A method for designing a metasurface for expanding beam deflection and a preparation method thereof

Abstract

The application discloses a metasurface design method for expanding beam deflection and a preparation method thereof, and relates to the technical field of integrated photonic devices. The method comprises the following steps: simulating the light propagation trajectory by using a ray tracing method to obtain an initial light mapping position; setting a target constraint of light beam emission; the target constraint is that the light beam deflection angle is expanded by 3 times, and the light beam divergence angle amplification multiple is less than 3 times; according to the emission target, the polynomial coefficients in the metasurface phase formula are calculated by using a damped least square method and an evaluation function; the target phase of the metasurface structure is determined according to the polynomial coefficients in the metasurface phase formula, and the final metasurface unit structure pattern is determined according to the phase distribution at different positions of the metasurface. The application can realize the expansion of the beam turning angle while reducing the deterioration of the divergence angle.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of integrated photonics, in particular to a method for designing a metasurface for expanding beam deflection and a method for preparing the same. BACKGROUND

[0002] With the development of laser technology and beam manipulation technology, beam steering has played an important role in many fields, such as laser scanning, optical imaging, laser radar, etc. The main method currently used to expand the beam steering angle is to use a lens assembly composed of a pair of aspherical concave and convex lenses, which has the problems of being bulky, large in volume and not conducive to integration. In recent years, as a new type of optical device, metasurfaces can control the propagation direction of light waves at the nanoscale through precise design of microstructures. Using metasurfaces as a replacement for traditional lenses is a feasible method, but most of the current research on using metasurfaces to expand the beam steering range uses a double-layer metasurface structure to simulate a lens assembly. The transmittance of the double-layer metasurface structure is lower than that of a single-layer structure, and the precise alignment of the upper and lower metasurface structures is a challenge for process processing. At the same time, in the current research on expanding the beam steering range, the beam divergence angle after the expansion of the steering range will expand by the same multiple, which will affect the beam quality. SUMMARY

[0003] The purpose of the present application is to provide a method for designing a metasurface for expanding beam deflection and a method for preparing the same, which can achieve the expansion of the beam steering angle while reducing the deterioration of the divergence angle.

[0004] To achieve the above-mentioned purpose, the present application provides the following solutions:

[0005] A method for designing a metasurface for expanding beam deflection, comprising:

[0006] Simulating the trajectory of light propagation using ray tracing method to obtain the initial light mapping position;

[0007] Setting a target constraint for the light beam exit; the target constraint is to expand the beam deflection angle by 3 times while the beam divergence angle is expanded by less than 3 times;

[0008] According to the exit target, the polynomial coefficients in the metasurface phase formula are calculated using the damped least squares method and the evaluation function;

[0009] The target phase of the metasurface structure is determined according to the polynomial coefficients in the metasurface phase formula, and the final metasurface unit structure pattern is determined according to the phase distribution at different positions of the metasurface.

[0010] Optionally, the step of simulating the trajectory of light propagation using ray tracing method to obtain the initial light mapping position specifically comprises:

[0011] Construct a light beam transmission structure;

[0012] Based on the light beam transmission structure, the light propagation trajectory is simulated by using the ray tracing method to obtain an initial light ray mapping position.

[0013] Optionally, the construction process of the light beam transmission structure is:

[0014] A first surface, a second surface, a third surface, a fourth surface and a fifth surface are sequentially placed on the light beam transmission path; wherein the incident light is emitted from the first surface, a multiple structure group is inserted at the second surface, including three plane mirrors rotating at different angles around the center, the mirrors reflect the incident light to different positions of the next surface by rotating at different angles, forming a one-dimensional linear scanning, and the light is emitted to the fifth surface after being regulated by the third surface and the fourth surface.

[0015] Optionally, the polynomial coefficients in the super surface phase formula specifically include:

[0016]

[0017] Wherein, Φ is the target phase, M is the diffraction order, N is the number of polynomial coefficients, ρ is the normalized polar coordinate aperture coordinate, A i is the coefficient of the 2i power of ρ.

