Metasurface design method for expanding light beam deflection and preparation method thereof

Abstract

The invention discloses a metasurface design method for expanding light beam deflection and a preparation method thereof, and relates to the technical field of integrated photonic devices. The method comprises the following steps: simulating a 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 divergence angle magnification factor is smaller than 3 times while the light beam deflection angle is enlarged by 3 times; calculating a polynomial coefficient in a metasurface phase formula by using a damping least square method and an evaluation function according to the emergent target; and determining a target phase of the metasurface structure according to a polynomial coefficient in the metasurface phase formula, and determining a final metasurface unit structure pattern according to phase distribution at different positions of the metasurface. According to the invention, deterioration of a divergence angle can be reduced while a light beam steering angle is expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated photon devices, and particularly to a method for designing a metasurface for expanding beam deflection and a preparation method thereof. Background Art

[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, lidar, etc. At present, the main method for expanding the beam steering angle is to use a lens assembly composed of a pair of aspherical concave lenses and convex lenses, which has problems such as heavy components, large volume, and being not conducive to integration. In recent years, as a new type of optical device, the metasurface can control the propagation direction of light waves at the nanoscale through precise design of microscopic structures. Using the metasurface as a substitute for traditional lenses is a feasible method, but most of the current research on using the metasurface to expand the beam steering range uses a double-layer metasurface structure to imitate the lens assembly. The transmittance of the double-layer metasurface structure is lower than that of the single-layer structure, and the precise alignment of the upper and lower layer metasurface structures is a challenge for the processing technology. At the same time, in the current research on expanding the beam steering range, the beam divergence angle after the steering range is expanded will increase by the same multiple, which will affect the beam quality. Summary of the Invention

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

[0004] To achieve the above purpose, the present invention provides the following scheme:

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

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

[0007] Setting the target constraint for the beam output; the target constraint is that while expanding the beam deflection angle by 3 times, the magnification of the beam divergence angle is less than 3 times;

[0008] According to the output target, calculating the polynomial coefficients in the metasurface phase formula by using the damped least squares method and the evaluation function;

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

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

[0011] Construct a beam transmission structure;

[0012] Based on the beam transmission structure, use the ray tracing method to simulate the ray propagation trajectory and obtain the initial ray mapping position.

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

[0014] Place the first, second, third, fourth, and fifth surfaces in sequence on the beam transmission path; among them, the incident light exits from the first surface, and a multiple structure group is inserted at the second surface, including three planar mirrors that rotate different angles around their own centers. By rotating different angles, the incident light is reflected to different positions on the subsequent surface, forming a one-dimensional linear scan, and then exits to the fifth surface after being regulated by the third and fourth surfaces.

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

[0016]

[0017] Among them, 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, and A i is the coefficient of the polynomial expansion of the 2i-th power of ρ.

[0018] The present invention also provides a method for fabricating a metasurface for expanding beam deflection. Based on the metasurface unit structure pattern obtained by the above metasurface design method, it includes:

[0019] Obtain a silicon substrate and perform ultrasonic cleaning using acetone, isopropyl alcohol, and deionized water;

[0020] Spin-coat a positive photoresist on the cleaned and dried silicon substrate using a spin coater at a speed of 5000 rpm / s for a spin coating time of 65 s, and then place the sample on a hot plate at 115 °C for 1 min of pre-baking;

[0021] Transfer the metasurface unit structure pattern to the photoresist using laser direct writing technology and then develop it in the developer for 40 s. After the development is completed, place the sample on a hot plate at 110 °C for 3 min of post-baking;

[0022] Use electron beam evaporation to deposit a 50-nm-thick Cr layer on the sample surface as a hard mask for subsequent ICP etching. Place the sample deposited with Cr material in an acetone solution for stripping. After the photoresist is stripped, a patterned Cr mask is left on the silicon substrate surface;

[0023] Use inductively coupled plasma etching technology to etch the silicon substrate, adopt the pseudo Bosch process of simultaneous passivation and etching, and use SF 6 and C4 F 8 Mix and introduce them into the chamber for etching. After etching, the nanocolumn array is transferred onto the superstrate material silicon;

[0024] Finally, wet-etch for 10 min in ammonium cerium nitrate solution to remove the Cr on the upper surface of the silicon nanocolumns, then clean the sample to remove impurities on the sample surface, and complete the preparation of the metasurface.

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

[0026] Optionally, the developer uses RZX3080.

