Arbitrary polarization conversion metasurface and method for performing polarization conversion on arbitrary polarized light

Abstract

This application relates to an arbitrary polarization conversion metasurface and a method for performing polarization conversion on arbitrary polarized light. The method includes: realizing the conversion of arbitrary polarized light by combining a depolarizing metasurface and a polarizer metasurface. The first step is to design a depolarizing metasurface, whose function is to depolarize the incident light with arbitrary polarization and convert it into completely unpolarized light, that is, natural light. The second step is to design a polarizer metasurface to convert the natural light generated in the first step into linearly polarized light with a fixed oscillation direction. The metasurface designed by the present invention is simple to process, has a low cost, and has a stable transmission efficiency, and can ensure the generation of a certain fixed polarized light under the condition of arbitrary polarized light incidence.

Description

Technical Field

[0001] The present application relates to the technical field of metasurface design, and particularly to an arbitrary polarization conversion metasurface and a method for polarizing and converting arbitrary polarized light. Background Art

[0002] The control of polarization is of great significance in the fields of communication, imaging, detection, and quantum. Traditional polarization conversion methods usually require the use of multiple optical elements, such as half-wave plates, quarter-wave plates, and polarizers, etc., resulting in a relatively complex optical path and being not conducive to the integration of the system. A polarization controller is an integrated polarization control device, which usually includes elements such as rotatable cascaded half-wave plates and quarter-wave plates. After determining the polarization state of the incident light, the wave plates are rotated accordingly through electrical adjustment to output the required polarized light. However, the polarization controller needs to determine the polarization state of the input light in advance to make corresponding responses and does not have the ability of real-time control. The problem of polarization maintenance during long-distance light transmission is also an important research topic in polarization optics. For light propagating through ordinary optical fibers and complex dielectric environments, the polarization state may be disturbed due to the birefringence effect of the material or repeated reflection and diffraction, resulting in inaccurate transmitted polarization. Polarization-maintaining optical fibers can transmit light without changing its polarization state by utilizing the amplified birefringence effect. However, for some large systems or applications that require long-distance polarized light transmission, relatively long polarization-maintaining optical fibers are usually required, which will lead to high costs.

[0003] A metasurface is a two-dimensional surface composed of an artificially constructed periodic unit arrangement, which can manipulate light at the sub-wavelength scale. Due to the miniaturization and real-time response characteristics of the metasurface, domestic and foreign institutions have conducted extensive research on metasurfaces for controlling arbitrary polarization. However, the existing technology still has problems with poor adaptability to the polarization conversion of arbitrarily polarized incident light. Summary of the Invention

[0004] Based on this, in view of the above technical problems, it is necessary to provide an arbitrary polarization conversion metasurface that can convert arbitrary polarized light into a specific polarized light and a method for polarizing and converting arbitrary polarized light.

[0005] An arbitrary polarization conversion metasurface, characterized in that the metasurface includes: a depolarizing metasurface and a polarizer metasurface; the depolarizing metasurface and the polarizer metasurface are integrated on the front and back sides of a processing substrate;

[0006] The depolarizing metasurface is used to depolarize and convert arbitrarily polarized incident light into completely unpolarized light;

[0007] The polarizer metasurface is used to convert the completely unpolarized light into linearly polarized light with a fixed oscillation direction.

[0008] In one embodiment, the depolarizing metasurface is realized by a large number of periodically arranged but randomly rotated rectangular Si nanocolumns etched on a SiO2 substrate.

[0009] In one embodiment, the polarizer of the polarizer metasurface is composed of a periodic metal grating.

[0010] In one embodiment, the polarizer metasurface allows polarized light perpendicular to the grating to pass through, while blocking polarized light parallel to the grating.

[0011] In one embodiment, the incident light with arbitrary polarization is linearly polarized light emitted by a polarized laser emitter and generated after passing through a half-wave plate or a quarter-wave plate.

[0012] A method for polarization conversion of incident light with arbitrary polarization, characterized in that the method includes:

[0013] Preparing a depolarizing metasurface and a polarizer metasurface;

[0014] Integrating the depolarizing metasurface and the polarizer metasurface on the front and back sides of a processing substrate;

[0015] Converting the incident light with arbitrary polarization into completely unpolarized light through the depolarizing metasurface;

[0016] Converting the completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface.

[0017] In one embodiment, it further includes: preparing a depolarizing metasurface by a large number of periodically arranged but randomly rotated rectangular Si nanocolumns etched on a SiO2 substrate.

