Terahertz dielectric folded planar metasurface light collimator and application

Abstract

The application discloses a terahertz dielectric folded planar metasurface light collimator and application. The light collimator comprises a top planar metasurface and a bottom planar metasurface which are arranged in parallel and are composed of dielectric materials; the top planar metasurface and the bottom planar metasurface have a first interval distance; the top planar metasurface comprises two grating structures with a second interval distance, which are used for selecting a desired linearly polarized terahertz wave; the bottom planar metasurface comprises 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 in the transverse axis direction and the longitudinal axis direction; and a terahertz source is arranged at the center of the bottom support plate.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano optical control technology, specifically relating to terahertz dielectric folded planar metasurface optical collimators and their applications. Background Technology

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

[0003] Metasurfaces are two-dimensional metamaterials with subwavelength thickness, allowing for flexible control of 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 mirrors. Terahertz devices based on planar metasurfaces, with their advantages of small profile and lightweight design, can replace traditional bulky terahertz devices. Summary of the Invention

[0004] To achieve a compact structure and reduce the propagation space of terahertz devices, thereby improving their performance, this invention provides a terahertz dielectric folded planar metasurface optical collimator and its application.

[0005] The technical solution adopted in this invention is:

[0006] In a first aspect, embodiments of the present invention provide a terahertz dielectric folded planar metasurface optical collimator, comprising a top planar metasurface and a bottom planar metasurface made of dielectric material arranged in parallel.

[0007] The top planar metasurface and the bottom planar metasurface have a first interval distance;

[0008] The top planar metasurface includes two grating structures with a second spacing distance for selecting the passage of desired linearly polarized terahertz waves;

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

[0010] The plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the horizontal and vertical axes.

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

[0012] In one or more alternative embodiments, the lattice period of the grating structure along the horizontal direction of the emitted wave is greater than the second spacing distance, which is greater than the thickness of the grating structure.

[0013] In one or more alternative embodiments, the plurality of rectangular anisotropic dielectric resonators are arranged along a terahertz source ring array.

[0014] In one or more alternative 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 more alternative embodiments, the top planar metasurface comprises two silicon substrates, each of which has the grating structure disposed in the center.

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

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

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

[0019] In one or more alternative embodiments, the lattice size of the bottom planar metasurface is 100 μm.

[0020] In one or more alternative embodiments, the top planar metasurface and the bottom planar metasurface are made of monocrystalline silicon.

[0021] In one or more alternative embodiments, the top planar metasurface and the bottom planar metasurface are supported and fixed together by a bracket.

[0022] Secondly, embodiments of the present invention provide a terahertz imaging device, characterized in that it uses the aforementioned terahertz dielectric folded planar metasurface optical collimator.

[0023] Thirdly, embodiments of the present invention provide a wireless terahertz communication device, characterized in that it uses the aforementioned terahertz dielectric folded planar metasurface optical collimator.

[0024] Fourthly, embodiments of the present invention provide a terahertz nondestructive testing device, characterized in that it uses the aforementioned terahertz dielectric folded planar metasurface optical collimator.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] The terahertz dielectric folded planar metasurface optical collimator provided in this invention comprises two planar metasurfaces made of dielectric material, spaced a first distance and arranged parallel to each other. The bottom planar metasurface is used for reflection gain, and the top planar metasurface is used for polarization collimation. Terahertz waves emitted from a terahertz source are polarized. Terahertz waves that meet the polarization requirements are emitted from the grating structure of the top planar metasurface, while other terahertz waves that do not meet the polarization requirements continue to be reflected between the two planar metasurfaces until they are polarized to meet the requirements. This converts the divergent terahertz waves emitted from the terahertz source into a set of parallel terahertz waves with consistent polarization directions, which are then radiated into the target space. By continuously reflecting the terahertz waves between the two planar metasurfaces, the propagation path of the terahertz waves is folded between the two planar metasurfaces. Compared to the existing technology that uses long propagation distances for propagation and re-modulation reflection, this significantly reduces the free-space propagation distance of the terahertz waves, reduces the propagation space, greatly reduces the volume of the optical collimator, and improves the radiation efficiency of the optical collimator. This not only facilitates the miniaturization of terahertz systems using this optical collimator, but also improves the accuracy of terahertz systems due to the efficient control of terahertz waves, since the output power of the terahertz source is much lower than that of microwaves.