[0027] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0028] The present invention discloses a metasurface design method for expanding beam deflection and its preparation method. The method includes simulating the light propagation trajectory by using the ray tracing method to obtain the initial light mapping position; setting the target constraint for beam emission; the target constraint is to expand the beam deflection angle by 3 times while the magnification of the beam divergence angle is less than 3 times; according to the emission target, using the damped least squares method and the evaluation function to calculate the polynomial coefficients in the metasurface phase formula; determining the target phase of the metasurface structure according to the polynomial coefficients in the metasurface phase formula, and determining the final metasurface unit structure pattern according to the phase distribution at different positions of the metasurface. The present invention can achieve reducing the deterioration of the divergence angle while expanding the beam steering angle. Description of the Drawings

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

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

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

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

[0033] Figure 4 This is the two-dimensional far-field distribution diagram obtained by simulation at incident angles of 0°, 10°, and 20° in this embodiment; among them, part (a) is the two-dimensional far-field distribution diagram obtained by simulation at an incident angle of 0°, part (b) is the two-dimensional far-field distribution diagram obtained by simulation at an incident angle of 10°, and part (c) is the two-dimensional far-field distribution diagram obtained by simulation at an incident angle of 20°.

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

[0035] Reference numerals: 100, the first surface; 101, the second surface; 102, the third surface; 103, the fourth surface; 104, the fifth surface. Detailed implementation manners

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] The purpose of the present invention is to provide a metasurface design method for expanding beam deflection and its preparation method, which can achieve the expansion of the beam steering angle while reducing the deterioration of the divergence angle.

[0038] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0039] As Figure 1 shown, the present invention provides a metasurface design method for expanding beam deflection, which can use a single-layer metasurface structure to expand the incident light deflection angle by a certain multiple. Beam control technology requires a large field of view angle to expand the viewing range of optical instruments on the one hand, and high resolution to increase the accuracy and precision of measurements on the other hand. In the embodiments of the present invention, by simultaneously constraining the numerical values of the deflection angle and the divergence angle of the outgoing light passing through the metasurface, the phase distribution of the metasurface is obtained, realizing the expansion of the beam steering angle while reducing the deterioration of the divergence angle, and providing a feasible method for realizing high-quality wide-angle beam steering.

[0040] Specifically, as Figure 1As shown, the incident light incident at a certain angle from the incident surface can have its deflection angle enlarged by a certain multiple after passing through the metasurface. To scatter the incident light and modify its propagation properties, the optical metasurface includes a plurality of high-refractive-index nanostructures extending from the substrate surface.

[0041] The angle of beam deflection achieved by the metasurface array and the phase gradient of the metasurface satisfy the generalized Snell's law of refraction. By designing the unit structures at different positions of the metasurface, the functions we designed can be achieved. The phase calculation formula of the metasurface structure is calculated by expanding in polynomial power series:

[0042]

[0043] where 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, and A i is the coefficient of the polynomial expansion of the 2i-th power of ρ.

[0044] The coefficients of each polynomial in the metasurface phase formula are obtained by optimizing the deflection angle and divergence angle of the light emerging from the metasurface. In this embodiment, the ray tracing method and the damped least squares method are used as examples to optimize the metasurface phase. The selected design method is only an example, and other design methods can also be used with the same concept to design the metasurface for expanding beam deflection. Figure 2 To simulate the light propagation trajectory of the optical metasurface in the present invention 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 sequentially placed on the light beam transmission path; among them, the incident light exits from the first surface 100, and a multi-structure group is inserted at the second surface 101, including three plane mirrors rotating at different angles around their own centers. By rotating at different angles, the incident light is reflected to different positions on the subsequent surface, forming a one-dimensional linear scan, and exits to the fifth surface 104 after being regulated by the third surface 102 and the fourth surface 103.

[0045] The incident light exits from surface 100 and is incident on surface 102 with a certain incident angle after being reflected by surface 101. A multi-structure group is inserted at surface 101, including three plane mirrors rotating at different angles around their own centers. By rotating at different angles, the incident light is reflected to different positions on the subsequent surface, forming a one-dimensional linear scan. Three different beams of light behind surface 101 represent different incident angles, which are 0°, 10°, and 20° respectively. In this embodiment, the divergence angle carried by the incident light is 1°. Surface 102 is the lower surface of the metasurface substrate, surface 103 is the nanostructure surface of the metasurface carrying the phase distribution, and the incident light exits to the 104 image surface after being regulated by surface 103. The exit angle and divergence angle of the exit light can be analyzed on surface 104.

[0046] Constraining and optimizing the light at different angles on the opposite surface 104 can obtain the phase distribution of surface 103. The design goal of this embodiment is to triple the beam deflection angle while the magnification factor of the beam divergence angle is less than 3 times. To achieve this goal, one method that can be used is: using the damped least squares method to optimize the system design and using the evaluation function operand to achieve and evaluate the final goal of optimization. The evaluation function operands for controlling the deflection angle and divergence angle can be RAID and GBSD. RAID represents the angle between the incident light and the specified surface normal at the wavelength defined by the wavelength, with the unit of degree; GBSD specifies the tilted Gaussian beam divergence angle of the light emerging from the specified surface, with the unit of radian.

