Metasurface structure based on high-order topological vector beam multiplexing and method for increasing holographic multiplexing channel number

Abstract

The invention discloses a metasurface structure based on high-order topological vector beam multiplexing and a method for increasing the number of holographic multiplexing channels. The metasurface structure comprises a substrate, a medium microstructure and a vector polarized light beam, the dielectric microstructure is established on the substrate; the medium microstructure is a metasurface array structure formed by periodically arranging a plurality of high-reflectivity medium metasurface units, the high-reflectivity medium metasurface units are geometrically-shaped cubes, and a vector polarized light beam is vertically incident on the metasurface array structure. According to the method, the defects of the existing multiplexing holographic technology based on the orbital angular momentum are mainly overcome, the selective isolation degree between vector vortex light with different parameters is improved by utilizing the topological orthogonal characteristics of the vector vortex light beams, and the selectivity independent of the orbital angular momentum is realized; and holographic imaging of the target can be carried out without assistance of a space dot matrix filter plate.

Description

Technical Field

[0001] The present invention relates to the technical field of metasurface light field regulation, and specifically refers to high-dimensional multi-channel holography that relies on the topological orthogonality of vector light beams, and in particular to a metasurface structure based on high-order topological vector light beam multiplexing and a method for increasing the number of holographic multiplexing channels. Background Art

[0002] Dielectric metasurfaces are optical components composed of subwavelength-scale nanostructures that scatter light. By precisely controlling the material, structure, shape, and orientation of each unit, dielectric metasurfaces can achieve precise control of optical signals. Due to the extremely small size of dielectric metasurfaces, they can be designed to be much smaller than traditional optical components to perform multiple functions such as spatial multiplexing, polarization multiplexing, and wavelength multiplexing. This miniaturized design enables it to perform complex optical operations in a very limited space. Dielectric metasurfaces have significant advantages in terms of versatility, compactness, small size, light weight, and high integration. In addition, it is compatible with traditional nanofabrication processes, which makes it easier to integrate into existing systems in practical applications. These characteristics make dielectric metasurfaces show great potential and application value in the design of mid-infrared enhanced imaging optical systems, especially in the pursuit of miniaturization and high efficiency. Orbital angular momentum, as one of the characteristic dimensions of light, makes light carry a spiral phase, and because it physically has infinite orthogonal spiral modes, it becomes a method to carry high-capacity information transmission channels. Many studies have demonstrated the potential of the exotic Hall effect in optical vector modes, opening up new directions for the design and application of future multifunctional optical devices. Starting from the orbital angular momentum dimension of light, the development of high-capacity holographic multiplexing has gradually taken shape, and then a multi-dimensional joint multiplexing technology of different dimensions of light and orbital angular momentum has been developed. However, it is necessary to perform spatial frequency sampling of the digital hologram in momentum space in order to retain the orbital angular momentum characteristics of the incident light and selectively present a holographic image matching the orbital angular momentum. However, this phase-only hologram does not take into account the precise convolution of the orbital angular momentum spiral wavefront during propagation, resulting in extremely strong crosstalk in the orbital angular momentum multiplexing. Therefore, it is necessary to add a filter plate to the imaging surface for perfect imaging. Summary of the invention

[0003] The purpose of the present invention is to provide a metasurface structure based on high-order topological vector beam multiplexing and a method for increasing the number of holographic multiplexing channels, which mainly overcomes the defects of the existing holographic technology based on orbital angular momentum multiplexing. It uses the topological orthogonal characteristics of the vector vortex beam itself to improve the selective isolation between vector vortex lights with different parameters, and realizes a technology that does not rely on orbital angular momentum selectivity and can perform target holographic imaging without the assistance of a spatial lattice filter plate.

[0004] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

[0005] A metasurface structure based on high-order topological vector beam multiplexing, characterized in that it includes a substrate, a dielectric microstructure, and a vector polarized beam;

[0006] The medium microstructure is established on a substrate; the medium microstructure is a metasurface array structure formed by periodically arranging a number of high-reflectivity medium metasurface units, the high-reflectivity medium metasurface units are geometric cubes, and a vector polarized light beam is vertically incident on the metasurface array structure.

[0007] The metasurface structure based on high-order topological vector beam multiplexing is characterized in that the material of the substrate is silicon, the material of the medium microstructure is silicon, and the wavelength of the vector polarized beam is 3 to 5 microns.

[0008] The metasurface structure based on high-order topological vector beam multiplexing is characterized in that the geometric cube is an elliptical cylinder, the major axis diameter and the minor axis diameter of the elliptical cylinder are variables, and the period of each elliptical cylinder and the height of the elliptical cylinder remain equal.

