Terahertz filter based on multilayer metasurface and processing method

Abstract

The invention provides a terahertz filter based on a multi-layer metasurface and a processing method, and can be applied to the technical field of metasurfaces. The filter comprises M layers of composite metasurfaces and M-1 layers of cycloolefin copolymer films, each layer of the M-1 layers of cycloolefin copolymer films is located between every two adjacent layers of composite metasurfaces, each layer of composite metasurface comprises a first layer of polyimide film, a metasurface and a second layer of polyimide film, the metasurface is composed of a plurality of metal microstructures, the metal microstructures are periodically and uniformly arranged, the metasurface is located between the first polyimide film layer and the second polyimide film layer, and M is a positive integer larger than 1.

Description

Technical Field

[0001] The present disclosure relates to the field of metasurface technology, and more specifically, to a terahertz filter based on a multi-layer metasurface and a processing method thereof. Background Art

[0002] The magnetic polarization of matter is formed by the spin of atomic orbital current or asymmetric electrons. The resonance phenomenon of magnetic systems usually occurs in extremely low frequency bands, and the terahertz magnetic effect of natural materials is very weak. Compared with other bands, terahertz waves have unique properties such as high bandwidth, high penetration, high spatial resolution, and transient properties. Some mature devices cannot be simply transplanted to the terahertz band for application. Therefore, the development of new terahertz devices, such as polarizers, filters, absorbers, high-performance modulators, etc., is crucial for customizing the required electromagnetic properties and synthesizing complex waveforms.

[0003] Filters are used in various measurement systems to selectively block or transmit the required frequency band of electromagnetic waves, filter out noise interference in information transmission, or extract signals at the target frequency. They play an important role in various fields such as wireless communication systems, radar systems, navigation systems, electronic countermeasures, and signal perception. They are divided into many types according to the frequency selection function, such as bandpass, bandstop, low-pass, and high-pass. Especially in communication systems, filters are indispensable frequency selection devices in modern information and communication systems, and their role directly affects the quality of the entire system. Terahertz waves have low energy and are easily interfered with. Therefore, terahertz filters have a very important impact on terahertz emission, reception, transmission sensing, etc., and are the most basic type of terahertz devices.

[0004] In the process of realizing the concept of the present disclosure, the inventors found that there are at least the following problems in the related art: the structure and use of the terahertz filter in the related art are limited and cannot perform filtering well. Summary of the invention

[0005] In view of this, the present disclosure provides a terahertz filter based on a multi-layer metasurface and a processing method.

[0006] One aspect of the present disclosure provides a terahertz filter based on a multilayer metasurface, comprising:

[0007] M layers of composite supersurface and M-1 layers of cyclic olefin copolymer (COC) film, wherein each of the M-1 layers of the COC film is located between two adjacent layers of the composite supersurface, each layer of the composite supersurface includes a first layer of polyimide (PI) film, a supersurface and a second layer of PI film, the supersurface is composed of a plurality of metal microstructures, the plurality of metal microstructures are periodically and uniformly arranged, the supersurface is located between the first layer of PI film and the second layer of PI film, and M is a positive integer greater than 1.

[0008] Another aspect of the present disclosure provides a method for processing the filter as described above, comprising:

[0009] A first PI film, a super surface and a second PI film are sequentially formed on a substrate to obtain a composite super surface, wherein the super surface is composed of a plurality of metal micro structures, and the plurality of metal micro structures are periodically and uniformly arranged;

[0010] The M layers of the above-mentioned composite metasurface and the M-1 layers of COC film are hot pressed at a preset temperature to obtain the filter as mentioned above, wherein each layer of the M-1 layers of the above-mentioned COC film is located between two adjacent layers of the above-mentioned composite metasurface, and M is a positive integer greater than 1.

[0011] According to the embodiments of the present disclosure, the filter of the present disclosure is realized by a method of alternating cascade of multi-layer composite metasurfaces and COC films, and the metasurface in each layer of the composite metasurface is composed of multiple metal microstructures, and the multiple metal microstructures are periodically and evenly arranged. The multi-layer metasurface can increase the adjustment freedom of the filter and can also achieve more filtering effects. The use of PI film as a flexible substrate for the metasurface can enable the device to have stretchable, bendable and foldable properties. The metasurface device is designed in a thin film material with high flexibility and stability, so that the device is durable and more compact; the COC film has high transparency and high Abbe number, excellent optical properties, small dielectric constant and dielectric tangent values, and low loss to terahertz frequency signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0013] Figure 1 A schematic diagram of a terahertz filter based on a multilayer metasurface according to an embodiment of the present disclosure is schematically shown;

[0014] Figure 2 Schematically shows a cross-sectional view of a composite metasurface according to an embodiment of the present disclosure;

[0015] Figure 3 A schematic diagram of the projection of the supersurface in the first layer of PI film according to an embodiment of the present disclosure is schematically shown.

[0016] Figure 4 A schematic diagram of an amplitude transmittance spectrum line of a terahertz filter simulation according to an embodiment of the present disclosure is schematically shown;

[0017] Figure 5 A flowchart schematically shows a method for processing a filter according to an embodiment of the present disclosure;

[0018] Figure 6 The schematic diagram of the processing flow of the terahertz filter according to the embodiment of the present disclosure is schematically shown;

[0019] Figure 7 The schematic diagram schematically shows the principle of the experimental test system according to the embodiment of the present disclosure;

[0020] Figure 8 Schematically showing a schematic diagram of amplitude transmittance spectra obtained by experimental measurement at 9 positions randomly selected from four terahertz filters according to an embodiment of the present disclosure;

[0021] Fig. 9 Schematically showing the amplitude transmittance spectra of four terahertz filters according to the embodiments of the present disclosure obtained by experimental measurement when terahertz waves are incident in different polarization directions;

[0022] Fig.10 Schematically showing the amplitude transmittance spectra of four terahertz filters according to the embodiments of the present disclosure obtained by experimental measurement when the terahertz wave is incident at different oblique angles;

[0023] Fig.11 The schematic diagram shows the amplitude transmittance spectra obtained by experimental measurement of four terahertz filters in normal and bent states according to the embodiments of the present disclosure. DETAILED DESCRIPTION

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the present disclosure. In the following detailed description, for ease of explanation, many specific details are set forth to provide a comprehensive understanding of the embodiments of the present disclosure. However, it is apparent that one or more embodiments may also be implemented without these specific details. In addition, in the following description, descriptions of known structures and technologies are omitted to avoid unnecessary confusion of the concepts of the present disclosure.