[0047] After optimization according to the design goal, the coefficient A of the polynomial in formula (1) is obtained i , and the expanded form of the final phase formula is: Φ = 2.104×10 2 ρ 2 +2.439×10 6 ρ 4 -2.096×10 8 ρ 6 +7.679×10 9 ρ 8 -1.283×10 11 ρ 10 +8.067×10 11 ρ 12 , and the results of the optimized evaluation function operands are shown in Table 1. It can be seen from the data that when the light incident on the metasurface is 0°, the outgoing light angle is 0°, and the divergence angle of the outgoing light is approximately equal to that of the incident light; for the incident angles of 10° and 20°, the outgoing angles are both expanded by about 3 times and the divergence angles remain basically unchanged, meeting the design goal.

[0048] Table 1 Results of operands optimized according to the design goal

[0049]

[0050] Designing a metasurface using the ray tracing method cannot directly establish a wavelength-scale structural model, but rather changes the propagation direction of light by advancing or delaying the phase of the local representation of the surface, which ignores other effects such as efficiency or multi-level diffraction. Therefore, after obtaining the phase distribution of the metasurface, the finite-difference time-domain method (FDTD) is used to perform 3D simulation on the metasurface. In this embodiment, the metasurface takes high-refractive-index Si material as an example and operates in the 10-μm band, and is composed of a periodic nano-pillar array with the same height as Figure 3As shown in (a), considering the wavelength and the Nyquist sampling law, 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 unit structure is enabled to obtain a phase control of 0 to 2π while achieving as high a transmittance as possible. The relationship between the radius of the unit structure and the transmittance and phase is as Figure 3 shown in (b), and the corresponding height H = 6.9 μm. The radius distribution of the unit structures at different positions of the metasurface is obtained by conversion based on the target phase distribution according to the pre-determined relationship between the radius and the phase. The radius distribution of the unit structures is as Figure 3 shown in (c).

[0051] As Figure 4 shown, the two-dimensional far-field distribution maps obtained by simulation at incident angles of 0°, 10°, and 20° are respectively presented. The corresponding deflection angles of the outgoing light are: 0°, 29.484°, and 59.544°. And Figure 5 is the relationship diagram between the normalized far-field light intensity and the angle corresponding to these three incident angles. From this, the divergence angle value of the outgoing light can be calculated (calculated by the beam diameter at the position where the light intensity drops to 1 / e of the central light intensity). Table 2 lists the specific values of the deflection angle and the divergence angle. Numerically, it can be seen that the enhanced beam-steering metasurface designed using the ray-tracing method can amplify the incident angle by 3 times and the amplification factor of the divergence angle is less than 3 times. 2

[0052] Table 2 Metasurface simulation results

[0053] Angle of incidence (°) Angle of deflection (°) Divergence angle (°) 0 0 1.152 10 29.484 1.503 20 59.544 2.430

[0054] In this technical solution, a method for fabricating an enlarged beam-deflecting metasurface device is also provided, which is characterized by including:

[0055] Obtain a silicon substrate, and perform ultrasonic cleaning using acetone, isopropyl alcohol, and deionized water to improve the adhesion between the photoresist and the substrate;

[0056] Spin-coat a positive photoresist on the cleaned and dried silicon substrate using a spin coater at a rotation speed of 5000 rpm / s for a spin-coating time of 65 s, and then place the sample on a hot plate at 115°C for 1 min of pre-baking;

[0057] Transfer the designed nano-column structure pattern onto the photoresist using laser direct writing technology and then develop it in the developer for 40 s. The nano-column radius distribution pattern is as Figure 3 shown. After the development is completed, place the sample on a hot plate at 110°C for 3 min of post-baking;

[0058] ​A 50-nm-thick Cr layer was deposited on the sample surface by electron beam evaporation as a hard mask for subsequent ICP etching. The sample coated with Cr material was placed in an acetone solution for stripping. After the photoresist was stripped, a patterned Cr mask was left on the surface of the silicon substrate.

[0059] The silicon substrate was etched using inductively coupled plasma etching technology. In the traditional Bosch process, etching and passivation steps were alternately performed periodically to obtain an etching structure with a high aspect ratio and good perpendicularity. However, when etching and passivation were alternately performed, the etching rates at the bottom and sidewalls were different, and wavy sidewalls were easily formed. To avoid this problem, in this embodiment, a pseudo-Bosch process with simultaneous passivation and etching was adopted, and SF 6 and C 4 F 8 were mixed and introduced into the chamber for etching to explore the effects of parameters such as ICP power, RF power, and gas flow ratio on silicon etching.

