Terahertz dielectric folded plane metasurface optical collimator and application

Abstract

The invention discloses a terahertz dielectric folded plane metasurface optical collimator and an application thereof. The optical collimator comprises a top plane metasurface and a bottom plane metasurface which are arranged in parallel and are made of dielectric materials. A first spacing distance is formed between the top plane metasurface and the bottom plane metasurface; the top plane metasurface comprises two grating structures with a second spacing distance, and the grating structures are used for selecting expected linear polarization terahertz waves to pass through; the bottom plane metasurface comprises a bottom surface supporting plate and a plurality of rectangular anisotropic dielectric resonators arranged on the bottom surface supporting plate; the plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the transverse axis direction and the longitudinal axis direction; and a terahertz source is arranged at the center of the bottom surface supporting plate.

Description

Technical Field

[0001] The present invention belongs to the field of micro-nano optical control technology, and in particular relates to a terahertz dielectric folded planar metasurface optical collimator and its application. Background Art

[0002] Terahertz (THz) waves have attractive properties such as wide bandwidth, non-ionization, fingerprints of various materials, making them attractive for various applications, including ultra-high-speed wireless communications, non-destructive testing, security screening, and medical imaging. However, due to the low output power of THz sources and the high attenuation of THz waves by the atmosphere, efficient THz devices are needed to collimate the beam and minimize the radiated power in unwanted directions. In conventional THz systems, bulky dielectric lenses and reflectors are the two most common THz devices used to collimate the waves from the THz source in the transmitter or to focus the incident waves into the detection module in the receiver.

[0003] Metasurfaces are two-dimensional metamaterials with subwavelength thickness that can flexibly control the phase, amplitude, and polarization characteristics of electromagnetic waves. In recent years, various terahertz devices based on planar metasurfaces have been reported, such as terahertz absorbers, terahertz beam splitters, terahertz polarizers, and terahertz reflectors. Terahertz devices based on planar metasurfaces can replace traditional bulky terahertz devices with their small profile and lightweight advantages. Summary of the invention

[0004] In order to achieve a compact structure of a terahertz device and reduce the propagation space of the terahertz device, and improve the performance of the terahertz device, an embodiment of the present invention provides a terahertz dielectric folded planar metasurface light collimator and application thereof.

[0005] The technical solution adopted by the present invention is:

[0006] In a first aspect, an embodiment of the present invention provides a terahertz dielectric folded planar metasurface light collimator, comprising a top planar metasurface and a bottom planar metasurface composed of a dielectric material arranged in parallel;

[0007] The top planar hypersurface has a first separation distance from the bottom planar hypersurface;

[0008] The top planar metasurface includes two grating structures with a second separation distance for selectively passing a desired linearly polarized terahertz wave;

[0009] The bottom plane metasurface includes a bottom support plate and a plurality of rectangular anisotropic dielectric resonators arranged on the bottom support plate;

[0010] The plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the horizontal axis direction and the vertical axis direction;

[0011] A terahertz source is arranged at the center of the bottom support plate.

[0012] In one or some optional embodiments, a lattice period of the grating structure along a horizontal direction of the outgoing wave is greater than the second spacing distance and greater than a thickness of the grating structure.

[0013] In one or some optional embodiments, the plurality of rectangular anisotropic dielectric resonators are arranged along a terahertz source annular array.

[0014] In one or some optional embodiments, the plurality of rectangular anisotropic dielectric resonators have the same height, and the plurality of rectangular anisotropic dielectric resonators have different lengths and widths.

[0015] In one or some optional embodiments, the top plane metasurface includes two silicon substrates, and the grating structure is arranged in the middle of each silicon substrate;

[0016] The two silicon substrates are buckled together to form a cavity in a region corresponding to the grating structure, and the height of the cavity is equal to the second spacing distance.

[0017] In one or some optional embodiments, a chromium thin film layer and a gold thin film layer are sequentially provided on the bottom surface of the bottom plane superplane;

[0018] The thickness ratio of the chromium thin film layer to the gold thin film layer is 2:200.

[0019] In one or some optional embodiments, the lattice size of the bottom plane metasurface is 100 μm.

[0020] In one or some optional embodiments, the material of the top plane supersurface and the bottom plane supersurface is single crystal silicon.

[0021] In one or some optional embodiments, the top plane supersurface and the bottom plane supersurface are supported and fixed by a bracket.

