Method for designing large-aperture multi-wavelength polarization division multiplexing metalens

Abstract

The present invention relates to a method for designing a large-aperture multi-wavelength polarization division multiplexing metalens, comprising the following steps: using a polarization-sensitive metasurface structure as a metasurface unit; performing parameter scanning by means of simulation software to obtain the relationships between phase regulation and transmittance regulation capabilities of the metasurface unit and parameters of the metasurface unit, and forming a parameter space; calculating a phase at a given position (x, y) on a metalens, and searching the parameter space for a proper structure for filling, to obtain metasurface arrays each working at a single wavelength; and sparsifying two metasurface arrays each working at the single wavelength, and integrating the metasurface arrays onto one metasurface, to obtain the multi-wavelength polarization division multiplexing metasurface lens that can achieve expected functions at two working wavelengths. The present invention can expand the aperture of the metalens without increasing the complexity of the system.

Description

A design method for large-aperture multi-wavelength polarization multiplexing metalens Technical Field

[0001] The present invention relates to the field of micro-nano photonics technology, and in particular to a design method for a large-aperture multi-wavelength polarization multiplexing super lens. Background Art

[0002] With the advancement of science and technology, the demand for high-performance lenses in optical systems is increasing, especially in the fields of high-resolution imaging, deep space exploration, and high-precision measurement. Traditional optical lenses can achieve high-quality focusing or other functions at specific wavelengths, but there are some limitations for multi-wavelength applications. Existing large-aperture lens technology is mainly based on the principles of refraction and reflection. However, when it is necessary to process multi-wavelength light, chromatic aberration will occur due to the different refractive indices of light of different wavelengths, which may lead to a decrease in imaging quality. To solve this problem, some multi-element lens combinations have been developed, but this increases the complexity and cost of the system. In addition, polarization multiplexing technology has a wide range of applications in optical communications, remote sensing, and medical imaging. However, traditional polarization multiplexing technology requires additional polarization elements, which may make the system more complex and heavy.

[0003] An optical metasurface is a two-dimensional, planar optical element composed of a large number of tiny, compact microstructures. Each microstructure is carefully designed to individually modulate incident light, enabling the entire metasurface to modulate and control optical properties such as light's propagation direction, polarization state, and wavefront phase. Its physical principles are based on diffraction theory and phase modulation.

[0004] Diffraction is the bending of light as it passes around an edge or obstacle. It occurs when light waves are subject to interference from different directions during propagation. In optical metasurfaces, the periodic arrangement of microstructures leads to the diffraction effect. When a plane wave is incident perpendicularly on an optical metasurface, it is reflected and scattered by the microstructures. Due to the periodicity of the microstructures, the incident light is diffracted, and the phase and amplitude of the different diffracted waves are affected by the geometric information of the microstructures. By precisely designing and manufacturing the microstructures on a metasurface, it is possible to reflect or transmit light in specific directions and modulate the wavefront phase of the light.

[0005] Current metalens technology can achieve subwavelength resolution, but most metalens designs are optimized for a single wavelength and present challenges in large-aperture applications.

[0006] Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a design method for a large-aperture multi-wavelength polarization multiplexing metalens, which can expand the aperture of the metalens without increasing the complexity of the system.

[0008] The technical solution adopted by the present invention to solve the technical problem is to provide a large-aperture multi-wavelength polarization multiplexing metalens design method, including the following steps:

[0009] A polarization-sensitive metasurface structure is used as a metasurface unit;

[0010] Performing parameter scanning through simulation software to obtain the relationship between the phase control and transmittance control capabilities of the metasurface unit and the parameters of the metasurface unit, and forming a parameter space;

[0011] Calculating the phase at a given position (x, y) on the metalens and finding a suitable structure to fill it from the parameter space to obtain a metasurface array operating at a single wavelength;

[0012] Two metasurface arrays operating at a single wavelength are thinned out and integrated onto a metasurface to obtain a multi-wavelength polarization multiplexing metasurface lens that achieves the expected functions at both operating wavelengths.