[0025] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0026] All terms (including technical and scientific terms) used herein have the meanings commonly understood by those skilled in the art unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0027] When using expressions such as "at least one of A, B, and C, etc.", they should generally be interpreted according to the meaning of the expression commonly understood by those skilled in the art (for example, "a system having at least one of A, B, and C" should include but is not limited to a system having A alone, B alone, C alone, A and B, A and C, B and C, and / or A, B, C, etc.).

[0028] In the embodiments of the present disclosure, the collection, updating, analysis, processing, use, transmission, provision, disclosure, storage, etc. of the data involved (for example, including but not limited to user personal information) are in compliance with the provisions of relevant laws and regulations, are used for legitimate purposes, and do not violate public order and good morals. In particular, necessary measures are taken for user personal information to prevent illegal access to user personal information data and maintain the security of user personal information and network security.

[0029] In the embodiments of the present disclosure, the user's authorization or consent is obtained before obtaining or collecting the user's personal information.

[0030] In the relevant technologies, when facing the actual needs of aviation, aerospace and other fields, it is not only necessary to solve the demand for terahertz devices from scratch, but also to put forward higher requirements for devices in other aspects. For example, in terms of load, terahertz devices are required to have the characteristics of small size, light weight, high integration, etc., and in terms of performance, the devices are required to have advantages such as high pressure resistance, strong flexibility and high efficiency.

[0031] Metasurface materials composed of artificial subwavelength unit structures can achieve abnormal electromagnetic properties that natural materials cannot achieve, such as invisibility cloaks, negative refractive index, complete absorption, hyperbolic dispersion, etc., which has brought development to terahertz science and technology. The research on metasurface materials has opened up new ways and methods for the research of new terahertz devices, and has become the main means of developing terahertz devices, with important application potential and value. As a two-dimensional manifestation of metasurface materials, metasurfaces are two-dimensional arrays with special artificial structures. Through carefully designed microstructures, they can achieve the regulation of the phase and amplitude of the incident terahertz wave. This regulation capability not only optimizes the propagation characteristics of terahertz waves, such as increasing penetration depth and reducing energy loss, but also achieves the extension of propagation distance by changing the propagation path. The combination of metasurface materials and terahertz devices provides a solution to overcome a series of challenges of terahertz devices. Specifically, metasurface materials can be used as enhancers or resonant cavities in the terahertz frequency band, thereby significantly enhancing the output power of the terahertz source and ensuring the stability of the signal by reducing the scattering and absorption of the wave. In addition, by designing a specific metasurface structure, it is possible to achieve efficient capture and focusing of terahertz waves, thereby improving the signal-to-noise ratio and sensitivity of the detector. Not only that, metasurface materials also demonstrate great potential in the design of new terahertz devices. It can be used to manufacture polarization converters, modulators, filters, waveguides, and absorbers, which have the advantages of smaller size, ultra-thinness, higher efficiency, and wider operating frequency band. More importantly, the design of metasurface materials can be optimized for specific application environments, effectively reducing the impact of environmental factors on terahertz wave transmission and detection. For example, in complex environments, the metasurface can adjust its structure to adapt to different temperature and humidity conditions to ensure the stable operation of terahertz devices. In addition, the dynamic regulation of the metasurface makes it possible to adjust the real-time performance of terahertz devices, making the system more flexible and further enhancing its practicality and functionality.

[0032] As the electromagnetic environment becomes increasingly complex, realizing high-performance terahertz filters to reduce the system's signal attenuation and suppress various interference signals is one of the keys to promoting the development and application of terahertz devices.

[0033] In the related art, most terahertz devices are processed on the surface of solid wafers. Although the introduction of microstructures increases the design freedom of the structure, it also makes the working characteristics of the terahertz device itself restricted by the properties of the solid wafer, such as fragility and small effective area. In practical applications, the device may be damaged due to severe impact; some bandpass and low-pass filter designs, although achieving relatively good results, these filters are not robust under changes in incident polarization, incident angle and degree of bending; the filter design of a single-layer metal hole structure, although not fragile and has polarization-independent characteristics, its single-layer microstructure design brings limitations to the designability of the filtering function, such as the filtering boundary is not steep enough, the filtering bandwidth and depth cannot be independently controlled, etc., and the single-layer metal hole structure can only design bandpass filters, not low-pass filters.

[0034] In view of this, an embodiment of the present disclosure provides a terahertz filter based on a multi-layer metasurface, comprising M layers of composite metasurfaces and M-1 layers of COC films, wherein each layer of the M-1 layers of COC films is located between two adjacent layers of composite metasurfaces, each layer of the composite metasurface comprises a first layer of PI film, a metasurface and a second layer of PI film, the metasurface is composed of multiple metal microstructures, the multiple metal microstructures are periodically and uniformly arranged, the metasurface is located between the first layer of PI film and the second layer of PI film, and M is a positive integer greater than 1.

[0035] Figure 1 A schematic diagram of a terahertz filter based on a multilayer metasurface according to an embodiment of the present disclosure is schematically shown.

[0036] like Figure 1 As shown, the terahertz filter 100 based on the multi-layer metasurface includes M layers of composite metasurface 110 and M-1 layers of COC film 120, wherein each layer of the M-1 layers of COC film 120 is located between two adjacent layers of composite metasurface 110, and M is a positive integer greater than 1.

[0037] COC film uses COC material, which is usually copolymerized by cycloolefin and other types of monomers (such as olefins or alkanes). COC material has excellent heat resistance, chemical stability and surface flatness, and COC material has high transparency, and has a very low loss in the visible light, ultraviolet light and terahertz bands (the loss at 1 THz is less than 0.3cm −1 ), has a constant refractive index in a broadband range and is particularly suitable as a spacer for terahertz devices.

[0038] Figure 2 A cross-sectional view of a composite metasurface according to an embodiment of the present disclosure is schematically shown.

[0039] like Figure 2As shown, each layer of the composite metasurface 200 includes a first layer of PI film 210, a metasurface 220 and a second layer of PI film 230, and the metasurface 220 is located between the first layer of PI film 210 and the second layer of PI film 230. Each layer of the metasurface 220 is composed of multiple metal microstructures, and the multiple metal microstructures are periodically and uniformly arranged.

[0040] According to the embodiments of the present disclosure, the first layer of PI film and the second layer of PI film use PI material as a substrate to support the metal structure. PI material has excellent thermal and chemical stability, mechanical properties and electrical insulation properties. This material usually exists in the form of a linear polymer, and its characteristics include high strength, high modulus and high glass transition temperature. Therefore, the supersurface device processed on the PI film can still maintain its performance in complex mechanical environments such as stretching, bending or folding, and can protect the integrity of the structure in subsequent processing. The supersurface is sandwiched together by two layers of PI films to form a composite supersurface, and multiple metal microstructures are periodically and evenly arranged to form the supersurface. The first layer of PI film, the supersurface and the second layer of PI film together constitute a composite structure of PI-supersurface-PI sandwich, that is, a composite supersurface.