[0022] In a second aspect, an embodiment of the present invention provides a terahertz imaging device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface light collimator.

[0023] In a third aspect, an embodiment of the present invention provides a wireless terahertz communication device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface optical collimator.

[0024] In a fourth aspect, an embodiment of the present invention provides a terahertz nondestructive testing device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface light collimator.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The terahertz dielectric folded planar metasurface optical collimator provided by the embodiment of the present invention has two planar metasurfaces composed of dielectric material with a first spacing distance and are arranged in parallel, the bottom planar metasurface is used for reflection gain, and the top planar metasurface is used for polarization collimation. The terahertz wave is emitted by the terahertz source, and the terahertz wave that meets the polarization requirements can be emitted from the grating structure of the top planar metasurface, while other terahertz waves that do not meet the polarization requirements will continue to be reflected between the two planar metasurfaces until the polarization meets the requirements. Thus, the divergent terahertz waves emitted by the terahertz source are converted into a group of parallel terahertz waves with consistent polarization directions, which are radiated to the target space. By the way that the terahertz wave is continuously reflected between the two planar metasurfaces, the propagation path of the terahertz wave is folded between the two planar metasurfaces. Compared with the method of propagating through a long propagation distance and then modulating the reflection in the prior art, the free space propagation distance of the terahertz wave is greatly reduced, the propagation space of the terahertz wave is reduced, the volume of the optical collimator is greatly reduced, and the radiation efficiency of the optical collimator is improved. This is not only conducive to the miniaturization of the terahertz system using the optical collimator, but also, since the output power of the terahertz source is much lower than the microwave range, efficient terahertz wave control can also improve the accuracy of the terahertz system.

[0027] The terahertz dielectric folded planar metasurface optical collimator provided in an embodiment of the present invention realizes the function of a Bragg polarizer by setting the top planar metasurface to include two grating structures with a second spacing distance. By adjusting the installation direction of the grating structure of the top planar metasurface, it is possible to select the linearly polarized terahertz wave passing therethrough, thereby obtaining any desired parallel terahertz wave with consistent polarization direction.

[0028] The terahertz dielectric folded planar metasurface optical collimator provided by the embodiment of the present invention is provided with a bottom planar metasurface including a bottom support plate and a plurality of rectangular anisotropic dielectric resonators provided on the bottom support plate, and the phase discontinuity and polarization control are realized by adjusting the plurality of rectangular anisotropic dielectric resonators to have different resonant frequencies along the horizontal axis direction and the longitudinal axis direction, and the u polarization (vertical direction of the outgoing wave) of the terahertz wave emitted by the terahertz source is converted into an orthogonal v polarization (horizontal direction of the outgoing wave), so as to realize the control of polarization characteristics. In addition, by directly integrating the terahertz source into the bottom support plate, the terahertz wave does not need to propagate through free space, but is directly emitted from the bottom planar metasurface, which greatly reduces the free space propagation distance of the terahertz wave and reduces the propagation space of the terahertz wave. At the same time, directly integrating the light source into the bottom support plate can make the structure of the device more compact and the fixed connection more firm.

[0029] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The schematic diagram of the structure of the silicon-based terahertz folded metasurface and the calculation results of the phase distribution required for the bottom plane metasurface;

[0031] Figure 2 Schematic diagram of the design and simulation response results of the top plane metasurface.

[0032] Figure 3 The preparation process and microscope images of the top plane metasurface. (ah) Preparation process of double-layer silicon grating metasurface.

[0033] Figure 4 Design and simulation response diagram of the bottom plane metasurface.

[0034] Figure 5 Fabrication and microscopy images of the bottom metasurface.

[0035] Figure 6 This is an experimental and simulation demonstration diagram of the folding supersurface of the present invention. DETAILED DESCRIPTION

[0036] The inventors have found that although a lot of research has been done on the design and manufacture of planar metasurface terahertz devices, there are still great obstacles to using planar metasurface terahertz devices to significantly reduce the volume of the terahertz system. Because the overall volume of the terahertz system is mainly determined by the propagation space between the terahertz source / receiver and the terahertz device. Although the terahertz device of the planar metasurface can significantly reduce the thickness of traditional geometric lenses and reflectors, it cannot reduce the propagation distance to the terahertz source / receiver. The long propagation distance in free space will cause the terahertz system to be bulky. This actually hinders the practical application of planar metasurface terahertz devices in terahertz systems. To this end, an embodiment of the present invention provides a terahertz dielectric folded planar metasurface optical collimator and application.