[0027] The terahertz dielectric folded planar metasurface optical collimator provided in this embodiment of the invention realizes the Bragg polarizer function 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 on the top planar metasurface, the linearly polarized terahertz wave can be selected to pass through, thereby obtaining any parallel terahertz wave with the same polarization direction as desired.

[0028] The terahertz dielectric folded planar metasurface optical collimator provided in this invention comprises a bottom planar metasurface including a bottom support plate and multiple rectangular anisotropic dielectric resonators disposed on the bottom support plate. Phase discontinuity and polarization control are achieved by adjusting the different resonant frequencies of the multiple rectangular anisotropic dielectric resonators along the horizontal and vertical axes. This converts the u-polarization (vertical direction of the emitted wave) of the terahertz wave emitted from the terahertz source into orthogonal v-polarization (horizontal direction of the emitted wave), thus controlling the polarization characteristics. Furthermore, by directly integrating the terahertz source into the bottom support plate, the terahertz wave does not need to propagate through free space and is directly emitted from the bottom planar metasurface, greatly reducing the free space propagation distance and propagation space of the terahertz wave. Simultaneously, directly integrating the light source into the bottom support plate allows for a more compact device structure and a more robust fixed connection.

[0029] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Attached Figure Description

[0030] Figure 1 A schematic diagram of the structure of a silicon-based terahertz folded metasurface and a schematic diagram of the calculated phase distribution of the required bottom planar metasurface;

[0031] Figure 2 This is a schematic diagram showing the design and simulation response results of the top planar metasurface.

[0032] Figure 3 The fabrication process diagram and microscope image are shown for the top planar metasurface. (ah) Fabrication process of the double-layer silicon grating metasurface.

[0033] Figure 4 The design and simulation response diagrams for the bottom planar metasurface are shown.

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

[0035] Figure 6 These are experimental and simulation diagrams illustrating the folded metasurface of this invention. Detailed Implementation

[0036] The inventors have discovered that despite extensive research into the design and fabrication of terahertz devices using planar metasurfaces, achieving a significant reduction in the size of terahertz systems using these devices remains a major obstacle. This is because the overall size of a terahertz system is primarily determined by the propagation space between the terahertz source / receiver and the terahertz device. While planar metasurface terahertz devices can significantly reduce the thickness of traditional geometric lenses and mirrors, they cannot reduce the propagation distance to the terahertz source / receiver. A long propagation distance in free space results in a large terahertz system size. This effectively hinders the practical application of planar metasurface terahertz devices in terahertz systems. Therefore, this invention provides a terahertz dielectric folded planar metasurface optical collimator and its application.

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

[0038] Example 1

[0039] This invention provides a terahertz dielectric folded planar metasurface optical collimator, with reference to... Figures 1 to 3 As shown, it includes a top planar metasurface (also referred to as the top metasurface) and a bottom planar metasurface (also referred to as the bottom metasurface) made of dielectric material arranged in parallel.

[0040] The top planar metasurface and the bottom planar metasurface have a first interval distance;

[0041] The top planar metasurface includes two grating structures with a second spacing distance for selecting the passage of desired linearly polarized terahertz waves;

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

[0043] The plurality of rectangular anisotropic dielectric resonators have different resonant frequencies along the horizontal and vertical axes.

[0044] A terahertz source (i.e., ...) is located at the center of the bottom support plate. Figure 1 (The light source in the middle).

[0045] The terahertz dielectric folded planar metasurface optical collimator provided in this invention comprises two planar metasurfaces made of dielectric material, spaced a first distance and arranged parallel to each other. The bottom planar metasurface is used for reflection gain, and the top planar metasurface is used for polarization collimation. Terahertz waves emitted from a terahertz source are polarized. Terahertz waves that meet the polarization requirements are emitted from the grating structure of the top planar metasurface, while other terahertz waves that do not meet the polarization requirements continue to be reflected between the two planar metasurfaces until they are polarized to meet the requirements. This converts the divergent terahertz waves emitted from the terahertz source into a set of parallel terahertz waves with consistent polarization directions, which are then radiated into the target space. By continuously reflecting the terahertz waves between the two planar metasurfaces, the propagation path of the terahertz waves is folded between the two planar metasurfaces. Compared to the existing technology that uses long propagation distances for propagation and re-modulation reflection, this significantly reduces the free-space propagation distance of the terahertz waves, reduces the propagation space, greatly reduces the volume of the optical collimator, and improves the radiation efficiency of the optical collimator. This not only facilitates the miniaturization of terahertz systems using this optical collimator, but also improves the accuracy of terahertz systems due to the efficient control of terahertz waves, since the output power of the terahertz source is much lower than that of microwaves.