[0018] The application also provides a super surface preparation method for expanding light beam deflection, based on the super surface unit structure pattern obtained by the above super surface design method, comprising:

[0019] A silicon substrate is obtained, and ultrasonic cleaning is performed using acetone, isopropyl alcohol and deionized water;

[0020] After the cleaned and dried silicon substrate, a spin coater is used to spin the positive photoresist at a speed of 5000 rpm / s for 65 s, and then the sample is placed on a hot plate at 115 DEG C for 1 min pre-baking;

[0021] After the super surface unit structure pattern is transferred to the photoresist by laser direct writing technology, it is developed in the developing solution for 40 s, and after the development is completed, the sample is placed on a hot plate at 110 DEG C for 3 min post-baking;

[0022] A layer of 50 nm thick Cr is evaporated on the surface of the sample using electron beam evaporation as a hard mask for subsequent ICP etching, and the sample with evaporated Cr material is placed in an acetone solution for stripping, and the photoresist is stripped to leave a patterned Cr mask on the surface of the silicon substrate;

[0023] The silicon substrate is etched by using an inductively coupled plasma etching technology, a pseudo-Bosch process of passivation and etching is carried out, SF6 and C4F8 are mixed and input into a chamber for etching, and after etching, the nano pillar array is transferred to a metasurface material silicon;

[0024] Finally, the silicon nano pillar is removed from the surface of the Cr in the cerium ammonium nitrate solution for 10 min, then the sample is cleaned to remove the impurities on the surface of the sample, and the preparation of the metasurface is completed.

[0025] Optionally, the photoresist is S1805 positive photoresist.

[0026] Optionally, the developer is RZX3080.

[0027] According to the specific embodiments of the present application, the following technical effects are disclosed:

[0028] The application discloses a metasurface design method for expanding light beam deflection and a preparation method thereof, and the method comprises the following steps: simulating a light ray propagation track by using a light ray tracing method to obtain an initial light ray mapping position; setting a target constraint of light beam emission; the target constraint is that the light beam deflection angle is expanded by 3 times while the light beam divergence angle amplification multiple is less than 3 times; calculating polynomial coefficients in a metasurface phase formula by using a damped least square method and an evaluation function according to the emission target; determining a target phase of a metasurface structure according to the polynomial coefficients in the metasurface phase formula, and determining a final metasurface unit structure pattern according to the phase distribution at different positions of the metasurface. The application can expand the light beam deflection angle while reducing the deterioration of the divergence angle. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative labor.

[0030] Figure 1 It is a metasurface function diagram in the embodiment;

[0031] Figure 2 It is a schematic diagram of simulating the light ray propagation track of the optical metasurface in the present application by using the light ray tracing method in the embodiment;

[0032] Figure 3 It is a metasurface diagram in the embodiment; wherein, (a) part is a schematic diagram of a periodic nano pillar array with the same height, (b) part is a schematic diagram of the relationship between the unit structure radius and the transmittance and the phase, and (c) part is a schematic diagram of the final unit structure distribution.

[0033] Figure 4 Fig. 2 shows the simulated two-dimensional far-field distribution diagram at 0°, 10° and 20° incident angles in this embodiment; wherein, part (a) is the simulated two-dimensional far-field distribution diagram at 0° incident angle, part (b) is the simulated two-dimensional far-field distribution diagram at 10° incident angle, and part (c) is the simulated two-dimensional far-field distribution diagram at 20° incident angle;

[0034] Figure 5 Fig. 3 shows the relationship diagram of normalized far-field light intensity and angle corresponding to 0°, 10° and 20° incident angles in this embodiment; wherein, part (a) is the relationship diagram of normalized far-field light intensity and angle corresponding to 0° incident angle, part (b) is the relationship diagram of normalized far-field light intensity and angle corresponding to 10° incident angle, and part (c) is the relationship diagram of normalized far-field light intensity and angle corresponding to 20° incident angle.

[0035] Fig. 1 shows the schematic diagram of the super surface structure according to the present application. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the 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 skilled in the art without creative labor fall within the scope of the present application.

[0037] The purpose of the present application is to provide a super surface design method for expanding light beam deflection and a preparation method thereof, which can achieve the expansion of light beam turning angle while reducing the deterioration of divergence angle.

[0038] In order to make the above-mentioned purposes, characteristics and advantages of the present application more obvious and easy to understand, the present application will be further described in detail below with reference to the drawings and specific embodiments.

[0039] As shown in Figure 1 The present application provides a super surface design method for expanding light beam deflection, which can use a single-layer super surface structure to expand the incident light deflection angle by a certain multiple. On the one hand, light beam control technology needs a large field of view to expand the field of view of optical instruments, and on the other hand, it needs high resolution to increase the accuracy and precision of measurement. The embodiments of the present application constrain the values of the deflection angle and the divergence angle of the outgoing light after passing through the super surface to obtain the phase distribution of the super surface, which can achieve the expansion of light beam turning angle while reducing the deterioration of divergence angle, and provides a feasible method for realizing wide-angle light beam turning with high quality.