[0037] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0038] Embodiment 1

[0039] The embodiment of the present invention provides a terahertz dielectric folded planar metasurface optical collimator, referring to Figures 1 to 3 As shown, it includes a top plane metasurface (also referred to as the top metasurface) and a bottom plane metasurface (also referred to as the bottom metasurface) composed of a dielectric material arranged in parallel;

[0040] The top planar hypersurface has a first separation distance from the bottom planar hypersurface;

[0041] The top planar metasurface includes two grating structures with a second separation distance for selectively passing a desired linearly polarized terahertz wave;

[0042] The bottom plane metasurface includes a bottom support plate and a plurality of rectangular anisotropic dielectric resonators arranged on the bottom support plate;

[0043] The plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the horizontal axis direction and the vertical axis direction;

[0044] A terahertz source (i.e. Figure 1 light source in the image).

[0045] The terahertz dielectric folded planar metasurface optical collimator provided by the embodiment of the present invention has two planar metasurfaces composed of dielectric material with a first spacing distance and are arranged in parallel, the bottom planar metasurface is used for reflection gain, and the top planar metasurface is used for polarization collimation. The terahertz wave is emitted by the terahertz source, and the terahertz wave that meets the polarization requirements can be emitted from the grating structure of the top planar metasurface, while other terahertz waves that do not meet the polarization requirements will continue to be reflected between the two planar metasurfaces until the polarization meets the requirements. Thus, the divergent terahertz waves emitted by the terahertz source are converted into a group of parallel terahertz waves with consistent polarization directions, which are radiated to the target space. By the way that the terahertz wave is continuously reflected between the two planar metasurfaces, the propagation path of the terahertz wave is folded between the two planar metasurfaces. Compared with the method of propagating through a long propagation distance and then modulating the reflection in the prior art, the free space propagation distance of the terahertz wave is greatly reduced, the propagation space of the terahertz wave is reduced, the volume of the optical collimator is greatly reduced, and the radiation efficiency of the optical collimator is improved. This is not only conducive to the miniaturization of the terahertz system using the optical collimator, but also, since the output power of the terahertz source is much lower than the microwave range, efficient terahertz wave control can also improve the accuracy of the terahertz system.

[0046] The terahertz dielectric folded planar metasurface optical collimator provided in an embodiment of the present invention realizes the function of a Bragg polarizer by setting the top planar metasurface to include two grating structures with a second spacing distance. By adjusting the installation direction of the grating structure of the top planar metasurface, it is possible to select the linearly polarized terahertz wave passing therethrough, thereby obtaining any desired parallel terahertz wave with consistent polarization direction.

[0047] The terahertz dielectric folded planar metasurface optical collimator provided by the embodiment of the present invention realizes phase discontinuity and polarization control by setting a bottom planar metasurface including a bottom support plate and a plurality of rectangular anisotropic dielectric resonators arranged on the bottom support plate, and adjusting the plurality of rectangular anisotropic dielectric resonators to have different resonant frequencies along the horizontal axis direction and the vertical axis direction, and converts the u polarization of the terahertz wave emitted by the terahertz source into an orthogonal v polarization, thereby realizing the control of polarization characteristics. In addition, by directly integrating the terahertz source into the bottom support plate, the terahertz wave does not need to propagate through free space, but is directly emitted from the bottom planar metasurface, which greatly reduces the free space propagation distance of the terahertz wave and reduces the propagation space of the terahertz wave. At the same time, directly integrating the light source into the bottom support plate can make the structure of the device more compact and the fixed connection more firm.

[0048] In the embodiment of the present invention, the dielectric material used in the top plane metasurface and the bottom plane metasurface may be single crystal silicon. The single crystal silicon used in the embodiment of the present invention may have a dielectric constant ε r Higher than 10, absorption coefficient α≤10^-3 / cm, resistivity ≥10^3Ω·cm, single crystal silicon of non-dispersive material.

[0049] In an optional embodiment, the top plane metasurface includes two silicon substrates, and the grating structure is arranged in the middle of each silicon substrate;

[0050] The two silicon substrates are buckled together to form a cavity in a region corresponding to the grating structure, and the height of the cavity is equal to the second spacing distance.