[0009] The metasurface structure based on high-order topological vector beam multiplexing is characterized in that: the elliptical cylinder is periodically extended along two mutually perpendicular directions, which are defined as the x-axis and the y-axis respectively, and the two extension directions are perpendicular to each other; when a plane wave with a polarization direction along the x-axis is irradiated to the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry the phase distribution determined by the size of the long axis of the elliptical cylinder; when a plane wave with a polarization direction along the y-axis is irradiated to the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry the phase distribution determined by the size of the short axis of the elliptical cylinder. Combined with the high-dimensional degree of freedom information carried by the vector polarized light beam (3) itself, a low-crosstalk and high-channel information carrier can be realized.

[0010] The metasurface structure based on high-order topological vector beam multiplexing is characterized in that the range of variation of the major axis diameter and the minor axis diameter of the elliptical cylinder is 300 nanometers to 1200 nanometers, the unit period is 1.5 micrometers to 2 micrometers, the number of elliptical cylinders is 1200×1200, and the total length is 1800 micrometers×1800 micrometers.

[0011] The metasurface structure based on high-order topological vector beam multiplexing is characterized in that the height of the elliptical cylinder is 4 microns.

[0012] A method for increasing the number of holographic multiplexing channels by using a metasurface structure based on high-order topological vector beam multiplexing, characterized in that it comprises the following steps:

[0013] Step A: Selecting appropriate materials of substrate and dielectric microstructure according to the incident light band of the vector polarized light beam;

[0014] Step B: Simulate the phase and intensity change database of light when incident plane light passes through units of different sizes according to the selected material;

[0015] Step C: Using the gradient descent algorithm to obtain the required metasurface array phase information, then finding the corresponding unit in the database, and obtaining the simulation result through simulation calculation;

[0016] Step D: Processing the metasurface array structure;

[0017] Step E: Perform experimental effect test on the designed metasurface array structure. If the achieved result does not meet the requirements, return to step B, adjust and optimize the metasurface parameters, and repeat steps DE until the experimental results meet the requirements.

[0018] The method is characterized in that: in the step D, electron beam exposure, dry etching or wet etching is used to process the super surface array structure.

[0019] The method is characterized in that: after step D and before step E, the surface of the metasurface array structure is cleaned with ultraviolet ozone.

[0020] When a designated incident vector polarized beam is irradiated onto the metasurface structure of the high-order vector topological multiplexing, the holographic image presented by the outgoing beam in the Fourier plane is highly consistent with the preset image. When the incident beam is changed, different images are presented to achieve the effect of vector polarization multiplexing. The outstanding feature is that the image does not rely on the sampling of the spatial lattice, which greatly reduces the size of the metasurface and opens up the operability of infrared polarized light at the sub-wavelength scale.

[0021] Based on the spatially structured vector vortex beam, we studied a metasurface holography based on high-order vector topological multiplexing in the mid-infrared band. The design uses three different vector vortex multiplexing metasurface holograms, with a total of 12 independently regulated holographic channels.

[0022] The present invention mainly aims to overcome the defects of the existing orbital angular momentum multiplexing holographic technology, and provide a metasurface holographic multiplexing device structure and preparation based on high-order topology and superposition vector beams. By breaking the problem that the existing orbital angular momentum multiplexing holography needs to perform spatial frequency sampling of the digital hologram in the momentum space in order to retain the orbital angular momentum characteristics of the incident light and selectively present the holographic image matching the orbital angular momentum, a metasurface hologram is obtained that does not require lattice sampling in frequency space, and therefore no longer relies on orbital angular momentum selectivity, and can perform target holographic imaging without the assistance of a filter plate. This greatly simplifies the optical path system and the design process, and promotes the development of integrated and miniaturized optical devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the metasurface structure based on high-order topological vector beam multiplexing of the present invention;

[0024] Figure 2 The x-direction phase and x-direction transmittance database of the superatom of the present invention, and the y-direction phase and x-direction transmittance database;

[0025] Figure 3 is a design flow chart of the super surface structure of the present invention;

[0026] Figure 4 3 is a schematic diagram of the metasurface holographic results of three different vector beam multiplexing provided by an embodiment of the present invention, where l represents the topological charge value. DETAILED DESCRIPTION

[0027] See also Figure 1 The present invention discloses a metasurface structure based on high-order topological vector beam multiplexing, which includes a substrate 1, a dielectric microstructure 2, and a vector polarized beam 3; the dielectric microstructure 2 is established on the substrate 1; the dielectric microstructure 2 is a metasurface array structure formed by a periodic arrangement of a number of high-reflectivity dielectric metasurface units, the high-reflectivity dielectric metasurface unit is a geometric cube, and the vector polarized beam 3 is vertically incident on the metasurface array structure.