[0040] Specifically,Figure 1 As shown, the incident light incident at a certain angle from the incident surface can expand the deflection angle of the outgoing light by a certain multiple after passing through the super surface. In order to scatter the incident light and modify its propagation properties, the optical super surface includes a plurality of high refractive index nano structures extending from the surface of the substrate.

[0041] The angle of beam deflection realized by the super surface array satisfies the generalized Snell refraction law with the phase gradient of the super surface, and by designing the unit structure at different positions of the super surface, the function designed by us can be realized. The phase calculation formula of the super surface structure is calculated by polynomial power series expansion:

[0042]

[0043] Wherein, Φ is the target phase, M is the diffraction order, N is the number of polynomial coefficients, ρ is the normalized polar coordinate aperture coordinate, A i is the coefficient of the 2i power polynomial expansion of ρ.

[0044] The coefficients of each polynomial in the super surface phase formula are obtained by optimizing the deflection angle and divergence angle of the outgoing light of the super surface. In this embodiment, ray tracing method and damped least square method are used to optimize the super surface phase. The selected design method is only an example, and other design methods can also be used to design the expanded beam deflection super surface using the same concept. Figure 2 To simulate the light propagation trajectory of the optical super surface in the application using the ray tracing method. A first surface 100, a second surface 101, a third surface 102, a fourth surface 103 and a fifth surface 104 are placed in turn on the beam transmission path; wherein the incident light is emitted from the first surface 100, and a multiple structure group is inserted at the second surface 101, including three plane mirrors rotating at different angles around their own centers. The mirrors reflect the incident light to different positions of the next surface by rotating at different angles, forming a one-dimensional linear scan, which is emitted to the fifth surface 104 after being regulated by the third surface 102 and the fourth surface 103.

[0045] The incident light is emitted from the surface 100, and after being reflected by the surface 101, it is incident on the surface 102 with a certain incident angle. A multiple structure group is inserted at the surface 101, including three plane mirrors rotating at different angles around their own centers. The mirrors reflect the incident light to different positions of the next surface by rotating at different angles, forming a one-dimensional linear scan. The different three beams of light after the surface 101 represent different incident angles, which are 0°, 10° and 20° respectively. In this embodiment, the incident light carries a divergence angle of 1°. The surface 102 is the lower surface of the super surface substrate, the surface 103 is the nano structure surface carrying the phase distribution of the super surface, and the incident light is emitted to the 104 image surface after being regulated by the surface 103. The outcoming angle and divergence angle of the outgoing light can be analyzed at the surface 104.

[0046] The phase distribution of the surface 103 can be obtained by constraining and optimizing the light rays of different angles on the surface 104, and the design goal of the embodiment is to expand the beam deflection angle by 3 times while the beam divergence angle is less than 3 times, in order to achieve this goal, one method that can be used is to use a damped least squares method to optimize the system design, and use evaluation function operators to achieve and evaluate the final goal of optimization. The evaluation function operator for controlling the deflection angle and the divergence angle can be RAID and GBSD, RAID represents the angle between the incident light and the specified surface normal at the wavelength defined wavelength, in degrees; GBSD specifies the tilt Gaussian beam divergence angle of the outgoing light of the surface, in radians.

[0047] According to the design goal, the coefficients A of the polynomial in formula (1) are optimized i The final phase formula expansion is: Φ = 2.104 x 10 2 ρ 2 + 2.439 x 10 6 ρ 4 - 2.096 x 10 8 ρ 6 + 7.679 x 10 9 ρ 8 - 1.283 x 10 11 ρ 10 + 8.067 x 10 11 ρ 12 The evaluation function operator results after optimization are shown in Table 1, from the data, it can be seen that when the light incident to the super surface is 0°, the outgoing light angle is 0°, and the divergence angle of the outgoing light is approximately equal to the divergence angle of the incident light; for the incident angles of 10° and 20°, the outgoing angles are expanded by about 3 times and the divergence angles are basically unchanged, which meets the design goal.