[0018] In one embodiment, it further includes: preparing a polarizer metasurface from a periodic metal grating.

[0019] In one embodiment, it further includes: before converting the incident light with arbitrary polarization into completely unpolarized light through the depolarizing metasurface, linearly polarized light emitted by a polarized laser emitter is generated into incident light with arbitrary polarization after passing through a half-wave plate or a quarter-wave plate.

[0020] In one embodiment, it further includes: after converting the completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface, the linearly polarized light is generated into incident light with arbitrary polarization after passing through a half-wave plate or a quarter-wave plate.

[0021] The above-mentioned arbitrary polarization conversion metasurface and the method for converting arbitrary polarized light can realize the conversion of arbitrary polarized light by combining a depolarizing metasurface and a polarizer metasurface. The first step is to design a depolarizing metasurface, which is used to depolarize the incident light with arbitrary polarization and convert it into completely unpolarized light, that is, natural light. The second step is to design a polarizer metasurface to convert the natural light generated in the first step into linearly polarized light with a fixed oscillation direction. The metasurface designed in the present invention is simple to process, has a low cost, and has a stable transmission efficiency, and can ensure that a certain fixed polarized light is generated under the condition of incident light with arbitrary polarization. Brief Description of the Drawings

[0022] Figure 1 Schematic diagram of an arbitrary polarization conversion metasurface in an embodiment;

[0023] Figure 2 Schematic diagram of a depolarizing metasurface in an embodiment, where a is a schematic diagram of periodically arranged but randomly rotated rectangular Si nanocolumns, and b is a schematic diagram of the random rotation degree of the nanocolumn array;

[0024] Figure 3 Schematic diagram of a polarizer in an embodiment;

[0025] Figure 4 Schematic diagram of the experimental process for converting arbitrary polarized light into linearly polarized light in an embodiment;

[0026] Figure 5 Schematic diagram of the change trend of normalized power with polarization angle in an embodiment;

[0027] Figure 6 is a scanning electron microscope image of a metasurface sample and the polarization state test results for converting arbitrary polarized light into linearly polarized light in a specific embodiment, where a is the scanning electron microscope image of the depolarizing metasurface, b is the scanning electron microscope image of the polarizer metasurface, and c is the schematic diagram of the change trend of the normalized power with polarization angle for converting arbitrary polarized light into linearly polarized light. Detailed Description of the Embodiments

[0028] In order to make the purpose, technical solutions and advantages of the present application clearer, the following further details the present application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0029] An arbitrary polarization conversion metasurface, characterized in that the metasurface includes: a depolarizing metasurface and a polarizer metasurface; the depolarizing metasurface and the polarizer metasurface are integrated on the front and back sides of the processing substrate;

[0030] The depolarizing metasurface is used to depolarize the incident light with arbitrary polarization and convert it into completely unpolarized light;

[0031] The polarizer metasurface is used to convert completely unpolarized light into linearly polarized light with a fixed oscillation direction.

[0032] In one embodiment, as Figure 1 is a schematic diagram of an arbitrary polarization conversion metasurface. A depolarizing metasurface is fabricated by etching a large number of periodically arranged but randomly rotated rectangular Si nanocolumns on a SiO2 substrate, and a polarizer metasurface is fabricated by a periodic aluminum grating, which are used to remove the polarization characteristics of incident light and re-polarize it respectively. Integrating the depolarizing metasurface and the polarizer metasurface on the front and back sides of a quartz crystal can achieve the integration of the polarization conversion structure. To depolarize any incident light into statistically unpolarized light, the depolarizing metasurface is realized by etching a large number of periodically arranged but randomly rotated rectangular Si nanocolumns on a SiO2 substrate, as Figure 2 (a) shows. For an array of 200×200 cells, the degree of random rotation is represented by Figure 2 (b), and it can be observed that its rotation angle distribution is uniform, meeting the random condition.