[0051] In an optional embodiment, the lattice period of the grating structure along the horizontal direction of the outgoing wave is greater than the second spacing distance and greater than the thickness of the grating structure. By performing polarization control with such a grating structure, high reflection of u-polarized incident light and low reflection of v-polarized incident light can be achieved, such as Figure 1 As shown, the u-polarized terahertz wave is constantly reflected, while the v-polarized terahertz wave is emitted from the top plane metasurface, thereby converting the divergent terahertz waves emitted by the terahertz source into a group of parallel terahertz waves with the same polarization direction. In the embodiment of the present invention, the filling rate of the above grating structure can be between 0.4 and 0.6 to achieve polarization control.

[0052] In an alternative embodiment, referring to Figure 1 As shown, the multiple rectangular anisotropic dielectric resonators are arranged along a terahertz source ring array.

[0053] In an optional embodiment, the plurality of rectangular anisotropic dielectric resonators have the same height, and the plurality of rectangular anisotropic dielectric resonators have different lengths and widths.

[0054] Reference Figure 3 As shown in a, multiple rectangular anisotropic dielectric resonators of the bottom plane metasurface are located on the bottom support plate. The heights of the rectangular anisotropic dielectric resonators are the same, and the lattice size of each rectangular anisotropic dielectric resonator is p = 100 μm.

[0055] In a specific embodiment, since the thickness of the bottom support plate is very thin, in order to prevent the terahertz wave reflected by the top plane metasurface from being transmitted, a chromium film layer and a gold film layer are sequentially provided on the back of the bottom support plate; wherein the thickness ratio of the chromium film layer to the gold film layer is 2:200. Through the arrangement, the chromium film layer and the gold film layer form a reflection surface to prevent the terahertz wave from being transmitted, thereby reducing the loss.

[0056] In the embodiment of the present invention, each rectangular anisotropic dielectric resonator has a different length dx and width dy to achieve phase discontinuity. For polarization control, different resonant frequencies along the horizontal axis x and the vertical axis y are achieved by using anisotropic dielectric resonators, and the bottom plane metasurface converts the u polarization of the incident terahertz wave into an orthogonal v polarization to achieve polarization operation. The collimated light beam with v polarization from the bottom plane metasurface is irradiated again, passes through the top plane metasurface, and radiates into free space.

[0057] In a specific embodiment, the top planar super surface and the bottom planar super surface can be supported and fixed by a bracket.

[0058] In order to provide a more detailed description of the terahertz dielectric folded planar metasurface optical collimator provided by the embodiment of the present invention, a specific implementation method of the optical collimator is used as an example for description. Figure 1 As shown, to simplify device characterization, a second coordinate system (u, v, z) is defined by rotating the main coordinate system (x, y, z) 45° along the z-axis clockwise.

[0059] In this embodiment, the single crystal silicon dielectric constant ε r =11.7, absorption coefficient α=10^-3 / cm, resistivity=10^3Ω·cm.

[0060] Reference Figure 2As shown in a, the top plane metasurface is composed of two identical silicon grating structures, and the second spacing distance is s. It is assumed that the top plane metasurface is infinite in the u direction and infinite period in the v direction. The operating frequency of the optical collimator designed in the embodiment of the present invention is 1THz. Therefore, the inventor sets the lattice period Λ of the Si grating along the v direction (i.e., along the horizontal direction of the outgoing wave) to 100μm, the operating frequency to 1THz, the thickness of each layer of silicon grating to d, and the silicon filling rate to r. By modeling each dielectric grating as an anisotropic uniform plate, the equivalent model is obtained as follows Figure 2 b, where the equivalent dielectric constants in the u and v directions are:

[0061]

[0062] In the formula, ε r is the equivalent dielectric constant when the silicon filling rate is r;

[0063] The reflection parameters of different interfaces of the planar metasurface under U-type and V-type polarization incidence are:

[0064]

[0065] Where M is the total number of air layers, k i is the free space propagation number of the ith interface, ρ i is the intrinsic reflection coefficient on the i-th interface, where ρ i for:

[0066]