[0046] The terahertz dielectric folded planar metasurface optical collimator provided in this embodiment of the invention realizes the Bragg polarizer function 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 on the top planar metasurface, the linearly polarized terahertz wave can be selected to pass through, thereby obtaining any parallel terahertz wave with the same polarization direction as desired.

[0047] The terahertz dielectric folded planar metasurface optical collimator provided in this invention comprises a bottom planar metasurface including a bottom support plate and multiple rectangular anisotropic dielectric resonators disposed on the bottom support plate. Phase discontinuity and polarization control are achieved by adjusting the different resonant frequencies of the multiple rectangular anisotropic dielectric resonators along the horizontal and vertical axes, converting the u-polarization of the terahertz wave emitted from the terahertz source into orthogonal v-polarization, thus controlling the polarization characteristics. Furthermore, by directly integrating the terahertz source into the bottom support plate, the terahertz wave does not need to propagate through free space and is directly emitted from the bottom planar metasurface, greatly reducing the free space propagation distance and propagation space of the terahertz wave. Simultaneously, directly integrating the light source into the bottom support plate allows for a more compact device structure and a more robust fixed connection.

[0048] In this embodiment of the invention, the dielectric material used for the top and bottom planar metasurfaces can be single-crystal silicon. The single-crystal silicon used in this embodiment of the invention can have a dielectric constant ε. r A single-crystal silicon material with an absorption coefficient α ≤ 10^-3 / cm and a resistivity ≥ 10^3Ω·cm is a non-dispersive material.

[0049] In an optional embodiment, the top planar metasurface comprises two silicon substrates, each of which has the grating structure disposed in the center.

[0050] The two silicon substrates are fastened together to form a cavity in the 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 emitted wave is greater than the second spacing distance, which is greater than the thickness of the grating structure. Polarization control using such a grating structure can achieve high reflection for u-polarized incident light and low reflection for v-polarized incident light, such as... Figure 1 As shown, u-polarized terahertz waves are continuously reflected, while v-polarized terahertz waves are emitted from the top planar metasurface, thus converting the divergent terahertz waves emitted by the terahertz source into a set of parallel terahertz waves with consistent polarization directions. In this embodiment of the invention, the fill factor of the grating structure can be between 0.4 and 0.6 to achieve polarization control.

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

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

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

[0055] In one specific embodiment, because the bottom support plate is very thin, to prevent the transmission of terahertz waves reflected from the top planar metasurface, a chromium thin film layer and a gold thin film layer are sequentially provided on the back side of the bottom support plate; wherein the thickness ratio of the chromium thin film layer to the gold thin film layer is 2:200. By setting it up, the chromium thin film layer and the gold thin film layer form a reflective surface, preventing the transmission of terahertz waves, thereby reducing losses.

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

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

[0058] To provide a more detailed explanation of the terahertz dielectric folded planar metasurface optical collimator provided in this embodiment of the invention, a specific implementation of the optical collimator will be used as an example below. (Refer to...) Figure 1 As shown, in order to simplify device characterization, a second coordinate system (u,v,z) is defined by rotating the principal coordinate system (x,y,z) clockwise by 45° along the z-axis.

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

[0060] Reference Figure 2As shown in Figure a, the top planar metasurface is composed of two identical silicon grating structures with a second spacing of s. It is assumed that the top planar metasurface is infinitely large in the u direction and infinitely periodic in the v direction. The optical collimator designed in this embodiment operates at a frequency of 1 THz. Therefore, the inventors set the lattice period Λ of the Si grating along the v direction (i.e., along the horizontal direction of the emitted wave) to be 100 μm, the operating frequency to be 1 THz, the thickness of each silicon grating layer to be d, and the silicon fill rate to be r. By modeling each dielectric grating as an anisotropic uniform plate, the equivalent model is obtained as follows: Figure 2 As shown in b, the equivalent dielectric constants in the u and v directions are respectively:

[0061]

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

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

[0064]

[0065] In the formula, M is the total number of layers including air layers, and k i Let ρ be the free space propagation number at the i-th interface. i Let be the intrinsic reflection coefficient at the i-th interface, where ρ i for:

[0066]

[0067] Based on this, the functional relationship between transmission amplitude and grating geometry was analyzed for different second spacing distances s, silicon grating thickness d, and silicon fill rate r of the top planar metasurface, resulting in the transmission coefficient data cloud map as shown below. Figure 2 As shown in c, according to Figure 2 By selecting the optimal value and rounding it, we can determine that when the top planar metasurface parameters (r,s,d) are within the range of (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 planar metasurface parameters (r,s,d) are selected as (0.4~0.6, 30μm~65μm, 15μm~35μm), 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 for polarization collimation. Among them, the top planar metasurface parameters with the optimal transmission coefficient are (r,s,d) = (0.5, 40μm, 20μm), such as... Figure 2 The white star is shown in c.

[0068] The top planar metasurface is prepared based on the top planar metasurface parameters (r,s,d) = (0.5, 40μm, 20μm). The preparation process may include the following steps:

[0069] 1.1 A 40μm thick, undoped, double-sided polished silicon wafer was cleaned with O2 plasma in a deep reactive ion etching (DRIE) system until it became hydrophilic.

[0070] 1.2. Spin-coat the front side of the silicon wafer prepared in step 1.1 with a photoresist, such as... Figure 3 As shown in figure a, a cavity pattern is generated using optical lithography, followed by development, hardening, and removal of residual layers, as shown in figure a. Figure 3 As shown in b;

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

[0072] 1.4. Remove the photoresist 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 photoresist, and generate a grating pattern [(r,d)=(0.5,20μm)] using optical lithography. Then perform development, hardening, and removal of residual layers. Use this as a mask to etch the pattern on the silicon wafer, as shown below. Figure 3 As shown in e;

[0074] 1.6. The silicon wafer prepared in step 1.5 is etched 20 μm through the silicon wafer using the Bosch process via DRIE etching, as shown below. Figure 3 As shown in f;

[0075] 1.7. Remove the photoresist from the silicon wafer prepared in step 1.6 using a plasma ashing system;

[0076] 1.8 Repeat steps 1.1 to 1.6 above to obtain two identical silicon linear polarizers with 20μm thick gratings and 20μm 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 with s = 40 μm, thus creating a top planar metasurface, as shown. Figure 3 As shown in h.

[0078] Microscopic images of the prepared top-plane metasurface observed under a high-magnification electron microscope are shown below. Figure 3 i to Figure 3 As shown in l.

[0079] In this embodiment of the invention, the photoresist used can be SPR 6112B photoresist. Of course, other types and specifications of photoresists in the prior art can also be used, and no specific limitation is made here.

[0080] Regarding the bottom planar metasurface, the inventors discovered through repeated experiments that when the diameter is 9mm and the distance between the two planar metasurfaces is 2mm, it is possible to reduce the free-space propagation distance and propagation space of terahertz waves. Simultaneously, it is convenient to integrate the light source onto the bottom planar metasurface, thereby reducing the size of the optical collimator. Therefore, in this embodiment of the invention, the inventors chose to design the bottom planar metasurface with a diameter of 9mm and a first interval distance of 2mm between the two planar metasurfaces. The required phase distribution calculation results are as follows... Figure 1 As shown. Using the terahertz source as the phase reference point, and based on geometric optics, the required compensation phase shift for each dielectric resonator on the bottom planar metasurface can be calculated using 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] By combining the phase distribution calculation results, the Cartesian coordinates of each dielectric resonator can be calculated.

[0084] Assuming the height of all dielectric resonator elements on the bottom hyperplane is 60 μm, the thickness of the bottom support plate is 15 μm, and the lattice size is 100 μm, then for laterally arranged dielectric resonator elements, their length dx equals the lattice size, i.e., 100 μm; and for longitudinally arranged dielectric resonator elements, their width dy equals the lattice size, i.e., 100 μm. Using the formulas, the dimensions of each dielectric resonator, i.e., length dx and width dy, can be calculated:

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

[0086] Where f is the resonant frequency, in this embodiment of the 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] Based on the calculated length dx and width dy of each dielectric resonator, the number and arrangement of the dielectric resonators can be designed and optimized in electromagnetic simulation software. The outer contour of the verified dielectric resonators is used as the etching pattern for the bottom planar metasurface. The electromagnetic simulation software can be any ANSYS series software. In this embodiment of the invention, the Maxwell module in ANSYS ElectronicsDesktop is used to implement the simulation process. Specific implementation details can be found in the prior art and will not be repeated here.