[0060] During the experiment, other parameters were fixed, and the ICP power was changed from 300 - 800 W. When the ICP power increased from 300 W to 500 W, the etching rate increased and the sidewall inclination angle decreased. This was because electron collisions accelerated with the increase of ICP power, promoting the ionization of the etching gas and accelerating the etching process. When the ICP power continued to increase, the etching rate no longer increased and even decreased, and the etching quality deteriorated because the high ICP power promoted the ionization of the passivation gas, forming a passivation film that hindered the chemical etching reaction. When the RF power was increased from 30 W to 70 W, the etching rate gradually increased until the RF power increased to 50 W. After exceeding 50 W, the etching rate decreased and the sidewall inclination angle increased.

[0061] Observing the etching results, it can be found that severe lateral etching occurred under the metal mask. This was because the positive charges in the metal mask layer caused CF x + to deflect, resulting in a thinner passivation layer on the top sidewall, thereby increasing the lateral etching in this area. Keeping other parameters unchanged and increasing the gas flow rate of C 4 F 8 weakened the lateral etching under the metal mask, indicating that this defect was mainly affected by the thickness of the passivation layer. After ICP etching, the nano-pillar array was transferred to the silicon of the metasurface material;

[0062] Finally, the Cr on the upper surface of the silicon nano-pillars was removed by wet etching in an ammonium cerium nitrate solution for 10 min, and then the sample was cleaned to remove the impurities on the sample surface, completing the preparation of the metasurface.

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

[0064] In this specification, the various embodiments are described in a progressive manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts among the embodiments can be referred to each other.

[0065] Specific examples are used in this article to elaborate on the principles and implementation manners of the present invention. The description of the above embodiments is only for helping to understand the core idea of the present invention; at the same time, for those of ordinary skill in the art, based on the idea of the present invention, there will be changes in the specific implementation manners and application scopes. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A metasurface design method for expanding beam deflection, characterized in that: include: The ray tracing method is used to simulate the light propagation trajectory and obtain the initial light mapping position; Set the target constraints for the beam emission; The target constraint is to increase the beam deflection angle by 3 times while the beam divergence angle is magnified by less than 3 times; According to the emission target, the polynomial coefficients in the metasurface phase formula are calculated using a damped least squares 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.

2. The method for designing a metasurface for expanding beam deflection according to claim 1, characterized in that: The method of simulating the light propagation trajectory by using the ray tracing method to obtain the initial light mapping position specifically includes: Constructing beam delivery structures; Based on the light beam transmission structure, the ray tracing method is used to simulate the light propagation trajectory to obtain the initial light mapping position.

3. The method for designing a metasurface for expanding beam deflection according to claim 2, characterized in that: 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 sequentially placed on a beam transmission path; wherein incident light is emitted from the first surface (100), and a multiple structure group is inserted at the second surface (101), comprising three plane reflectors rotating at different angles around their own centers, and the reflectors reflect the incident light to different positions of the latter surface by rotating at different angles, forming a one-dimensional linear scan, and then emitted to the fifth surface (104) after being regulated by the third surface (102) and the fourth surface (103).

4. The method for designing a metasurface for expanding beam deflection according to claim 1, characterized in that: The polynomial coefficients in the metasurface phase formula specifically include: where Φ is the target phase, M is the diffraction order, N is the number of polynomial coefficients, ρ is the normalized polar aperture coordinate, and A i are the coefficients of the term expansion of the coefficients of ρ raised to the 2ith power.

5. A method for preparing a metasurface for enlarging beam deflection, based on a metasurface unit structure pattern obtained by the metasurface design method according to any one of claims 1 to 4, characterized in that: include: Obtain a silicon substrate and perform ultrasonic cleaning using acetone, isopropanol, and deionized water; The cleaned and dried silicon substrate was spin-coated with positive photoresist using a coating machine at a speed of 5000 rpm / s for 65 s, and then the sample was placed on a hot plate at 115°C for 1 min pre-baking; The metasurface unit structure pattern was transferred to the photoresist using laser direct writing technology and then developed in a developer for 40 seconds. After the development was completed, the sample was placed on a hot plate at 110°C and baked for 3 minutes. A 50nm thick Cr layer was deposited on the sample surface by 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 surface of the silicon substrate. The silicon substrate is etched using inductively coupled plasma etching technology. A pseudo-Bosch process in which passivation and etching are performed simultaneously is used. A mixture of SF6 and C4F8 is passed into the chamber for etching. After etching, the nanopillar array is transferred to the metasurface material silicon. Finally, the Cr on the surface of the silicon nanorods was removed by wet etching in ammonium cerium nitrate solution for 10 min. Then the sample was cleaned to remove impurities on the surface of the sample to complete the preparation of the supersurface.

6. The method for preparing a metasurface for enlarging beam deflection according to claim 5, characterized in that: The photoresist is S1805 positive photoresist.

7. The method for preparing a metasurface for enlarging beam deflection according to claim 5, characterized in that: The developer is RZX3080.

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