[0013] The polarization-sensitive metasurface structure is an elliptical cylinder metasurface structure.

[0014] The elliptical cylinder metasurface structure includes a base and an elliptical cylinder, the base is a cubic structure, and the elliptical cylinder is fixed at the center of the top surface of the base; the column height of the elliptical cylinder and the period of the metasurface unit are fixed; the metasurface unit parameters are the major and minor axes of the elliptical cylinder.

[0015] The phase at a given position (x, y) on the metalens is determined by Calculated, where is the phase at a given position (x, y) on the metalens, λ is the operating wavelength, F is the designed focal length, L is the topological charge number, which is used to distinguish the function under different polarization incident light, and θ is the azimuthal coordinate, which is expressed as:

[0016] The method thins out two metasurface arrays operating at a single wavelength and integrates them onto a metasurface to obtain a multi-wavelength polarization multiplexing metasurface lens that achieves the intended function at both operating wavelengths. Specifically, the method comprises: analyzing the gradient distribution of the theoretical phase distribution on the metasurface, selecting a thinning scheme that best suits the function, thinning the two metasurface arrays operating at a single wavelength using the best-suited thinning scheme, alternately arranging the two thinned-out metasurface arrays operating at a single wavelength in a radial direction, and integrating them onto a metasurface to form a composite array, wherein the width of each ring of the composite array is fixed. Beneficial effects

[0017] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: the present invention adopts a spatial multiplexing method to solve the chromatic aberration problem of multi-wavelength light without the need for additional polarization elements, and expands the aperture of the super lens. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 is a flow chart of a method for designing a large-aperture multi-wavelength polarization multiplexing metalens according to an embodiment of the present invention;

[0019] FIG2 is a schematic structural diagram of a metasurface unit according to an embodiment of the present invention;

[0020] FIG3 is a diagram showing simulation results of a 200 μm aperture metalens structure under 3.75 μm incident light in an embodiment of the present invention;

[0021] FIG4 is a diagram showing simulation results of a 200 μm aperture metalens structure under 4.25 μm incident light in an embodiment of the present invention;

[0022] FIG5 is an example diagram of analyzing sector-shaped and ring-shaped thinning spatial multiplexing schemes according to an embodiment of the present invention;

[0023] FIG6 is a diagram showing simulation results of a 200 μm aperture metalens structure after spatial multiplexing under incident light of 3.75 μm according to an embodiment of the present invention;

[0024] FIG7 is a diagram showing simulation results of a 200 μm aperture metalens structure after spatial multiplexing under incident light of 4.25 μm according to an embodiment of the present invention;

[0025] FIG8 is a diagram showing simulation results of a 500 μm aperture metalens structure after spatial multiplexing under incident light of 3.75 μm according to an embodiment of the present invention;

[0026] FIG9 is a diagram showing simulation results of a 500 μm aperture superlens structure after spatial multiplexing under incident light of 4.25 μm according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0028] The present invention relates to a method for designing a large-aperture, multi-wavelength, polarization-multiplexed metalens. The core of this method is spatial multiplexing, the principle of which is: given the full positional and geometric information of multiple metalens arrays, if the metalens units in any of these metalens arrays can be grouped according to a certain arrangement rule so that each group can precisely and independently express the complete function of the metalens, the array can be sparsely populated by reducing the number of groups. These sparsely populated metalens arrays with different functions can then be combined into a new full-aperture metalens array to achieve spatial multiplexing. Each group of metalens units that can precisely and independently express the complete function is called a complete function group.

[0029] As shown in Figure 5, analysis of the theoretical phase compensation formula for the metalens shows that the shape of the complete functional group for the focusing function is a radial column passing through the center of the metalens, i.e., fan-shaped thinning; the shape of the complete functional group for the vortex light generation function is concentric circles of the metalens, i.e., annular thinning. When adopting the functional design of focusing + dual vortex light generation, the focusing function has a more relaxed requirement for spatial multiplexing, so the complete functional group for the dual vortex light function is selected as the complete functional group for this embodiment, and annular thinning is performed before the ring is formed.