[0033] The principle of depolarization of the depolarizing metasurface lies in that by introducing a phase delay of π between the fast axis and the slow axis, each unit cell of the depolarizer can be regarded as a half-wave plate unit. For linearly polarized light with a polarization direction at an angle α relative to the fast axis, the half-wave plate rotates its transmitted polarization direction by an angle of 2α, while for circularly polarized light, it converts the incident light into a polarization state with the opposite sense of rotation, and the phase of the outgoing light is accompanied by a phase delay related to α, similar to the function of the Pancharatnam-Berry (PB) phase metasurface. Considering that any polarization can be decomposed into the superposition of two orthogonal linear polarizations, by constructing randomly arranged silicon nanocolumns, the incident light with any polarization can be converted into a mixed state of random polarization states. To further explain the depolarization principle, the Stokes vector is calculated according to the following formula:

[0034]

[0035] where E x and E y are the electric field strengths along the x-axis and y-axis respectively, and δ is the phase difference between these two fields. I represents the total light intensity, Q and U are the direction and intensity of the linearly polarized components, and V is the intensity of the circularly polarized part. For linearly polarized light, the incident Stokes vector S in can be expressed as

[0036]

[0037] Therefore, the output Stokes vector S out of the light passing through the depolarizer can be obtained by introducing a Mueller matrix M with the following form

[0038]

[0039] where θ n is the rotation angle of the nth nanorod. Due to the randomness of the rotation angle, the output Stokes vector can be calculated as the statistical average of the transmitted light of all units. As a result, when the number of array elements N is large enough, the Stokes vector of the transmitted light becomes:

[0040]

[0041] This equation indicates that the output light is completely unpolarized light, that is, the depolarizer completely removes the polarization information.

[0042] In one embodiment, as Figure 3 shown, the polarizer of the polarizer metasurface is composed of a periodic metal grating. The polarizer metasurface allows the polarized light perpendicular to the grating to pass through, while blocking the polarized light parallel to the grating.

[0043] In one embodiment, the incident light of any polarization is generated by linearly polarized light emitted by a polarized laser emitter and passing through a half-wave plate or a quarter-wave plate.

[0044] A method for polarization conversion of arbitrary polarized light, characterized in that the method includes:

[0045] Preparing a depolarizing metasurface and a polarizer metasurface;

[0046] Integrating the depolarizing metasurface and the polarizer metasurface on the front and back sides of the processing substrate;

[0047] Converting the incident light of any polarization into completely unpolarized light through the depolarizing metasurface;

[0048] Converting the completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface.

[0049] The above-mentioned arbitrary polarization conversion metasurface and the method for polarization conversion of arbitrary polarized light realize the conversion of arbitrary polarized light by combining a depolarizing metasurface and a polarizer metasurface. The first step is to design a depolarizing metasurface, whose function is to depolarize the incident light of any polarization and convert it into completely unpolarized light, that is, natural light. The second step is to design a polarizer metasurface to convert the natural light generated in the first step into linearly polarized light with a fixed oscillation direction. The metasurface designed by the present invention is simple to process, has a low cost, and has a stable transmission efficiency, and can ensure the generation of a certain fixed polarized light under the condition of incident light of any polarization.

[0050] In one embodiment, it further includes: preparing a depolarizing metasurface by etching a large number of periodically arranged but randomly rotated rectangular Si nanorods on a SiO2 substrate.

[0051] In one embodiment, it further includes: preparing a polarizer metasurface from a periodic metal grating.

[0052] In one embodiment, it further includes: after converting completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface, generating output light with arbitrary polarization after passing the linearly polarized light through a half-wave plate or a quarter-wave plate.

[0053] After converting arbitrarily polarized incident light into linearly polarized light with a fixed oscillation direction, if other forms of polarized light are needed, a half-wave plate and a quarter-wave plate can be placed at the output end to achieve the generation of arbitrarily polarized light.

[0054] In a specific embodiment, as Figure 4 shown, it is a schematic diagram of the experimental process for converting arbitrarily polarized light into linearly polarized light. The polarized laser emitter emits linearly polarized light, and vertically polarized light is generated after polarization splitting. Using a half-wave plate or a quarter-wave plate, the linearly polarized light can be converted into arbitrarily polarized light, such as linearly polarized light with different polarization directions, elliptically polarized light, and circularly polarized light. The converted light is focused by a lens and then irradiated onto the metasurface designed by the present invention, and the output light is stable vertically polarized light. By rotating the half-wave plate or the quarter-wave plate, observe and record the output optical power of the photodetector to judge the polarization characteristics of the output light.

[0055] Figure 5 are the transmission curves of linearly polarized light with different polarization directions generated by a half-wave plate after passing through a linear polarizer. The transmission power varies with the polarization angle θ p showing a cos 2 θ p variation trend, where θ p is the polarization angle of the polarized light.