[0067] Based on this, the functional relationship between the transmission amplitude and the grating geometric size is determined when the top plane metasurface has different second spacing distances s, the thickness d of the silicon grating, and the silicon filling rate r, and the transmission coefficient data cloud diagram is obtained as follows Figure 2 c, according to Figure 2 c Selecting the best value and rounding it off can determine that when the top plane metasurface parameters (r, s, d) = (0.4 ~ 0.6, 30μm ~ 65μm, 15μm ~ 35μm), the reflection attenuation in the v direction is less than -1db, and the reflection attenuation in the u direction is greater than -10db. Therefore, when the top plane metasurface parameters (r, s, d) = (0.4 ~ 0.6, 30μm ~ 65μm, 15μm ~ 35μm) are selected, the reflection attenuation in the v direction is less than 10%, and the reflection attenuation in the u direction is greater than 90%, which meets the requirements of polarization collimation. Among them, the top plane metasurface parameters with the best transmission coefficient are (r, s, d) = (0.5, 40μm, 20μm), such as Figure 2 As shown by the white star in c.

[0068] According to the top plane super surface parameters (r, s, d) = (0.5, 40 μm, 20 μm), the top plane super surface is prepared, and the preparation process may include the following steps:

[0069] 1.1. Clean a 40 μm thick, undoped, double-sided polished silicon wafer using O2 plasma in a deep reactive ion etching (DRIE) system until it becomes hydrophilic;

[0070] 1.2. Spin-coat the front side of the silicon wafer obtained in step 1.1 with a resist, such as Figure 3 As shown in a, the cavity pattern is generated by optical lithography technology, and then developed, hardened and the residual layer is removed, as shown in Figure 3 As shown in b;

[0071] 1.3. The silicon wafer prepared in step 1.2 is etched by DRIE with a Bosch process at s / 2, i.e. 20 μm, to form a cavity, such as Figure 3 As shown in c;

[0072] 1.4. Remove the resist from the silicon wafer prepared in step 1.3 using a plasma ashing system, such as Figure 3 As shown in d;

[0073] 1.5. Spin-coat the back of the silicon wafer prepared in step 1.4 with a resist, and generate a grating pattern [(r, d) = (0.5, 20 μm)] using optical lithography technology, followed by development, hardening, and removal of the residual layer, and use it as a mask to etch a pattern on the silicon wafer. Figure 3 As shown in e;

[0074] 1.6. The silicon wafer prepared in step 1.5 is etched by DRIE by Bosch process for 20 μm until it penetrates the silicon wafer. Figure 3 f shown;

[0075] 1.7. Using a plasma ashing system to remove the resist from the silicon wafer prepared in step 1.6;

[0076] 1.8. Repeat steps 1.1 to 1.6 to obtain two identical silicon linear polarizers with 20 μm thick gratings and 20 μm thick cavities, such as Figure 3 As shown in g;

[0077] 1.9. Align two silicon linear polarizers and stack them with their backs facing each other to form a cavity of s = 40 μm, and obtain a top planar metasurface, such as Figure 3 h.

[0078] The micrograph of the prepared top plane super surface is observed under an ultra-high magnification electron microscope. Figure 3 i to Figure 3 l shown.

[0079] In the embodiment of the present invention, the resist used may be SPR 6112B photoresist. Of course, photoresists of other types and specifications in the prior art may also be used, which are not specifically limited here.

[0080] For the bottom plane metasurface, the inventors found through repeated experiments that when the diameter is 9mm and the distance between the two plane metasurfaces is 2mm, it is possible to reduce the free space propagation distance of the terahertz wave and the propagation space of the terahertz wave. At the same time, the light source can be easily integrated into the bottom plane metasurface to reduce the volume of the light collimator. Therefore, in the embodiment of the present invention, the inventors chose to design the bottom plane metasurface with a diameter of 9mm and a first spacing distance of 2mm between the two plane metasurfaces. The required phase distribution calculation results are as follows: Figure 1 Taking the terahertz source as the phase reference point, according to geometric optics, the required compensatory phase shift of each dielectric resonator of the bottom plane metasurface can be calculated by the following formula:

[0081]

[0082] Where k0 is the free space propagation number, (x i ,y i ,z i ) is the Cartesian coordinate of the i-th dielectric resonator, and D is twice the first spacing distance.

[0083] Combined with the phase distribution calculation results, the Cartesian coordinates of each dielectric resonator can be calculated.