[0030] As shown in FIG1 , this embodiment specifically includes the following steps:

[0031] Step 1: Use a polarization-sensitive metasurface structure as a metasurface unit.

[0032] To achieve polarization multiplexing, this embodiment uses a polarization-sensitive metasurface structure as the basic unit. This polarization-sensitive metasurface structure is an elliptical cylinder metasurface structure, as shown in Figure 2. The elliptical cylinder metasurface structure comprises a cubic base A and an elliptical cylinder B fixed at the center of the base's top surface. The height H of the elliptical cylinder B and the period P of the metasurface unit are fixed, while the major and minor axes of the elliptical cylinder can vary within an appropriate range.

[0033] Step 2: Perform parameter scanning through simulation software to obtain the relationship between the phase control and transmittance control capabilities of the metasurface unit and the parameters of the metasurface unit, and form a parameter space.

[0034] The metalens designed in this embodiment can realize different light modulation functions under the conditions of different polarized light incidence, specifically, it presents a focusing function (topological charge number = 0) when X-polarized light is incident, and presents a vortex light generation function (topological charge number = 2) when Y-polarized light is incident. Therefore, this embodiment uses the major and minor axes of the elliptical cylinder as variables, and uses simulation software to perform parameter scanning to obtain the relationship between the phase control and transmittance control capabilities of the metasurface unit and the major and minor axes of the ellipse, thereby forming a parameter space. When the incident wavelength is 3.75μm and 4.25μm, the average transmittance is 79.07% and 86.5% respectively, and a phase coverage range of 2*2π can be achieved.

[0035] Step 3: Calculate the phase at a given position (x, y) on the metalens and find a suitable structure to fill it from the parameter space to obtain a metasurface array operating at a single wavelength.

[0036] The design of the metalens in this embodiment is based on the principle of transmission phase. The microstructure at each position of the metasurface is designed to achieve focusing and vortex light generation under different polarization conditions. To achieve focusing and vortex beam generation, the phase distribution at a given position (x, y) on the metalens can be expressed as: in, is the phase at a given position (x, y) on the metalens, λ is the operating wavelength, F is the design focal length, and L is the topological charge number, which is used to distinguish the functions under different polarized light incidences. The corresponding formula is different for L values, specifically: L = 0 for X-polarized light incidence, L = 2 for Y-polarized light incidence, and θ is the azimuth coordinate, expressed as: According to this formula, the phase of a given position on the metalens can be obtained, so that a suitable structure can be found in the parameter space to fill it.

[0037] At this point, simulation software can be used to model and simulate the single-wavelength polarization multiplexing metalens designed above. During modeling and simulation, the metalens operating at single wavelengths (3.75μm and 4.25μm) are first modeled separately. The PML boundary conditions are selected at the boundary, and the simulation is started after setting appropriate simulation parameters. In order to shorten the simulation time and improve the simulation efficiency, a small-aperture (200μm) metalens structure is initially used for simulation under 3.75μm incident light, and the simulation results shown in Figure 3 are obtained. The simulation results under 4.25μm incident light are shown in Figure 4.

[0038] According to the above simulation results, the metasurface lens achieves the expected functions at the set focal plane (z = 600μm) under the incidence of polarized light of the respective operating wavelengths, namely focusing (under the incidence of x-polarized light) and vortex light generation (under the incidence of y-polarized light).

[0039] Step 4: Thin out the two metasurface arrays operating at a single wavelength and integrate them onto a metasurface to obtain a multi-wavelength polarization multiplexing metasurface lens that achieves the expected function at both operating wavelengths. The specific method is: analyze the gradient distribution of the theoretical phase distribution on the metasurface, select the most suitable thinning scheme for the function (sector thinning or ring thinning as shown in Figure 5), thin out the two metasurface arrays operating at a single wavelength using the most suitable thinning scheme, and alternately arrange the two thinned metasurface arrays operating at a single wavelength in the radial direction and integrate them onto a metasurface to form a composite array, where the ring width of each ring of the composite array is fixed.