[0041] According to an embodiment of the present disclosure, the composite metasurface and the COC thin film are placed alternately to form a relatively compact multi-layer thin film metasurface terahertz filter.

[0042] According to the embodiments of the present disclosure, the filter of the present disclosure is realized by a method of alternating cascade of multi-layer composite metasurface and COC film. The metasurface in each layer of the composite metasurface is composed of multiple metal microstructures, and the multiple metal microstructures are arranged uniformly and periodically. The multi-layer metasurface can increase the adjustment freedom of the filter and can also achieve more filtering effects. The use of PI film as a flexible substrate for the metasurface can enable the device to have stretchable, bendable and foldable properties. The metasurface device is designed in a thin film material with high flexibility and stability, so that the device is durable and more compact; the COC film has high transparency and high Abbe number, excellent optical properties, small dielectric constant and dielectric tangent, and low loss to terahertz frequency signals.

[0043] Figure 3 A schematic diagram of the projection of the supersurface in the first layer of PI film according to an embodiment of the present disclosure is schematically shown.

[0044] According to an embodiment of the present disclosure, a terahertz filter based on a multilayer metasurface may include a low-pass filter and a band-pass filter. In the case where the filter is a low-pass filter, such as Figure 3 As shown in a, the projection shape of the super surface in each composite super surface in the first layer of PI film is a square array, and the projection shape of the metal microstructure in the first layer of PI film is a square in the square array. In the case where the filter is a bandpass filter, as shown in Figure 3 As shown in middle b, the projection shape of the supersurface in each composite supersurface in the first layer of PI film is an array of squares and square rings, and the projection shape of the metal microstructure in the first layer of PI film is squares and square rings in the array of squares and square rings.

[0045] For low-pass filters, a broadband band-stop design method can be adopted. By designing the position and width of the passband and the band-stop, the low-pass function can be realized within the operating frequency range (the passband position is at low frequency, and the stopband position is at high frequency). The transmission characteristics of general metal microstructures are manifested as a resonance valley, mostly in a V-shape. From the filtering effect, it can be regarded as a band-stop filter, but it does not conform to the optimal step-shaped filtering boundary. Therefore, the present disclosure introduces a design scheme of multi-layer metasurfaces (composed of periodically arranged metal microstructures) and multi-layer thin film cascades, and utilizes the coupling of interlayer structures and the multiple reflections brought by the interlayer thin film optical cavity to widen the resonance, on the one hand increasing the range of the band-stop, and on the other hand increasing the speed of the boundary amplitude change. In order to ensure that the filtering effect has a polarization-independent response under normal incidence, the specific metal microstructure will adopt an isotropic structure, such as Figure 3 As shown in a, the projection shape of the metasurface in the first layer of PI film is a square array (it can also be designed as a circle). Then it is stacked in multiple layers in the vertical direction, and the design effect is optimized by designing the size (side length, thickness, etc.) of the metal microstructure in the metasurface, the periodic arrangement (square lattice, triangle lattice, hexagonal lattice, etc.), the thickness and number of layers of the film material, etc., so as to achieve the filtering index.

[0046] For the bandpass filter, its design concept is the same as that of the bandstop filter, but the specific design must adopt a structure that is complementary to the bandstop filter, that is, a metal hole structure. According to the Babinet principle, the transmission effects of the two complementary metal structures are complementary in the spectrum, so the transmission spectrum of the metal hole structure is a transmission peak, but the transmission peak of the single-layer metasurface is mostly Λ-shaped, the passband transmittance varies greatly, and the filtering boundary is not steep enough. The present disclosure also adopts a design scheme of alternating cascades of multi-layer metasurfaces and multi-layer thin films, and utilizes the coupling of the interlayer structure and the multiple reflections brought by the interlayer thin film optical cavity to increase the bandwidth of the passband. In order to ensure the polarization-independent characteristics of the bandpass filter under normal incidence, the present disclosure will also adopt an isotropic metal microstructure design scheme, and its structural schematic diagram is shown as follows. Figure 3 As shown in b.

[0047] According to the embodiment of the present disclosure, a multi-layer thin film multi-beam interference model is used to calculate the transmission spectrum of the terahertz filter of the multi-layer metasurface, and the non-uniform composite metasurface composed of the PI-metasurface-PI composite layer is equivalent to a uniform composite metasurface of the same thickness, and the definition is To characterize the effective complex refractive index of the equivalent uniform composite metasurface:

[0048] (1)

[0049] in, is the thickness of the equivalent uniform composite metasurface, is the wave number, and For non-uniform composite metasurfaces Parameters, then the terahertz filter consists of a refractive index of The equivalent uniform composite metasurface and refractive index are The COC films are composed of alternating cascades.

[0050] The transmission spectrum of a thin film system can be calculated using the transfer matrix method. It can be defined as

[0051] (2)

[0052] and are the complex electric field and complex magnetic field at the incident interface of the terahertz filter, and are the complex electric field and complex magnetic field at the other interface respectively. The transfer matrix of the homogeneous film connecting the two interface electric fields of the film is defined as:

[0053] (3)

[0054] in, and Respectively The complex refractive index and thickness of the layer, for a uniform COC film, is its refractive index, for the non-uniform PI-metasurface-PI composite layer, is its effective refractive index, For light and The angle between the normal lines of the layer interface is given by Snell's law and the angle of incidence It is calculated that, under normal incidence, , is the impedance of the layer.

[0055] For a terahertz filter containing a total of M layers of composite metasurfaces, the transfer matrix of the entire terahertz filter is the continuous product of the transfer matrices of each layer: The transmission coefficient of the terahertz filter is:

[0056] (4)

[0057] Among them, A, B, C, and D simply represent the The corresponding matrix elements are, is the air impedance .

[0058] According to an embodiment of the present disclosure, the material of the metal microstructure in the metasurface is aluminum. The frequency band range of the filter is 0.1-1 THz.

[0059] Metal microstructures will show resonance characteristics under external field excitation, and different resonance modes have different radiation characteristics and different coupling characteristics. Analyzing these characteristics will help to deeply understand and design the response of terahertz devices. For the design of terahertz filters based on multilayer metasurfaces, the coupling effect of interlayer structures and the multiple reflections brought by the thin film optical cavity between layers are usually utilized.