[0084] Assume that the height of the dielectric resonator elements of the bottom plane hyperplane is 60μm, the thickness of the bottom support plate is 15μm, and the lattice size is 100μm, that is, for the dielectric resonator elements arranged horizontally, their length dx is equal to the lattice size, that is, 100μm, and for the dielectric resonator elements arranged vertically, their width dy is equal to the lattice size, that is, 100μm. Combined with the formula, the size of each dielectric resonator, that is, the length dx and width dy, is calculated:

[0085] f=2c / (n)√(m^2 / dxi^2+n^2 / dyi^2);

[0086] Wherein, f is the resonant frequency. In the embodiment of the present invention, f=1 THz, c is the speed of light, n is the conductivity, dxi is the length of the i-th dielectric resonator, dyi is the width of the i-th dielectric resonator, and m and n are the mode index numbers of the electromagnetic wave in the x and y directions (m=1, 2, 3, ..., n=0, 1, 2, ...).

[0087] According to the calculated length dx and width dy of each dielectric resonator, the number and arrangement of dielectric resonators can be designed and optimized in the electromagnetic simulation software, and the outer contour of the dielectric resonator that has passed the verification is used as the etching pattern of the bottom plane super surface. The electromagnetic simulation software can be selected from the ANSYS series software. In the embodiment of the present invention, the Maxwell module in ANSYS Electronics Desktop is used to implement the simulation process. The specific implementation process can refer to the detailed description in the prior art, and will not be repeated here.

[0088] like Figure 4 As shown in a, each dielectric resonator of the bottom plane metasurface is located on the bottom support plate. The length of the anisotropic rectangular dielectric resonator is dx, the width is dy, and the height is h. All dielectric resonators have the same height, h = 60 μm, and are placed on the bottom support plate with a thickness of t = 15 μm. The lattice size of the dielectric resonator is p = 100 μm. Figure 4 b is a schematic diagram of the working principle of polarization conversion. The incident wave can be decomposed into two orthogonal components Ex and Ey. When the x-polarized illumination and the y-polarized illumination have the same reflection amplitude and π reflection phase difference, polarization rotation from the u direction to the v direction can be achieved. For the bottom plane metasurface prepared, the dielectric resonator surrounded by periodic boundary conditions along the x and y directions was modeled and simulated in COMSOL Multiphysics. The simulation results are shown in Figure 4 c and Figure 4 As shown in d. Figure 4 c is the simulated electric field distribution on the yz and xz planes under the incidence of a normal x-polarized plane wave, and the TE111 fundamental mode of the rectangular dielectric resonator is excited. Figure 4 d The simulated reflection coefficient f(dx,dy) is a function of the length dx and width dy of the dielectric resonator under x-polarization incidence at 1 THz, which reflects the changes of the simulated reflection amplitude and phase response of the dielectric resonator under the irradiation of the normal x-polarization plane wave with the width and length of the dielectric resonator. Figure 4 d It can be seen that all dielectric resonators can achieve low loss less than 0.25dB, showing their high efficiency performance.

[0089] According to the bottom plane super surface parameters h = 60 μm, p = 100 μm, the bottom plane super surface is prepared, and the preparation process includes the following steps:

[0090] 2.1. Thin the undoped double-sided polished silicon wafer with a thickness of 100 μm to 75 μm using silicon dry etching technology in a DRIE system;

[0091] 2.2. Spin-coat the front side of the silicon wafer prepared in step 2.1 with a resist, such as Figure 5As shown in a, a circular pattern with a diameter of 500 μm is generated, and then developed, hardened, and the residual layer is removed, and the pattern is etched on the silicon wafer as a mask. Figure 5 As shown in b;

[0092] 2.3. According to the etched circular pattern with a diameter of 500 μm, a circular through hole is etched on the silicon wafer prepared in step 2.2, such as Figure 5 c, and depositing 2 / 200nm chromium / gold films on the back of the silicon wafer respectively by thermal evaporation;

[0093] 2.4. Spin-coat the resist on the front side of the silicon wafer obtained in step 2.3 again, generate the super surface pattern obtained by simulation, and then develop, harden the film and remove the residual layer, as shown in FIG. Figure 5 As shown in d;

[0094] 2.5. The front side of the silicon wafer prepared in step 2.4 is etched by DRIE for 60 μm using the Bosch process. Figure 5 As shown in e;

[0095] 2.6. Remove the resist from the silicon wafer prepared in step 2.5 by using a plasma ashing system, such as Figure 5 f.