[0040] The 200μm aperture metalens after spatial multiplexing is simulated, and the simulation results under 3.75μm incident light are shown in Figure 6. The simulation results under 4.25μm incident light are shown in Figure 7.

[0041] The simulation results are consistent with expectations, demonstrating that the design is effective at a 200μm aperture. Due to limitations in simulation conditions, it is not possible to directly simulate large-aperture (centimeter-level) metalenses. Therefore, this embodiment expands the metalenses aperture to 500μm and performs simulations again. The simulation results for 3.75μm incident light are shown in Figure 8. The simulation results for 4.25μm incident light are shown in Figure 9.

[0042] It can be seen that after the aperture is expanded, the metasurface structure obtained using the above design method also has the function of multi-wavelength polarization multiplexing, proving that this design method can be extended to large apertures.

[0043] It is not difficult to find that the present invention adopts the spatial multiplexing method to solve the chromatic aberration problem of multi-wavelength light without the need for additional polarization elements, and expands the aperture of the metalens.

[0044] The processing flow of the metalens designed using this embodiment is as follows, which is mainly divided into the steps of Cr plating, glue coating, EBL, Cr etching, Si etching, and Cr removal in the processing order. First, Cr is plated on a double-polished silicon wafer as a hard mask, and photoresist is spin-coated. Then, the metasurface pattern is transferred to the hard mask using electron beam exposure (EBL) technology. EBL is a commonly used micro-nano processing technology that can achieve high-precision processing of the silicon wafer surface. Then, a high-precision ion beam etching (ICP) process is used for micro-nano processing to manufacture elliptical cylindrical metasurface units. ICP etching can achieve high precision and high processing quality, and can complete the processing in a relatively short time.

Claims

1. A method for designing a large-aperture multi-wavelength polarization multiplexing metalens, characterized in that: The following steps are involved: A polarization-sensitive metasurface structure is used as a metasurface unit; Perform parameter scanning through simulation software to obtain the relationship between the phase control and transmittance control capabilities of the metasurface unit and the parameters of the metasurface unit, and form a parameter space; Calculating the phase at a given position (x, y) on the metalens and finding a suitable structure to fill in the parameter space to obtain a metasurface array operating at a single wavelength; Two metasurface arrays operating at a single wavelength are thinned and integrated onto a metasurface to obtain a multi-wavelength polarization multiplexing metasurface lens that achieves the expected functions at both operating wavelengths.

2. The method for designing a large-aperture multi-wavelength polarization multiplexing superlens according to claim 1, characterized in that: The polarization-sensitive metasurface structure is an elliptical cylinder metasurface structure.

3. The method for designing a large-aperture multi-wavelength polarization multiplexing superlens according to claim 1, characterized in that: The elliptical cylinder metasurface structure comprises a base and an elliptical cylinder, the base is a cubic structure, and the elliptical cylinder is fixed at the center of the top surface of the base; the column height of the elliptical cylinder and the period of the metasurface unit are fixed; The metasurface unit parameters are the major semi-axis and the minor semi-axis of the elliptical cylinder.

4. The method for designing a large-aperture multi-wavelength polarization multiplexing superlens according to claim 1, characterized in that: The phase at a given position (x, y) on the metalens is given by Calculated, among which, is the phase at a given position (x, y) on the metalens, λ is the operating wavelength, F is the design focal length, L is the topological charge number, and θ is the azimuth coordinate, which can be expressed as:

5. The method for designing a large-aperture multi-wavelength polarization multiplexing superlens according to claim 1, characterized in that: The method thins out two metasurface arrays working at a single wavelength and integrates them onto a metasurface to obtain a multi-wavelength polarization multiplexing metasurface lens that realizes the expected functions at two working wavelengths. Specifically, the gradient distribution of the theoretical phase distribution on the metasurface is analyzed, and the most functionally suitable thinning scheme is selected. The two metasurface arrays working at a single wavelength are thinned using the most suitable thinning scheme respectively. The two thinned metasurface arrays working at a single wavelength are alternately arranged in a radial direction and integrated onto a metasurface to form a composite array, wherein the ring width of each ring of the composite array is fixed.

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