[0060] The coupled mode theory model is as follows:

[0061] (5)

[0062] in, Indicates the metal microstructure The complex amplitude of each resonant mode, Indicates The resonant frequency of each resonant mode, Indicates The intrinsic (absorption) loss of each resonant mode, Indicates The radiation loss of the resonant mode is and Respectively represent from The complex amplitude of the electromagnetic field incident and emitted from the port, Indicates and The coupling coefficient between the resonant modes, Indicates that from The field incident on the first port is The coupling coefficient of the resonant mode coupling is, Indicates The radiation field of the first resonant mode is The coupling coefficient from the channel to the far field is It represents the relationship between the transmitted field or reflected field and the incident field when there is no metal microstructure.

[0063] Using this model, the specific expressions of transmission response and reflection response can be calculated, namely: and By fitting the transmission and reflection spectra obtained by simulation and measurement, the characteristic parameters of each mode can be obtained, revealing the interaction process therein.

[0064] The multi-beam interference theoretical model is as follows:

[0065] (6)

[0066] in, , denote the transmission coefficients of the upper and lower structural interfaces, respectively. , Represent the reflection coefficients of the upper and lower structure interfaces, It represents the accumulated phase of the electromagnetic wave propagating one way between the two layers. The transmission and reflection responses of each interface microstructure are obtained by simulation or theoretical calculation, and the final transmission response can be calculated by substituting them into formula (6). The reflection response can also be obtained in a similar way.

[0067] The projection shape of the metasurface of the low-pass filter in the first layer of PI film of the disclosed embodiment is a square array, and the projection shape of the metasurface of the band-pass filter in the first layer of PI film is a square and square ring combination array, both of which are isotropic structures, and the final filter is a polarization-independent filter. Changing the incident angle of the electromagnetic wave and the bending degree of the device within a certain range will not have a significant impact on the filtering effect, and the designed filter has high robustness.

[0068] The terahertz filter designed in the embodiment of the present disclosure adopts a design method of alternating cascade of multi-layer composite metasurfaces and COC films. Due to the design of the multi-layer structure, the overall performance of the filter will be improved to a certain extent, such as a steeper falling edge and more obvious out-of-band suppression. More layers of metasurfaces mean more adjustment freedom, which also means that more filtering effects can be achieved; and because the film thickness is small, the design of the multi-layer metasurface will not increase the device thickness too much due to the increase in the number of layers, and it is robust to smaller processing size errors.

[0069] The disclosed embodiment arranges the metal microstructures in the metasurface periodically in the PI film, together forming a PI-metasurface-PI sandwich composite structure, i.e., a composite metasurface. The PI film has excellent properties such as stable chemical properties, high temperature resistance, low temperature resistance, radiation resistance, and good mechanical flexibility. The use of the PI film as a flexible substrate for the metasurface can enable the device to have stretchable, bendable, and foldable properties. The metasurface device is designed in a thin film material with high flexibility and stability, making the device more compact while being durable.

[0070] The disclosed embodiment uses COC film as a spacer in the terahertz region, and the COC film has a constant refractive index in a wide band. The COC film has high transparency and high Abbe number, and excellent optical properties; small dielectric constant and dielectric tangent values, and extremely low loss of terahertz frequency signals; good heat resistance, with a heat deformation temperature above 100 ° C; almost no water absorption, with very high water vapor barrier properties; good resistance to water-soluble chemicals, acids, alkalis and polar organic substances, and can withstand various sterilization methods.

[0071] The terahertz filter proposed in the embodiment of the present disclosure has a large area. When emitting terahertz waves, the larger the area of ​​the terahertz beam, the better the directivity; when receiving terahertz waves, the larger the filter aperture, the more terahertz signals can be collected. The frequency band range of the terahertz filter proposed in the embodiment of the present disclosure is 0.1-1 THz. Due to the large frequency band range, the filter adopts cascade technology. When the cutoff frequency of the low-pass filter and the center frequency of the band-pass filter are low, the period of the unit structure will be relatively large, and diffraction will occur at high frequencies, and the suppression effect of remote harmonics and spurious passbands is not ideal. A low-pass filter with a cutoff frequency at a higher frequency (such as a cutoff frequency of about 0.6 THz) can eliminate the influence of remote harmonics and spurious passbands. The cascade of bandpass and low-pass filters can improve the overall performance to a certain extent, such as improving high-frequency harmonic suppression and high-frequency selectivity.

[0072] According to the terahertz filter based on the multilayer metasurface in the embodiment of the present disclosure, a low-pass filter with cutoff frequencies of 0.22 THz and 0.35 THz and a band-pass filter with center frequencies of 0.22 THz and 0.35 THz are designed.

[0073] In the filter of the embodiment of the present disclosure, in the composite metasurface, the thickness of the metal microstructure in the metasurface is 195-205 nanometers, the thickness of the first PI film is 9-11 microns or 15-17 microns, the thickness of the second PI film is 9-11 microns or 15-17 microns; the thickness of the COC film is 49-51 microns or 79-81 microns. Those skilled in the art can choose other sizes as needed.

[0074] First, a low-pass filter with a cutoff frequency of 0.22 THz is introduced.

[0075] 0.22 THz is a low frequency in the terahertz frequency range of 0.1-1 THz, so the metal microstructure will interact with the terahertz electromagnetic wave only at a larger period. However, the larger the period of the metal microstructure, the more obvious the diffraction phenomenon at high frequencies. In order to reduce the diffraction effect and ensure that the low-pass filter has a good filtering effect at 0.1-1 THz, a composite metasurface composed of six layers of PI-metasurface-PI and five layers of COC film are alternately cascaded. The metal microstructure that makes up the metasurface is Figure 3 The square shown in a in the middle increases the bandwidth of the passband by using the coupling between the interlayer structures and multiple reflections between the layers. However, the period of the metal microstructure will be different. The metal microstructure with a larger period is to achieve a low-pass filtering effect with a cutoff frequency of 0.22 THz, and the metal microstructure with a smaller period is to suppress the diffraction effect at high frequencies. The metal microstructure with a larger period uses the boundary of the low pass to "cut" the high-frequency part of the low-pass filter with a cutoff frequency of 0.22 THz. The final effect further sharpens the boundary change on the right side of the passband, and also further suppresses the transmittance of the stopband, thereby improving the filtering effect. By establishing a structural model in the electromagnetic simulation software, changing the size of the metal microstructure (square), the thickness and period size of the PI film, and the thickness and period size of the COC film, a parameter scan is performed, and the transmission amplitude spectrum is observed in the range of 0.1-1 THz.