[0048] Table 1 Operator results after optimization according to the design goal

[0049]

[0050] The ray tracing method for designing the super surface cannot directly establish a wavelength-level structure model, but changes the propagation direction of the light by advancing or delaying the phase of the local representation of the surface, which ignores other influences such as efficiency or multi-level diffraction. Therefore, after obtaining the phase distribution of the super surface, the super surface is simulated by using the finite difference time domain method (FDTD). The super surface in the embodiment takes high refractive index Si material as an example, and works at 10 μm wavelength, which is composed of periodic nanocolumn arrays with the same height as Figure 3As shown in (a), the period of the unit structure is set to P = 3.7 μm, the radius of the cylinder ranges from 0.5 to 1.5 μm, and the phase control of the unit structure is 0 to 2π in the case of obtaining the highest transmittance as possible, the relationship between the radius of the unit structure and the transmittance and phase is as shown in (b). Figure 3 As shown in (b), the corresponding height H = 6.9 μm. The radius distribution of the unit structure at different positions of the metasurface is obtained by converting the target phase distribution according to the predetermined relationship between the radius and the phase, and the radius distribution of the unit structure is as shown in (c). Figure 3

[0051] As shown in (a), the corresponding height H = 6.9 μm. The radius distribution of the unit structure at different positions of the metasurface is obtained by converting the target phase distribution according to the predetermined relationship between the radius and the phase, and the radius distribution of the unit structure is as shown in (c). Figure 4 As shown in (a), the corresponding height H = 6.9 μm. The radius distribution of the unit structure at different positions of the metasurface is obtained by converting the target phase distribution according to the predetermined relationship between the radius and the phase, and the radius distribution of the unit structure is as shown in (c). Figure 5 As shown in (a), the corresponding height H = 6.9 μm. The radius distribution of the unit structure at different positions of the metasurface is obtained by converting the target phase distribution according to the predetermined relationship between the radius and the phase, and the radius distribution of the unit structure is as shown in (c). 2 As shown in (a), the corresponding height H = 6.9 μm. The radius distribution of the unit structure at different positions of the metasurface is obtained by converting the target phase distribution according to the predetermined relationship between the radius and the phase, and the radius distribution of the unit structure is as shown in (c).

[0052] Table 2: Simulation results of the metasurface

[0053] Incident angle (°) Deflection angle (°) Divergence angle (°) 0 0 1.152 10 29.484 1.503 20 59.544 2.430

[0054] In the technical solution, a preparation method of the light beam deflection metasurface device is also provided, and the method comprises the following steps:

[0055] The silicon substrate is obtained, and acetone, isopropyl alcohol and deionized water are used for ultrasonic cleaning to improve the adhesion of the photoresist and the substrate.

[0056] The cleaned and dried silicon substrate is spin-coated with a positive photoresist using a spin coater at a speed of 5000 rpm / s for 65 s, and then the sample is placed on a hot plate at 115°C for 1 min of pre-baking.

[0057] After the laser direct writing technology is used to transfer the designed nano-pillar structure pattern to the photoresist, the sample is developed in a developing solution for 40 s, and the radius distribution pattern of the nano-pillar is as shown in (a). Figure 3 After the development is completed, the sample is placed on a hot plate at 110°C for 3 min of post-baking.

[0058] ​A 50nm thick Cr layer is evaporated on the sample surface as a hard mask for subsequent ICP etching. The sample with the evaporated Cr material is placed in an acetone solution for stripping. After the photoresist is stripped, a patterned Cr mask is left on the silicon substrate surface;

[0059] The silicon substrate is etched using an inductively coupled plasma etching technique. The traditional Bosch process alternates between etching and passivation steps periodically to obtain an etching structure with a high aspect ratio and good verticality. However, the etching rate of the bottom and sidewall is different when etching and passivation are alternated, which can easily form a wavy sidewall. To avoid this problem, the pseudo-Bosch process is used in this embodiment, in which SF6 and C4F8 are mixed and introduced into the chamber for etching. The effects of ICP power, RF power, and gas flow ratio on silicon etching are investigated.

[0060] In the experiment, other parameters are fixed, and the ICP power is changed from 300W to 800W. When the ICP power increases from 300W to 500W, the etching rate increases and the sidewall angle decreases. This is because the electron collision accelerates with the increase of ICP power, which promotes the ionization of etching gas and accelerates the etching process. When the ICP power continues to increase, the etching rate no longer increases or even decreases, and the etching quality deteriorates. This is because high ICP power promotes the ionization of passivation gas, forming a passivation film that hinders the chemical etching reaction. When the RF power increases from 30W to 70W, the etching rate gradually increases until the RF power increases to 50W. When the RF power is greater than 50W, the etching rate decreases and the sidewall angle increases.