[0056] Figure 6 shows the SEM images of the metasurface sample processed using the principle of the present invention (Figure 6(a), Figure 6(b)) and the test results (6(c)). First, using a commercial linear polarizer as a reference, the measured extinction ratio reached 350:1. Then, this polarizer was replaced with the polarizer designed in the present invention, and the test was carried out using the same method. The extinction ratio was about 15:1, showing a large deviation from the designed value. The main reasons for the low extinction ratio of the designed polarizer are as follows: 1) processing errors; 2) the prepared area is small, and the focusing performance of the laser beam is not good, resulting in both the main polarization component and the cross-polarization component being able to transmit from around the sample, increasing the overall power and thus reducing the extinction performance. Using a commercial polarizer in combination with a depolarizing metasurface for testing, the extinction ratio decreased from 350:1 to about 5:1, indicating that the depolarizing metasurface effectively depolarized the incident linearly polarized light and reduced the degree of polarization. However, the center of the transmitted power fluctuation was about 0.2, showing a certain gap from the theoretical value of 0.5. This is because there are errors in the processing dimensions and the photoresist removal is not thorough, resulting in enhanced reflection and absorption and reduced transmittance. The extinction ratio of the polarization conversion metasurface was further reduced to about 1.7:1 because the designed linear polarizer had a low extinction ratio and a poor blocking effect on horizontally polarized light, resulting in the power received by the power meter being greater than the combination of the depolarizing metasurface and the commercial linear polarizer. In the vertical polarization direction, since the transmittance of the designed linear polarizer was lower than that of the commercial linear polarizer, the transmitted power decreased. The experimental results show that most of the polarized light in any incident polarization direction was converted into the set linearly polarized light, proving the correctness of the principle of the present invention.

[0057] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0058] The above-described embodiments merely represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. An arbitrary polarization conversion metasurface, characterized in that, The metasurface includes: a depolarizing metasurface and a polarizer metasurface; the depolarizing metasurface and the polarizer metasurface are integrated on the front and back sides of a processing substrate; The depolarizing metasurface is used to depolarize incident light of any polarization into completely unpolarized light; the depolarizing metasurface is fabricated by etching a large number of periodically arranged but randomly rotated rectangular Si nanocolumns on a SiO2 substrate; The polarizer metasurface is used to convert the completely unpolarized light into linearly polarized light with a fixed oscillation direction; the polarizer of the polarizer metasurface is composed of a periodic aluminum grating; the Stokes vector of the transmitted light output by the depolarizing metasurface is: where M is the Mueller matrix, S in is the incident Stokes vector, N is the number of array elements on the depolarizing metasurface, θ n is the rotation angle of the nth nanorod, and α is the angular magnitude of the polarization direction of the linearly polarized light relative to the fast axis angle.

2. The metasurface according to claim 1, characterized in that, The polarizer metasurface allows the polarization of light perpendicular to the grating to pass through, while blocking the polarization of light parallel to the grating.

3. The metasurface according to claim 1, characterized in that, The incident light of any polarization is linearly polarized light emitted by a polarized laser emitter and generated after passing through a half-wave plate or a quarter-wave plate.

4. A method for polarizing an arbitrary polarized light, characterized in that, The method includes: Fabricating a depolarizing metasurface and a polarizer metasurface; Integrating the depolarizing metasurface and the polarizer metasurface on the front and back sides of a processing substrate; the depolarizing metasurface is fabricated by etching a large number of periodically arranged but randomly rotated rectangular Si nanocolumns on a SiO2 substrate; Depolarizing incident light of any polarization into completely unpolarized light through the depolarizing metasurface; Converting the completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface; fabricating the polarizer metasurface includes: Fabricating the polarizer metasurface from a periodic aluminum grating; the Stokes vector of the transmitted light output by the depolarizing metasurface is: where M is the Mueller matrix, S in is the incident Stokes vector, N is the number of array elements on the depolarizing metasurface, θ n is the rotation angle of the n-th nanorod, and α is the angular magnitude of the polarization direction of the linearly polarized light with respect to the fast axis angle.

5. The method according to claim 4, characterized in that, Before depolarizing incident light of any polarization into completely unpolarized light through the depolarizing metasurface, it further includes: Generating incident light of any polarization by passing the linearly polarized light emitted by a polarized laser emitter through a half-wave plate or a quarter-wave plate.

6. The method according to claim 4, characterized in that, After converting the completely unpolarized light into linearly polarized light with a fixed oscillation direction through the polarizer metasurface, it further includes: Generating output light of any polarization by passing the linearly polarized light through a half-wave plate or a quarter-wave plate.

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