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

[0089] Based on the bottom planar metasurface parameters h = 60 μm and p = 100 μm, a bottom planar metasurface was prepared. The preparation process included the following steps:

[0090] 2.1. A 100μm thick, undoped, double-sided polished silicon wafer is thinned to 75μm using silicon dry etching technology in the DRIE system;

[0091] 2.2 Spin-coating the front side of the silicon wafer prepared in step 2.1 with a photoresist, such as... Figure 5As shown in figure a, a circular pattern with a diameter of 500 μm is generated, and then developed, hardened, and the residual layer is removed. This pattern is then used as a mask to etch the pattern onto the silicon wafer, as shown in figure a. Figure 5 As shown in b;

[0092] 2.3. Based on the etched circular pattern with a diameter of 500 μm, etch circular through-holes on the silicon wafer prepared in step 2.2, such as... Figure 5 As shown in c, 2 / 200 nm chromium / gold films were deposited on the back side of the silicon wafer by thermal evaporation;

[0093] 2.4. The silicon wafer prepared in step 2.3 is then spin-coated with resist again on its front side to generate the metasurface pattern obtained through simulation. This is followed by development, hardening, and removal of the residual layer, as shown below. Figure 5 As shown in d;

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

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

[0096] Microscopic images of the prepared bottom planar metasurface observed under a high-magnification electron microscope are shown below. Figure 5 g to Figure 5 As shown in j.

[0097] Finally, the terahertz source is installed into the through hole of the bottom planar metasurface. The top and bottom planar metasurfaces are fixed in parallel using a bracket, and the distance between the top and bottom planar metasurfaces is made to reach the first interval distance, i.e., 2 mm, to obtain the optical collimator.

[0098] The terahertz source used in this embodiment of the invention can be a Ti:sapphire femtosecond laser (center 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 resulting optical collimator was measured and analyzed using a spreader (VDI WR-1.0 spreader). The spreader included a source and a receiver to generate a known stimulus signal and determine the stimulus variation induced by the structure. A standard WR-1.0 open waveguide was directly connected to the bottom planar metasurface to feed the optical collimator. Simultaneously, synchronous simulations were performed using the commercial software package CST Studio Suite to evaluate the performance of the designed optical collimator. The reflection coefficient |S| was measured in CST Studio Suite using a standard WR-1.0 waveguide with waveport excitation to feed the optical collimator.11 | Based on simulation and measurement results at different frequencies, plot as follows Figure 6 The frequency reflection coefficient curve shown in figure a. (From...) Figure 6 It can be seen that within the operating frequency band of 0.9–1.1 THz, the measured |S 11 The voltage level is less than -10dB. This means that less than 10% of the energy is reflected back to the light source. The remaining 90% or more of the power is received / transmitted to the optical collimator. Therefore, the optical collimator provided in this embodiment of the invention (with an external cube volume of 9x9x2mm) 3 The radiation efficiency exceeds 90%. It is known that the efficiency of terahertz collimators described in existing technology is typically around 70% at the same operating frequency. For example, SYZhu, YLLi, KMLuk, SWPang, IEEE Trans. Antennas Propag. 2020, 68, 5945, describes an circumscribed cube with a volume of 3x3x0.77mm. 3 The radiation efficiency of the terahertz collimator is 73%. For example, XLYou, RTAko, WSLLEe, MXLow, M. Bhaskaran, S. Sriram, C. Fumeaux, W. Withayachumnankul, Adv. Opt. Mater. 2019, 7, 1900791. The literature states that the volume of the circumscribed cube is 50x50x150mm. 3 The terahertz collimator has a radiation efficiency of 71%. Therefore, compared with conventional terahertz collimators in the prior art, the optical collimator provided in this embodiment of the invention greatly improves the radiation efficiency.