[0076] The simulation process uses a frequency domain solver, and first sets the grid for discretization of the actual space. Frequency domain simulation generally sets the x and y directions as Unit Cell boundary conditions to describe a large range of microstructure arrays, and the z direction is set as an open boundary for feeding in and out electromagnetic waves. In order to fit the actual situation, that is, the radiation source and the detector are a certain distance away from the structure, it is necessary to set an air spacing in the propagation direction of the metasurface. The spacing value of 300 μm is comparable to the wavelength of the frequency band of interest. Floquent ports are used as detection and excitation sources, distributed at both ends of the structure in the z direction, and the excitation source is selected as the port at either end; at the same time, due to the four-fold rotational symmetry of the design of the metal microstructure, the filtering effect is polarization-independent, and the excitation mode is longitudinal electric field (TE) polarized incidence; for transmission filters that do not involve polarization conversion functions, the amplitude transmittance curve can be obtained by observing the results of the scattering parameters (Szmax, Szmin). A structural model is established in the electromagnetic simulation software. In order to achieve a filtering effect with a cutoff frequency of 0.22 THz, the metal microstructure period in the first three layers of the composite metasurface is set to (The period in both the x and y directions is P), the side length L of the square metal microstructure in the i-th (i=1, 2, 3) layer of the composite metasurface iThe length of the metal microstructures in the last three layers of the composite metasurface is set to 150 μm and 350 μm respectively in steps of 30 μm. , the side length L of the square metal microstructure in the i-th (i=4, 5, 6)-th layer of the composite metasurface i The thickness of the metal microstructure (aluminum, conductivity 3.56×10 7 The thickness of the 100 μm (S / m) film is set to 200 nm, the thickness of the two PI films is set to 5 μm, and the thickness of the COC film is set to 50 μm or 80 μm. After setting the structural parameters, perform parameter scanning. Finally, in the results of a series of parameter scanning, select the structure corresponding to the amplitude transmittance spectrum line that best meets the target.

[0077] Figure 4 The schematic diagram of the amplitude transmittance spectrum of the terahertz filter simulation according to the embodiment of the present disclosure is shown schematically. The amplitude transmittance of the low-pass filter simulation with a cut-off frequency of 0.22 THz is as follows: Figure 4 As shown in (a), the corresponding structural parameters are: L1=300 μm, L2=210 μm, L3=330 μm, L4=140 μm, L5=160 μm, L6=160 μm, the thickness of the two layers of PI film is 5 μm, and the thickness of the COC film is 50 μm.

[0078] Next, a low-pass filter with a cutoff frequency of 0.35 THz is introduced.

[0079] The design concept is the same as that of the low-pass filter with a cutoff frequency of 0.22 THz. In order to reduce the diffraction effect and ensure that the low-pass filter has a good filtering effect at 0.1-1 THz, the same design method of alternating cascade of a composite metasurface composed of six layers of PI-metasurface-PI and five layers of COC film is adopted. The metal microstructure that constitutes the metasurface adopts Figure 3 The square shown in a in the middle increases the bandwidth of the passband by using the coupling between the interlayer structures and multiple reflections between the layers. 0.35 THz is a low frequency in the range of 0.1-1 THz, so the metal microstructure will interact with the terahertz electromagnetic wave only at a larger period. However, the larger the period of the metal microstructure, the more obvious the diffraction phenomenon at high frequencies. In order to reduce the diffraction effect and ensure that the low-pass filter has a good filtering effect at 0.1-1 THz, multiple layers need to be cascaded. Figure 3The square metal microstructure shown in a. However, the period of the metal microstructure will be different. The metal microstructure with a larger period is to achieve a low-pass filtering effect with a cutoff frequency of 0.35 THz, and the metal microstructure with a smaller period is to suppress the diffraction effect at high frequencies. The metal microstructure with a larger period uses the boundary of the low pass to "cut" the high-frequency part of the low-pass filter with a cutoff frequency of 0.35 THz. The final effect further sharpens the boundary change on the right side of the passband, and also further suppresses the transmittance of the stopband, thereby improving the filtering effect.

[0080] The simulation model is set up the same as the filter with a cutoff frequency of 0.22 THz. The structural model is established in the electromagnetic simulation software, and the period of the metal microstructure in the first three layers of the composite metasurface is set to , the side length L of the square metal microstructure in the i-th (i=1, 2, 3)-th layer of the composite metasurface i The period of the metal microstructure in the last three layers of the composite metasurface is set to , the side length L of the square metal microstructure in the i-th (i=4, 5, 6)-th layer of the composite metasurface i The thickness of the metal microstructure (aluminum, conductivity 3.56×10 7 The thickness of the PI film (S / m) was set to 200 nm, the thickness of the two PI films was set to 5 μm, and the thickness of the COC film was set to 50 μm or 80 μm. After setting the structural parameters, parameter scanning was performed.

[0081] Finally, in the results of a series of parameter scans, the structure corresponding to the amplitude transmittance spectrum line that best meets the target is selected. The selected amplitude transmittance spectrum line is as follows Figure 4 As shown in (b), the corresponding structural parameters are: L1=180 μm, L2=210 μm, L3=240 μm, L4=160 μm, L5=160 μm, L6=140 μm. The thickness of the two layers of PI film is 5 μm, and the thickness of the COC film is 50 μm.

[0082] Next, a bandpass filter with a center frequency of 0.22 THz is introduced.

[0083] 0.22 THz is a low frequency in the range of 0.1-1 THz, so the metal microstructure will interact with the terahertz electromagnetic wave only at a larger period. However, the larger the period of the metal microstructure, the more obvious the diffraction phenomenon at high frequencies. In order to reduce the diffraction effect and ensure that the bandpass filter has a good filtering effect at 0.1-1 THz, this embodiment adopts a design method of alternating cascade of a composite metasurface composed of three layers of PI-metasurface-PI and two layers of COC film. The metal microstructure that constitutes the metasurface adopts Figure 3 The square and square ring combination shown in b increases the bandwidth of the passband by using the coupling between the interlayer structures and multiple reflections between the layers. In order to reduce the diffraction effect at high frequencies, a low-pass filter with a higher cutoff frequency is cascaded on this basis, which includes a composite metasurface composed of three layers of PI-metasurface-PI and two layers of COC film. The metal microstructure that makes up the metasurface is made of Figure 3 The square shown in a. The period of the metal microstructure in the two filters is different. Figure 3 The bandpass filter composed of the metal microstructure shown in b is designed to achieve a bandpass filtering effect with a center frequency of 0.22 THz. Figure 3 The low-pass filter composed of the metal microstructure shown in a is designed to suppress the diffraction effect at high frequencies. The final bandpass filter structure adopts a design method of alternating cascade of six layers of PI-metasurface-PI composite metasurface and five layers of COC film. The bandpass filter and the low-pass filter are cascaded, and the high-frequency part of the bandpass is "cut" using the boundary of the low-pass. The final effect is to further sharpen the boundary change on the right side of the passband, and at the same time, the transmittance of the stopband is further suppressed, thereby improving the filtering effect.