[0096] The bottom plane super surface micrograph prepared under super magnification electron microscope is shown in Figure 5 g to Figure 5 j shown.

[0097] Finally, the terahertz source is installed in the circular through hole of the bottom plane metasurface, and the top plane metasurface and the bottom plane metasurface are fixed in parallel using a bracket, so that the distance between the top plane metasurface and the bottom plane metasurface reaches the first spacing distance, i.e. 2 mm, to obtain a light collimator.

[0098] The terahertz source used in the embodiment of the present invention can be a titanium sapphire femtosecond laser (central wavelength 800nm, pulse width 35fs, repetition frequency 1kHz). Of course, other terahertz wave light sources in the prior art can also be used, and no specific limitation is made here.

[0099] The obtained optical collimator was measured and analyzed using an expander (VDI WR-1.0 expander). The expander contains a source and a receiver to generate a known stimulus signal and determine the stimulus changes caused by the structure, respectively. A standard WR-1.0 open waveguide is directly connected to the bottom planar metasurface to feed the optical collimator; at the same time, a commercial software package CST Studio Suite is used for synchronous simulation to evaluate the performance of the designed optical collimator. A standard WR-1.0 waveguide with waveport excitation is used in CST Studio Suite to feed the optical collimator to measure the reflection coefficient |S11 |, based on the simulation and measurement results at different frequencies, the following Figure 6 a shows the frequency reflection coefficient curve. Figure 6 It can be seen that within the working frequency band of 0.9 to 1.1 THz, the measured |S 11 | Less than -10dB. This means that less than 10% of the energy is reflected back to the light source. The remaining 90% of the power is received / transmitted to the light collimator. From the above, it can be seen that the light collimator provided by the embodiment of the present invention (the volume of the external cube is 9x9x2mm 3 ) has a radiation efficiency of more than 90%. It is known that the efficiency of the terahertz collimator recorded in the prior art is usually around 70% at the same operating frequency. For example, SYZhu, YLLi, KMLuk, SWPang, IEEE Trans.Antennas Propag.2020, 68, 5945. The volume of the circumscribed cube is 3x3x0.77mm 3 The radiation efficiency of the terahertz collimator is 73%. For example, XLYou, RTAko, WSLLlee, MXLow, M.Bhaskaran, S.Sriram, C.Fumeaux, W.Withayachumnankul, Adv.Opt.Mater.2019,7,1900791. The volume of the circumscribed cube is 50x50x150mm. 3 The radiation efficiency of the terahertz collimator is 71%. Therefore, compared with the conventional terahertz collimator in the prior art, the light collimator provided by the embodiment of the present invention greatly improves the radiation efficiency.

[0100] The gain describes the power transmitted from the peak radiation direction (the wide side direction in this design) U(θ0,φ0) to the isotropic source U0(θ0,φ0), that is, G = U(θ0,φ0) / U0(θ0,φ0). In order to study the effect of the distance D / 2 between the two metasurfaces on the gain performance, the optical collimator with different D / 2 values ​​was simulated using CST Studio Suite. The results are shown in the figure. Figure 6 As shown in b. Figure 6 b It can be seen that the gain of the optical collimator is very sensitive to the distance between the two planar metasurfaces. When D / 2 is greater than the expected value of 2mm, the gain in the lower frequency band decreases, but the simulation results in the medium and high frequency bands are consistent, and the gain effect is good, which is in line with expectations. The measured results show that when the actual installation position is D / 2 = 2mm, it is consistent with the simulation results when D / 2 = 2.2mm, indicating that the installation error result is within the error range of 0.2mm, and the gain effect still meets the actual use requirements. Figure 6 c and Figure 6d is the normalized far-field radiation pattern of the far-field E plane (vertical plane) and H plane (horizontal plane) simulated and measured at 1.05 THz, given by Figure 6 c and Figure 6 d A highly directive pencil-shaped beam can be observed. The measured half-power beamwidth, defined as the angle between the two directions where the radiation intensity is half of the beam maximum, is 2.3° and 2.1° in the E-plane and H-plane of the optical collimator, respectively. The maximum measured level of the side lobes, defined as the radiation lobes in any direction other than the expected lobe, is -15.9dB and -11.8dB in the E-plane and H-plane, respectively.