[0100] Gain describes the power transmitted from the peak radiation direction (wide-side direction in this design) U(θ0,φ0) to the isotropic source U0(θ0,φ0), i.e., G = U(θ0,φ0) / U0(θ0,φ0). To investigate the effect of the distance D / 2 between the two metasurfaces on gain performance, optical collimators with different D / 2 values ​​were simulated using CST Studio Suite. The results are as follows... Figure 6 As shown in b. From Figure 6 As can be seen from b, the gain of the optical collimator is highly sensitive to the distance between the two planar metasurfaces. When D / 2 is greater than the expected value of 2mm, the gain decreases in the lower frequency band, but the simulation results in the mid-to-high frequency band are in good agreement, and the gain effect is good, meeting expectations. Actual measurement results show that when the actual installation position is D / 2 = 2mm, the simulation results are in good agreement with those when D / 2 = 2.2mm, indicating that the installation error is within 0.2mm, and the gain effect still meets the actual usage requirements. Figure 6 c and Figure 6d represents the normalized far-field radiation maps of the E-plane (vertical plane) and H-plane (horizontal plane) simulated and measured at 1.05 THz, derived from... Figure 6 c and Figure 6 A highly directional pencil-shaped beam can be observed. The measured half-power beamwidth, defined as the angle between two directions where the radiation intensity is half the maximum beamwidth, is 2.3° and 2.1° in the E-plane and H-plane of the optical collimator, respectively. The maximum measured level of the sidelobes, defined as the radiation lobe in any direction other than the expected lobe, is -15.9 dB and -11.8 dB in the E-plane and H-plane, respectively.

[0101] Example 2

[0102] Based on the same inventive concept, this invention provides a terahertz imaging device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface optical collimator.

[0103] Example 3

[0104] Based on the same inventive concept, this invention provides a wireless terahertz communication device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface optical collimator.

[0105] Example 4

[0106] Based on the same inventive concept, this invention provides a terahertz nondestructive testing device, characterized in that it uses the above-mentioned terahertz dielectric folded planar metasurface optical collimator.

[0107] Unless otherwise stated, if any of the technical solutions disclosed in this invention specify a numerical range, then the disclosed numerical range is a preferred numerical range. Anyone skilled in the art should understand that the preferred numerical range is merely one among many feasible numerical values ​​that has a more obvious or representative technical effect. Because there are many numerical values, it is impossible to list them all. Therefore, this invention discloses only some numerical values ​​to illustrate the technical solutions of this invention. Furthermore, the numerical values ​​listed above should not constitute a limitation on the scope of protection of this invention.

[0108] If the terms "first" or "second" are used in this document to specify components, those skilled in the art should know that the use of "first" or "second" is merely for the purpose of distinguishing components in description, and unless otherwise stated, the above terms have no special meaning.

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

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A terahertz dielectric folded planar metasurface light collimator, characterized in that: The top and bottom planar metasurfaces are arranged in parallel and are made of dielectric material; The top planar metasurface and the bottom planar metasurface have a first interval distance; The top planar metasurface includes two grating structures with a second interval distance for selecting a desired linearly polarized terahertz wave; The bottom planar 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 resonance frequencies in the horizontal and vertical directions; A terahertz source is arranged at the center of the bottom support plate; The grating structure has a lattice period in the horizontal direction of the outgoing wave, which is greater than the second interval distance and the thickness of the grating structure; The plurality of rectangular anisotropic dielectric resonators are arranged in a circular array of terahertz sources; The plurality of rectangular anisotropic dielectric resonators have the same height, and the plurality of rectangular anisotropic dielectric resonators have different lengths and widths.

2. The terahertz dielectric folded planar metasurface light collimator of claim 1, wherein: The top planar metasurface includes two silicon substrates, and a grating structure is arranged in the middle of each silicon substrate; The two silicon substrates are clamped together, and a cavity is formed in the area corresponding to the grating structure, and the height of the cavity is equal to the second interval distance.

3. The terahertz dielectric folded planar metasurface light collimator of claim 1, wherein: A chromium thin film layer and a gold thin film layer are arranged in sequence on the bottom surface of the bottom planar metasurface; The thickness ratio of the chromium thin film layer and the gold thin film layer is 2:

200.

4. The terahertz dielectric folded planar metasurface light collimator of any one of claims 1-3, wherein: The top planar metasurface and the bottom planar metasurface are fixed and supported by a support.

5. A terahertz imaging device, characterized by, The terahertz dielectric folded planar metasurface light collimator of any one of claims 1-4 is used.

6. A wireless terahertz communication device, characterized by The terahertz dielectric folded planar metasurface light collimator of any one of claims 1-4 is used.

7. A terahertz non-destructive testing apparatus, characterized by, The terahertz dielectric folded planar metasurface light collimator of any one of claims 1-4 is used.

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