[0084] The simulation model is set up the same as the filter with a cutoff frequency of 0.22 THz. The structural model is established in the electromagnetic simulation software, and the metal microstructure period in the first three layers of the composite metasurface is set to , the side length L of the square metal microstructure in the i-th (i=1, 2, 3)-th layer of the composite metasurface i The same structural parameters are set, and the length of the inner side g of the metal square ring is gradually changed from 150 μm to 300 μm in steps of 30 μm. The length of the inner side g of the metal square ring is gradually changed from 300 μm to 450 μm in steps of 10 μm. The metal microstructure period in the latter three-layer composite metasurface is set to , the side length L of the square metal microstructure in the i-th (i=4, 5, 6)-th layer of the gold composite metasurface i The thickness of the metal microstructure (aluminum, conductivity 3.56×10 7The thickness of the PI film (S / m) was set to 200 nm, the thickness of the two PI films was set to 8 μm, and the thickness of the COC film was set to 50 μm or 80 μm. After setting the structural parameters, parameter scanning was performed.

[0085] Finally, in the results of a series of parameter scans, the structure corresponding to the amplitude transmittance spectrum line that best meets the target is selected. The selected amplitude transmittance spectrum line is as follows Figure 4 As shown in Figure c, the corresponding structural parameters are: L1=L2=L3=223 μm, g=445 μm, L4=191 μm, L5=223 μm, L6=191 μm, the thickness of the two layers of PI film is 8 μm, and the thickness of the COC film is 80 μm.

[0086] Finally, a bandpass filter with a center frequency of 0.35 THz is introduced.

[0087] 0.35 THz is a low frequency in the range of 0.1-1 THz, so the metal microstructure will interact with the terahertz electromagnetic wave only at a larger period. However, the larger the period of the metal microstructure, the more obvious the diffraction phenomenon at high frequencies. Referring to the design method of a bandpass filter with a center frequency of 0.22 THz, this embodiment is also a bandpass filter composed of a composite metasurface of three layers of PI-metasurface-PI and two layers of COC film, and a low-pass filter with a higher cutoff frequency composed of a composite metasurface of three layers of PI-metasurface-PI and two layers of COC film is cascaded.

[0088] The simulation model is set up the same as a low-pass filter with a cutoff frequency of 0.22 THz. The structural model is established in the electromagnetic simulation software, and the metal microstructure period in the first three layers of the composite metasurface is set to , the side length L of the square metal microstructure in the i-th (i=1, 2, 3)-th layer of the composite metasurface i The same structural parameters are set, and the step size is gradually changed from 100 μm to 200 μm in 20 μm steps. The size of the inner side length g of the metal square ring is gradually changed from 200 μm to 290 μm in 10 μm steps. The metal microstructure period of the last three layers of the composite metasurface is set to The side length Li of the square metal microstructure of the i-th (i=4, 5, 6)-th layer of the composite metasurface changes gradually from 100 μm to 140 μm in steps of 20 μm. The metal microstructure (aluminum, conductivity 3.56×10 7 The thickness of the PI film (S / m) was set to 200 nm, the thickness of the two PI films was set to 5 μm, and the thickness of the COC film was set to 50 μm or 80 μm. After setting the structural parameters, parameter scanning was performed.

[0089] Finally, in the results of a series of parameter scans, the structure corresponding to the amplitude transmittance spectrum line that best meets the target is selected. The selected amplitude transmittance spectrum line is as follows Figure 4 As shown in (d), the corresponding structural parameters are: L1=L2=L3=140 μm, g=280 μm, L4=120 μm, L5=140 μm, L6=120 μm, the thickness of the two layers of PI film is 5 μm, and the thickness of the COC film is 50 μm.

[0090] Figure 5 The flowchart of the method for manufacturing the filter according to the embodiment of the present disclosure is schematically shown.

[0091] like Figure 5 As shown, the method includes operations S510 to S520.

[0092] In operation S510, a first PI film layer, a supersurface, and a second PI film layer are sequentially formed on a substrate to obtain a composite supersurface, wherein the supersurface is composed of a plurality of metal microstructures, and the plurality of metal microstructures are periodically and uniformly arranged.

[0093] In operation S520, M layers of composite metasurface and M-1 layers of COC film are thermally pressed at a preset temperature to obtain a terahertz filter, wherein each layer of the M-1 layers of COC film is located between two adjacent layers of the composite metasurface, and M is an integer greater than 1.

[0094] According to the embodiments of the present disclosure, the COC film serves as the middle layer, and the upper and lower layers are respectively composite supersurfaces. The composite supersurface is composed of a supersurface sandwiched between two layers of PI films, and the supersurface is composed of multiple periodically arranged metal microstructures. The glass transition temperature of the COC material is 185°C. At this temperature, the COC material will be in a molten state and have a certain viscosity, which can tightly fix each layer of PI film. The melting point of the PI material is much higher than 185°C. At this temperature, the PI film will not undergo morphological changes, thereby ensuring the integrity of the metal microstructure. Therefore, 200°C can be selected as the hot pressing temperature for hot pressing the M-layer composite supersurface and the M-1-layer COC film.

[0095] According to the embodiments of the present disclosure, the composite metasurface and the COC film are placed alternately to form a relatively compact multi-layer thin film metamaterial. From a processing perspective, PI material is easier to process into a stable and high-precision metal structure than COC material, but from a loss perspective, it is significantly higher than COC material. This cross-layered design can reduce the impact of material loss while ensuring processing accuracy (COC thickness is greater than PI). Therefore, the embodiments of the present disclosure use COC material as a spacer. The composite metasurface composed of PI-metasurface-PI and the COC film are placed alternately and stacked precisely, and a relatively dense multi-layer thin film metamaterial is formed by hot pressing technology, and finally a terahertz filter consisting of a composite metasurface of M layers of PI-metasurface-PI and M-1 layers of COC film is obtained.

[0096] According to the embodiments of the present disclosure, a first layer of PI film, a super surface and a second layer of PI film are sequentially formed on a substrate to obtain a composite super surface, including: evenly coating PI gel on the substrate to form a first layer of PI film; forming a metal film by vacuum thermal evaporation on the first layer of PI film; evenly coating photoresist on the metal film, removing the metal area outside the metal microstructure on the metal film by exposure, development and etching, and then washing the surface of the metal film with acetone solution, isopropanol and deionized water to obtain a super surface; evenly coating PI gel on the super surface to form a second layer of PI film, and finally obtaining a composite super surface. According to the embodiments of the present disclosure, the size of terahertz band devices is generally in the micron range, so the device processing is usually completed using photolithography technology that has been widely used in semiconductor manufacturing processes. Photolithography technology is a processing technology that combines photographic replication and chemical etching processes to transfer pre-designed patterns on the mask to the substrate using light of a specific wavelength.