[0061] Observing the etching results, it can be found that the lateral etching under the metal mask is serious. This is because the positive charge in the metal mask layer causes CF x + The deflection causes the passivation layer under the top sidewall to thin, which increases the lateral etching in this area. When other parameters are kept constant, increasing the flow rate of C4F8 gas weakens the lateral etching under the metal mask, indicating that this defect is mainly affected by the thickness of the passivation layer. After ICP etching, the nanorod array is transferred to the metasurface material silicon;

[0062] Finally, the Cr on the surface of the silicon nanorod is removed by wet etching in a cerium ammonium nitrate solution for 10 minutes, and then the sample is cleaned to remove impurities on the sample surface, completing the preparation of the metasurface.

[0063] The photoresist used in this embodiment is S1805 positive photoresist, and the developer is RZX3080. It should be noted that this is only for reference and cannot be understood as a limitation on the scheme. Other photoresists and corresponding developers can also be used.

[0064] The various embodiments described in this specification are intended to be exemplary only. The various embodiments were chosen and described in order to best explain the principles of the application and its practical application, to thereby enable others skilled in the art to best utilize the application, various embodiments with various modifications as are suited to the particular use contemplated.

[0065] The principles and implementations of the present application have been described in the specification with reference to specific examples. The above description is only used to help understand the core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation and application range will be changed. In view of the above, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A metasurface design method for amplifying beam deflection, characterized in that, include: The initial ray mapping position is obtained by simulating the ray propagation trajectory using ray tracing. Set target constraints for beam emission; The target constraint is to increase the beam deflection angle by a factor of 3 while the beam divergence angle amplification factor is less than a factor of 3. Based on the launch target, the polynomial coefficients in the metasurface phase formula are calculated using the damped least squares method and the evaluation function. The target phase of the metasurface structure is determined based on the polynomial coefficients in the metasurface phase formula, and the final metasurface unit structure pattern is determined based on the phase distribution at different locations on the metasurface. The method of simulating the ray propagation trajectory of light to obtain the initial ray mapping position specifically includes: Construct a beam transmission structure; Based on the beam transmission structure, the initial ray mapping position is obtained by simulating the ray propagation trajectory using the ray tracing method. The construction process of the beam transmission structure is as follows: A first surface (100), a second surface (101), a third surface (102), a fourth surface (103), and a fifth surface (104) are placed sequentially along the beam transmission path. The incident light exits from the first surface (100), and a multi-structure group is inserted at the second surface (101), which includes three plane mirrors that rotate around their own center at different angles. By rotating at different angles, the mirrors reflect the incident light to different positions on the next surface, forming a one-dimensional linear scan. After being controlled by the third surface (102) and the fourth surface (103), the light exits to the fifth surface (104). The polynomial coefficients in the metasurface phase formula specifically include: ， in, Φ It is the target phase. M It is a diffraction order. N It is the number of polynomial coefficients. ρ These are normalized polar aperture coordinates. A i yes ρ 2 i The coefficients of the expansion of the power term.

2. A method for fabricating a metasurface to enhance beam deflection, based on a metasurface unit structure pattern obtained by the metasurface design method described in claim 1, characterized in that, include: A silicon substrate was obtained and ultrasonically cleaned using acetone, isopropanol, and deionized water. Positive photoresist was spin-coated onto the cleaned and dried silicon substrate using a spin coater at a speed of 5000 rpm / s for 65 s. The sample was then placed on a hot plate at 115 ℃ for 1 min of pre-baking. After transferring the metasurface unit structure pattern onto the photoresist using laser direct writing technology, the sample was developed in the developer for 40 seconds. After development, the sample was placed on a hot plate at 110 °C and baked for 3 minutes. A 50 nm thick Cr layer was deposited on the sample surface using electron beam evaporation as a hard mask for subsequent ICP etching. The sample with the Cr material deposited was placed in an acetone solution for stripping. After the photoresist was stripped, a patterned Cr mask was left on the silicon substrate surface. The silicon substrate was etched using inductively coupled plasma etching technology. A pseudo-Bosch process was used to simultaneously passivate and etch. SF6 and C4F8 were mixed and introduced into the chamber for etching. After etching, the nanopillar array was transferred onto the metasurface material silicon. Finally, the Cr on the surface of the silicon nanopillars was removed by wet etching in cerium ammonium nitrate solution for 10 min. Then, the sample was cleaned to remove impurities from the sample surface, thus completing the preparation of the metasurface.

3. The method for preparing a metasurface to amplify beam deflection according to claim 2, characterized in that, The photoresist used is S1805 positive photoresist.

4. The method for preparing a metasurface to amplify beam deflection according to claim 2, characterized in that, The developer used is RZX3080.

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