[0101] Embodiment 2

[0102] Based on the same inventive concept, an embodiment of the present invention provides a terahertz imaging device, characterized in that the above-mentioned terahertz dielectric folded planar metasurface light collimator is used.

[0103] Embodiment 3

[0104] Based on the same inventive concept, an embodiment of the present invention provides a wireless terahertz communication device, characterized in that the above-mentioned terahertz dielectric folded planar metasurface optical collimator is used.

[0105] Embodiment 4

[0106] Based on the same inventive concept, an embodiment of the present invention provides a terahertz nondestructive testing device, characterized in that the above-mentioned terahertz dielectric folded planar metasurface light collimator is used.

[0107] Unless otherwise stated, any technical solution disclosed in the present invention disclosed above, if it discloses a numerical range, then the disclosed numerical range is a preferred numerical range, and any technician in the field should understand that the preferred numerical range is only a numerical value with a more obvious technical effect or representative value among many implementable numerical values. Since there are too many numerical values ​​to be exhaustive, the present invention discloses some numerical values ​​to illustrate the technical solution of the present invention, and the numerical values ​​listed above should not constitute a limitation on the scope of protection of the present invention.

[0108] If the words "first", "second" and so on are used in this article to limit components, those skilled in the art should know that the use of "first" and "second" is only for the convenience of description to distinguish the components. Unless otherwise stated, the above words have no special meaning.

[0109] In addition, unless otherwise stated, the terms used to indicate positional relationships or shapes in any of the technical solutions disclosed in the present invention include states or shapes that are approximate, similar, or close to them.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or some technical features can be replaced by equivalents without departing from the spirit of the technical solution of the present invention, which should be included in the scope of the technical solution for protection of the present invention.

Claims

1. A terahertz dielectric folded planar metasurface optical collimator, characterized in that: comprising a top planar metasurface and a bottom planar metasurface composed of a dielectric material arranged in parallel; The top planar hypersurface has a first separation distance from the bottom planar hypersurface; The top planar metasurface includes two grating structures with a second separation distance for selectively passing a desired linearly polarized terahertz wave; The bottom plane metasurface includes a bottom support plate and a plurality of rectangular anisotropic dielectric resonators arranged on the bottom support plate; The plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the horizontal axis direction and the vertical axis direction; A terahertz source is arranged at the center of the bottom support plate.

2. The terahertz dielectric folded planar metasurface optical collimator according to claim 1, characterized in that: The lattice period of the grating structure along the horizontal direction of the outgoing wave is greater than the second spacing distance and greater than the thickness of the grating structure.

3. The terahertz dielectric folded planar metasurface optical collimator according to claim 1, characterized in that: The plurality of rectangular anisotropic dielectric resonators are arranged along a terahertz source ring array.

4. The terahertz dielectric folded planar metasurface optical collimator according to claim 1, characterized in that: The plurality of rectangular anisotropic dielectric resonators have the same height, and the plurality of rectangular anisotropic dielectric resonators have different lengths and widths.

5. The terahertz dielectric folded planar metasurface optical collimator according to claim 1, characterized in that: The top plane metasurface comprises two silicon substrates, and the grating structure is arranged in the middle of each silicon substrate; The two silicon substrates are buckled together to form a cavity in a region corresponding to the grating structure, and the height of the cavity is equal to the second spacing distance.

6. The terahertz dielectric folded planar metasurface optical collimator according to claim 1, characterized in that: A chromium thin film layer and a gold thin film layer are sequentially provided on the bottom surface of the bottom plane super plane; The thickness ratio of the chromium thin film layer to the gold thin film layer is 2:

200.

7. The terahertz dielectric folded planar metasurface optical collimator according to any one of claims 1 to 6, characterized in that: The top plane super surface and the bottom plane super surface are supported and fixed by a bracket.

8. A terahertz imaging device, characterized in that: A terahertz dielectric folded planar metasurface optical collimator is used as described in any one of claims 1 to 7.

9. A wireless terahertz communication device, characterized in that: A terahertz dielectric folded planar metasurface optical collimator is used as described in any one of claims 1 to 7.

10. A terahertz nondestructive testing device, characterized in that: A terahertz dielectric folded planar metasurface optical collimator is used as described in any one of claims 1 to 7.

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