[0097] According to an embodiment of the present disclosure, the metal microstructure constituting the super surface can be made of 200 nm thick aluminum through vacuum thermal evaporation, photolithography and etching processes. The specific steps can be referred to Figure 6 .

[0098] Figure 6 The figure schematically shows a processing flow diagram of a terahertz filter according to an embodiment of the present disclosure.

[0099] like Figure 6As shown, the first time PI gel is evenly coated: the substrate and PI film are pre-treated. Since the PI film is a flexible material, a solid block crystal substrate is required to provide support for it. In this embodiment, a silicon wafer is selected as a rigid support for the film. The preparation of the PI film uses PI gel as a precursor material. In order to obtain a uniform thickness, a glue spreader is required for spin coating. Adsorb the silicon wafer on the tray of the glue spreader, pour an appropriate amount of PI gel onto the surface of the silicon wafer, set the rotation speed and time to determine the thickness of the PI film adhered to the silicon wafer, and then place the silicon wafer on a hot plate. Use a step heating program to gradually increase the temperature so that the temperature is slowly and evenly transferred to ensure that the material achieves optimal performance at an appropriate cross-linking level, thereby obtaining a dense PI film.

[0100] Evaporation: Vacuum thermal evaporation is a surface coating process carried out in a vacuum environment. By heating to the evaporation temperature of the material, it is converted into a gaseous state and deposited on the surface of the substrate. This method is often used to prepare thin films, such as metal, oxide or other compound films, to change the surface characteristics of the substrate or increase its function. All filters in this embodiment use metal aluminum as the coating material. The heat generated by the heater resistor of the thermal evaporation machine after power is turned on makes the atoms of the aluminum evaporation material obtain sufficient kinetic energy and evaporate. The evaporation rate is controlled by controlling the temperature. The specific operation process is as follows: the silicon wafer with PI film obtained above is pasted on the tray of the thermal evaporation machine, and the tray is fixed in the vacuum chamber; the aluminum particles to be deposited are loaded into the evaporation source located in the vacuum chamber. The number of aluminum particles depends on the thickness of the evaporated metal film. The embodiment of the present disclosure uses a tungsten boat crucible evaporation source; the air in the operating chamber is evacuated to a lower pressure to avoid interference from gas molecules, thereby ensuring the uniformity of the deposited aluminum film; the aluminum particles in the tungsten boat are heated to their evaporation temperature. After evaporation, the aluminum particles form gaseous molecules, diffuse in the vacuum chamber, and finally deposit on the surface of the substrate; the air in the operating chamber is pressurized to restore to atmospheric pressure, the chamber door is opened, and the sample tray is taken out.

[0101] Spreading photoresist, heating and drying, exposure and development: Use a spreader to spread photoresist on the evaporated substrate. The related operations are similar to the spread process of PI gel. Considering factors such as the viscosity of the photoresist, the size of the substrate and the evaporation rate of the solvent, reasonable parameters are adjusted to ensure that the thickness of the photoresist after spread is about 3 μm. In order to make the polymer matrix in the photoresist more cross-linked, enhance its mechanical strength and stability, and thus help to keep the shape and size of the pattern unchanged in subsequent process steps such as development, heat curing operations are also required after spread. After the substrate surface returns to room temperature, it is transferred to the photolithography machine together with the mask, and the appropriate exposure time is set according to the performance parameters of the photolithography machine and the properties of the photoresist for exposure. The light-transmitting part of the mask will change the characteristics of the photoresist in the corresponding area, thereby leaving pattern information on the photoresist surface. After exposure, a development operation is performed, and the pattern on the mask is transferred to the photoresist during exposure. The exposed photoresist area will undergo chemical changes, causing the chemically changed photoresist to dissolve under the action of the developer, exposing the metal Al outside the metal microstructure, and a layer of photoresist is attached to the designed metal microstructure.

[0102] Etching and rinsing photoresist: The sample to be etched is immersed in an etching solution to dissolve the metal coating exposed to the air. Since the photoresist does not react with the etching solution, the metal Al not covered by the photoresist can be selectively etched away, and the metal microstructure covered by the photoresist will be retained. The etching solution selected in the embodiment of the present disclosure is made in a mixed solution of strong acid, weak acid and deionized water. Its chemical mechanism is to use a strong acid to undergo a replacement reaction with aluminum to form aluminum oxide, and then use a weak acid and deionized water to remove the aluminum oxide. When it is observed that the metallic luster on the surface of the metal layer placed in the etching solution completely disappears, the substrate is taken out. The photoresist on the surface of the metal microstructure is dissolved with acetone, and then rinsed with isopropyl alcohol and deionized water in sequence to finally obtain the designed metal microstructure.

[0103] Second PI gel coating: The second PI gel coating process is the same as the first. The etched silicon wafer is adsorbed on the tray of the gel coating machine and PI gel is coated on the metal microstructure surface again, so that the metal microstructure is wrapped between two layers of PI film. After thermal curing, the composite metasurface composed of PI-metasurface-PI can be torn off the silicon wafer, and the processing of one layer of the composite metasurface of the filter can be completed. According to the above process, the PI-metasurface-PI composite structure of each layer of the filter can be obtained according to the structural parameters of each filter design.

[0104] The composite metasurface and COC film are pressed together by hot pressing technology: Hot pressing technology is a process that uses a press and heating equipment to combine materials under high temperature and high pressure conditions. The principle is to use high temperature to promote plastic deformation and interatomic diffusion of materials. Hot pressing is usually carried out at a higher temperature, which helps the plastic deformation and interatomic diffusion of materials, while the high pressure environment helps to ensure close contact between materials. The combined effect of the two is to increase the synthetic strength of atoms in the material.

[0105] Through Figure 6 After the steps shown, a composite metasurface can be obtained. The composite metasurface composed of M layers of PI-metasurface-PI and M-1 layers of COC film are stacked in sequence according to the designed order, compacted tightly, and the air gaps between the layers of film are squeezed out to ensure the surface flatness of the filter. The filter is placed in a mold for fixing, and then placed in a blast drying oven for heating for more than 60 minutes. The stack is cooled at room temperature to obtain a filter device. The present disclosure selects hot pressing technology to prepare a multi-layer thin film metasurface filter device with strong robustness. Multiple single-layer thin film metasurfaces can be made into filters using only vacuum hot pressing. Figure 7 The schematic diagram schematically shows a principle diagram of an experimental test system according to an embodiment of the present disclosure.