[0028] In the present invention, the material of the substrate 1 is silicon or other high refractive index dielectric material, and its thickness can be regarded as semi-infinite thickness. The material of the dielectric microstructure 2 is silicon, or other material with high transmittance to mid-infrared light, and its thickness is 4 microns. The wavelength of the vector polarized light beam 3 is 3 to 5 microns.

[0029] In the present invention, the geometric cube may be an elliptical cylinder, the major axis diameter and the minor axis diameter of the elliptical cylinder are variables, and the period of each elliptical cylinder and the height of the elliptical cylinder are kept equal.

[0030] Furthermore, the elliptical cylinder is periodically extended along two mutually perpendicular directions, which are defined as the x-axis and the y-axis respectively, and the two extension directions are perpendicular to each other; when a plane wave with a polarization direction along the x-axis irradiates the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry a phase distribution determined by the size of the long axis of the elliptical cylinder; when a plane wave with a polarization direction along the y-axis irradiates the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry a phase distribution determined by the size of the short axis of the elliptical cylinder. Combined with the high-dimensional degree of freedom information carried by the vector polarized light beam 3 itself, a low-crosstalk and high-channel information carrier can be realized.

[0031] Furthermore, the major axis diameter and minor axis diameter of the elliptical cylinder vary in the range of 300 nanometers to 1200 nanometers, the unit period is 1.5 micrometers to 2 micrometers, the number of elliptical cylinders is 1200×1200, the total length is 1800 micrometers×1800 micrometers, and the height of the elliptical cylinder is 4 micrometers.

[0032] The present invention also discloses a method for increasing the number of holographic multiplexing channels by using a metasurface structure based on high-order topological vector beam multiplexing, which comprises the following steps:

[0033] Step A: selecting suitable materials of the substrate 1 and the dielectric microstructure 2 according to the incident light band of the vector polarized light beam 3;

[0034] Step B: Based on the selected material, simulate the phase and intensity changes of the incident plane light when it passes through 50×50 units of different sizes with the x and y directions varying from 300 nanometers to 1200 nanometers respectively. Figure 2 );

[0035] Step C: Use the gradient descent algorithm to obtain phase information. When the incident light information is known, the target image is presented at the specified spatial position through the optical diffraction process. During the iteration process, when the loss function is less than the specified size, the loop is jumped out. After obtaining the phase information of the metasurface array, the corresponding unit is found in the database, and the simulation result (such as Figure 3 );

[0036] Step D: Processing the metasurface array structure;

[0037] Step E: Perform experimental effect test on the designed metasurface array structure. If the achieved result does not meet the requirements, return to step B, adjust and optimize the metasurface parameters, and repeat steps DE until the experimental results meet the requirements.

[0038] In the step D, electron beam exposure, dry etching or wet etching is used to process the super surface array structure.

[0039] After step D and before step E, the surface of the metasurface array structure is cleaned with ultraviolet ozone.

[0040] The present invention is further described below in conjunction with embodiments and drawings.

[0041] Example

[0042] Step 1: First, it is necessary to scan and obtain the x-polarization target phase and y-polarization target phase that can be generated by high-reflectivity medium metasurface units (or meta-atoms) of different sizes. In this embodiment, the working wavelength of the light wave is 4 microns, the material of the meta-atom and the substrate is silicon, and the geometric shape of the meta-atom is a cube, such as Figure 1 As shown, the period P = 1.5 microns, the height H = 4 microns, and the length and width range from 300 nanometers to 1200 nanometers. The length and width of the super atom are equally spaced at 50 points, and a total of 2500 super atoms of different geometric sizes are selected. The three target wavelengths of light waves are irradiated with linear polarized light of x polarization and y polarization, respectively, to obtain the super atom's ability to manipulate light waves with a wavelength of 4 microns. Figure 2 The two pictures above are the x-phase and x-transmittance databases of meta-atoms, and the two pictures below are the y-phase and x-transmittance databases. In each database, the x-polarization phase and y-polarization target phase have achieved full coverage from -π to +π and the transmittance is above 65%.

[0043] Step 2: Apply the gradient descent algorithm to perform iterative calculations to obtain the phase distribution information of each target pattern, such as Figure 3 . The metasurface of this implementation case is designed to have a pixel size of 1200×1200 (1800 microns×1800 microns). First, the target pattern needs to be distributed according to different vector incident light and diffraction distances, and the digital images "A" to "L" are planned to be reproduced on the focal plane 2000 microns away from the metasurface. Then, the distance parameters and wavelength parameters corresponding to each target image are brought into the first-class Rayleigh-Sommerfeld formula to obtain the corresponding response function. Finally, the gradient descent algorithm is used for iterative calculation to obtain the required phase distribution information.

