can be explained by Figure 3 . Any polarization state can be described by a pair of orthogonal polarization bases of X polarization and Y polarization. The phase modulation amounts obtained after the X-polarized and Y-polarized lights are normally incident are the same, both being is the phase modulated by the metasurface, is the propagation phase difference between the liquid crystal with refractive index n o and the second dielectric material, is the propagation phase difference between the liquid crystal with refractive index n e and the second dielectric material layer. Therefore, it can be understood that for any normally incident polarized light, the designed device can give the same phase modulation and generate the same extraordinary reflected light switch.

[0054] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, shall be covered by the protection scope of the present invention.

Claims

1. A polarization-insensitive optical switch device based on a metasurface, characterized in that, It includes a bottom dielectric substrate (1), a metal reflection layer (2), a spacer layer (3), a nanocolumn structure (4), a first dielectric material layer (5), a second dielectric material layer (6), a nematic liquid crystal (7), a liquid crystal alignment layer (8), an ITO transparent electrode (9), and a top dielectric substrate (10); the metal reflection layer (2) is covered on the top of the bottom dielectric substrate (1); the spacer layer (3) is covered on the top of the metal reflection layer (2), and a number of nanocolumn structures (4) with sub-wavelength size are arrayed on the top of the spacer layer (3); the nanocolumn structures (4) are arranged periodically at an angle of 45 degrees or 135 degrees with the X-axis; the first dielectric material layer (5) coats the outside of the nanocolumn structures (4) and fills the gaps between the nanocolumn structures (4); the second dielectric material layer (6) is arranged at intervals on the top of the first dielectric material layer (5); the nematic liquid crystal (7) and the second dielectric material layer (6) are arranged alternately; the liquid crystal alignment layer (8) is covered on the top of the nematic liquid crystal (7); the ITO transparent electrode (9) is covered on the top of the liquid crystal alignment layer (8); the top dielectric substrate (10) is covered on the top of the ITO transparent electrode (9).

2. The polarization-insensitive optical switch device based on a metasurface according to claim 1, wherein The nanocolumn structure (4) is a columnar structure, and the cross-sectional shape of the nanocolumn structure (4) has anisotropy, being rectangular or elliptical or cross-shaped or L-shaped or crescent-shaped or a combined shape.

3. A polarization-insensitive optical switch device based on a metasurface according to claim 1, characterized in that, The material of the nanocolumn structure (4) is gold or silver or aluminum metal, or is a dielectric material such as titanium dioxide or silicon nitride or hafnium dioxide or silicon or zirconium dioxide or gallium nitride or gallium arsenide or zinc sulfide or aluminum nitride.

4. The polarization-insensitive optical switch device based on the metasurface according to claim 1, wherein The bottom dielectric substrate (1) and the top dielectric substrate (10) are made of a quartz substrate, a silicon substrate or a silicon oxide substrate.

5. The polarization-insensitive optical switch device based on a metasurface according to claim 1, wherein The metal reflection layer (2) is made of a gold or silver or aluminum metal material.

6. The polarization-insensitive optical switch device based on metasurface according to claim 1, wherein The spacer layer (3) is selected from a silicon dioxide or silicon nitride dielectric material.

7. The polarization-insensitive optical switch device based on metasurface according to claim 1, characterized in that, The first dielectric material layer (5) is hydrogen silsesquioxane or spin-on glass dielectric material.

8. The polarization-insensitive optical switch device based on a metasurface according to claim 1, wherein The second dielectric material layer (6) is polymethyl methacrylate or silicon dioxide or titanium dioxide or silicon nitride or hafnium dioxide or silicon dielectric material.

9. A method for implementing a polarization-insensitive optical switch device based on a metasurface according to any one of claims 1-8, characterized in that, It includes the following steps: S1: Obtain the phase response and polarization conversion efficiency of the nanocolumn structures (4) with different structural sizes at the designed wavelength through numerical simulation, and find out the nanocolumn structures (4) that can achieve the ideal polarization conversion function; optimize and obtain the thickness of the spacer layer (3) and the thickness of the first dielectric material layer (5) through numerical simulation; S2: The bottom dielectric substrate (1) is made of silicon material, the spacer layer (3) is made of silicon dioxide material, expose and develop on the bottom dielectric substrate (1) through laser direct writing technology, and prepare pixelated aluminum electrodes and alignment marks through thermal evaporation of aluminum and lift-off process, and sputter the spacer silicon dioxide; S3: Spin-coat PMMA photoresist on the spacer layer, use electron beam exposure technology for overetching exposure and then develop, to make a hole structure complementary to the nanocolumn structure array; S4: Use thermal evaporation technology to evaporate aluminum to fill the holes; S5: Obtain an array of nanocolumn structures (4) using a stripping technique, where the nanocolumn structures (4) are arranged periodically at 45 degrees or 135 degrees with respect to the X-axis; S6: The first dielectric material layer (5) uses HSQ, and HSQ is spin-coated on the nanocolumn structures (4) as a protective layer to coat the structure; S7: The second dielectric material layer (6) uses PMMA. Then, PMMA is spin-coated on the nanocolumn structures (4), and after using electron beam lithography technology for overlay exposure and development; S8: Select a transparent dielectric substrate with a layer of ITO electrode film grown on it as the top dielectric substrate (10), coat and align a liquid crystal alignment layer (8) on the ITO electrode film, place the two substrates opposite to each other, and finally pour liquid crystal between the two substrates and bond them into a cell to complete the encapsulation.

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