[0044] Step 3: Assign the target images to different polarization channels. Assign images "A", "C", "E", "G", "I", and "K" to the x-polarization-x-polarization state channel, and images "B", "D", "F", "H", "J", and "L" to the y-polarization-y-polarization channel. Bring the data of each channel into the corresponding Jones matrix function, and set the x-polarization target phase and the y-polarization target phase as optimization variables, and finally obtain the required x-polarization phase and y-polarization phase distribution information.

[0045] Step 4: Select meta-atoms based on the obtained x-polarization target phase and y-polarization target phase distribution information, and form a metasurface. Compare the obtained x-polarization target phase and y-polarization target phase distribution information with the phase and transmittance database of the corresponding wavelength, that is, for a certain pixel point, convert the corresponding x-polarization and y-polarization target light fields into complex function forms, and subtract the modulus from the data in the database. Since a single unit has two target phases, x-polarization and y-polarization, their results need to be added and the minimum value is taken to find the metaatom that best meets the requirements. The high-order topological charge vector holographic metasurface designed in this implementation case realizes holographic imaging results after the incidence of vector light beams with three different topological combinations. Figure 4 As shown, on the focal plane of f=2000 microns, 12 digital patterns from "A" to "L" are reproduced.

[0046] In summary, metasurface holography based on high-order vector topological multiplexing perfectly realizes 12 multi-dimensional multiplexing channels. The present invention has very important practical significance for the research on increasing the number of non-interleaved metasurface multi-dimensional multiplexing channels.

Claims

1. A metasurface structure based on high-order topological vector beam multiplexing, characterized in that: It includes a substrate (1), a dielectric microstructure (2), and a vector polarized light beam (3); The medium microstructure (2) is built on a substrate (1); the medium microstructure (2) is a metasurface array structure formed by a periodic arrangement of a plurality of high-reflectivity medium metasurface units, the high-reflectivity medium metasurface units are cubes in geometric shape, and a vector polarized light beam (3) is vertically incident on the metasurface array structure.

2. The metasurface structure based on high-order topological vector beam multiplexing according to claim 1, characterized in that: The material of the substrate (1) is silicon, the material of the medium microstructure (2) is silicon, and the wavelength of the vector polarized light beam (3) is 3 to 5 microns.

3. The metasurface structure based on high-order topological vector beam multiplexing according to claim 1, characterized in that: The cube of the geometric shape is an elliptical cylinder, the major axis diameter and the minor axis diameter of the elliptical cylinder are variables, while the period of each elliptical cylinder and the height of the elliptical cylinder are kept equal.

4. The metasurface structure based on high-order topological vector beam multiplexing according to claim 3, characterized in that: The elliptical cylinder is periodically extended along two mutually perpendicular directions, which are defined as the x-axis and the y-axis respectively, and the two extension directions are perpendicular to each other; when a plane wave with a polarization direction along the x-axis irradiates the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry a phase distribution determined by the size of the long axis of the elliptical cylinder; when a plane wave with a polarization direction along the y-axis irradiates the high-reflectivity medium metasurface unit in the positive direction along the z-axis, it will carry a phase distribution determined by the size of the short axis of the elliptical cylinder. Combined with the high-dimensional degree of freedom information carried by the vector polarized light beam (3) itself, a low-crosstalk and high-channel information carrier can be realized.

5. The metasurface structure based on high-order topological vector beam multiplexing according to claim 4, characterized in that: The major axis diameter and minor axis diameter of the elliptical cylinder vary in the range of 300 nanometers to 1200 nanometers, the unit period is 1.5 micrometers to 2 micrometers, the number of elliptical cylinders is 1200×1200, and the total length is 1800 micrometers×1800 micrometers.

6. The metasurface structure based on high-order topological vector beam multiplexing according to claim 5, characterized in that: The height of the elliptical cylinder is 4 micrometers.

7. A method for increasing the number of holographic multiplexing channels using the metasurface structure based on high-order topological vector beam multiplexing as described in claim 1 or 2 or 3 or 4 or 5 or 6, characterized in that: It includes the following steps: Step A: selecting suitable materials of the substrate (1) and the dielectric microstructure (2) according to the incident light band of the vector polarized light beam (3); Step B: Simulate the phase and intensity change database of light when incident plane light passes through units of different sizes according to the selected material; Step C: Using the gradient descent algorithm to obtain the required metasurface array phase information, then finding the corresponding unit in the database, and obtaining the simulation result through simulation calculation; Step D: Processing the metasurface array structure; Step E: Perform experimental effect test on the designed metasurface array structure. If the achieved result does not meet the requirements, return to step B, adjust and optimize the metasurface parameters, and repeat steps DE until the experimental results meet the requirements.

8. The method according to claim 7, characterized in that: In the step D, electron beam exposure, dry etching or wet etching is used to process the super surface array structure.

9. The method according to claim 7, characterized in that: After step D and before step E, the surface of the metasurface array structure is cleaned with ultraviolet ozone.

Images