[0106] like Figure 7 As shown in the figure, the experimental test system is a self-built time-domain spectroscopy system based on a photoconductive antenna. A femtosecond pulse laser with a repetition frequency of 80 MHz and a central wavelength of 800 nm is emitted by a titanium sapphire laser. After passing through a beam splitter, the femtosecond laser is divided into two beams, one of which (called the excitation pulse) is incident on the transmitting end of the system optical path. The femtosecond laser is collimated and focused on two parallel metal wires of the semiconductor substrate (low-temperature grown gallium arsenide crystal) of the photoconductive antenna, thereby exciting carriers. By applying a bias voltage of 70 V to the metal wires of the substrate, the carriers accelerate and radiate terahertz waves with a duration of picoseconds. The terahertz wave is focused by two off-axis parabolic mirrors to the placement area of ​​the terahertz filter to be tested, contacts the terahertz filter to be tested to carry information, and then passes through two parabolic mirrors to focus the terahertz wave on the photoconductive antenna at the receiving end. The other beam (called the measurement pulse) enters the time delay device. A displacement platform equipped with optical elements such as reflectors, corner cubes or hollow reflectors can be used as an optical delay line to change the time when the laser pulse reaches the receiving device relative to the terahertz pulse. By allowing the optical delay line to move continuously within a certain range, the electrical signal is measured as a function of the time delay, which is equivalent to the time-domain electric field of the terahertz pulse. After passing through the optical delay line, the femtosecond laser is collimated and focused onto the photoconductive antenna at the receiving end, and photogenerated carriers are generated on the substrate of this antenna. At this time, the photogenerated carriers move in a directional manner driven by the electric field of the terahertz wave to form a current. The acquired current signal is amplified by a current amplifier with an amplification factor of 10 7times, using a phase-locked amplifier to improve the signal-to-noise ratio of the electrical signal, and using a self-programmed program to present the collected time domain and frequency domain signals on the computer screen, thus realizing the whole process of femtosecond laser scanning the terahertz wave point by point in the time domain.

[0107] For the terahertz thin film low-pass filters with cutoff frequencies of 0.22 THz and 0.35 THz, and the terahertz thin film band-pass filter samples with center frequencies of 0.22 THz and 0.35 THz, the terahertz transmission time domain signals in their effective areas, the measured data and their processing are as follows:

[0108] For low-pass filters with cutoff frequencies of 0.22 THz and 0.35 THz, the terahertz wave transmission time domain signals at 9 randomly selected positions on each filter plane were measured, and the time domain signal of the terahertz wave propagating in the air was used as a reference. The time domain signal time length was 30 ps. The measured time domain signal was Fourier transformed to obtain the corresponding frequency domain information, and the amplitude information after Fourier transformation was taken. At each frequency position, the transmission amplitude of the filter was divided by the transmission amplitude of the air to obtain the amplitude transmittance spectrum of the filter, and the amplitude transmittance data in the range of 0.1 THz~1.0 THz were taken. Due to the limited frequency resolution, in the analysis process, in order to avoid large errors caused by inaccurate selection of frequency point positions, the amplitude transmittance data in the range of 0.1 THz~1.0 THz were interpolated by interpolation, and the frequency interval was 0.001 THz.

[0109] For low-pass filters with cutoff frequencies of 0.22 THz and 0.35 THz, the passband range is calculated within the range of 0.1 THz to 1 THz, and the end position fp1 of the passband is determined based on the square root of two of the maximum amplitude transmittance, that is, the passband range is: 0.1 THz~fp1. The passband average amplitude transmittance is calculated by adding the amplitude transmittance data within the passband range and taking the average value. The transition band range of the low-pass filter is calculated within the range of 0.1 THz to 1 THz, with 0.150 THz as the bandwidth B of the transition band. t , determine the transition band fp1~fb1 on the high frequency side of the passband, where fb1=fp1+B t The stopband range of the low-pass filter sample is calculated to be within the range of 0.1 THz to 1 THz. According to the stopband frequency point fb1 on the high-frequency side of the passband determined by the transition band, the stopband range is: fb1 to 1.0 THz. The stopband average amplitude transmittance is calculated by adding the amplitude transmittance data within the stopband range and taking the average value.

[0110] The testing and data processing of the bandpass filters with center frequencies of 0.22 THz and 0.35 THz are the same as those of the low-pass filters.

[0111] For bandpass filters with center frequencies of 0.22 THz and 0.35 THz, the passband range is calculated to be within the range of 0.1 THz to 1 THz. The passband starting positions fp1 (low frequency position) and fp2 (high frequency position) are determined based on the square root of two of the maximum amplitude transmittance, that is, the passband range is: fp1 to fp2. The bandwidth is calculated as (fp2-fp1), and the passband average amplitude transmittance is calculated by adding the amplitude transmittance data within the passband range and taking the average value. The stopband range of the bandpass filter sample is calculated to be within the range of 0.1 THz to 1 THz. The stopband frequencies fb1 (low frequency position) and fb2 (high frequency position) on both sides of the passband are determined based on the transition band. The stopband range is: 0.1 THz to fb1, and fb2 to 1.0 THz. The stopband average amplitude transmittance is the average value of the amplitude transmittance data within the stopband range.

[0112] Figure 8 The figure schematically shows the amplitude transmittance spectra obtained by experimental measurement at 9 positions randomly selected from four terahertz filters according to the embodiments of the present disclosure.

[0113] like Figure 8 As shown, Figure 8 In the figure, a is the amplitude transmittance spectrum obtained by experimental testing and data processing of a low-pass filter with a cutoff frequency of 0.22 THz. Figure 8 Figure b in the figure is the amplitude transmittance spectrum obtained by experimental testing and data processing using a low-pass filter with a cutoff frequency of 0.35 THz. Figure 8 (c) is the amplitude transmittance spectrum obtained by experimental testing and data processing of a bandpass filter with a center frequency of 0.22 THz. Figure 8 d in the figure is the amplitude transmittance spectrum obtained by experimental testing and data processing of a bandpass filter with a center frequency of 0.35 THz.

[0114] Fig. 9 The schematic diagram shows the amplitude transmittance spectra of four terahertz filters according to the embodiments of the present disclosure obtained by experimental measurement when terahertz waves are incident in different polarization directions.

[0115] like Fig. 9 As shown, the angle between the incident polarization direction of the terahertz electromagnetic wave and the horizontal direction is defined as , respectively measured The amplitude transmittance spectrum of each filter is shown below. Among them, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.22 THz is as follows Fig. 9 In a, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.35 THz is as follows Fig. 9In b, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.22 THz is shown in Fig. 9 In c, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.35 THz is as follows Fig. 9 From the test result diagram, it can be found that changing the polarization direction has almost no effect on the test results of the terahertz thin film low-pass filter and the band-pass filter. This is because the metal microstructures constituting the metasurface in the terahertz filter disclosed in the present invention are all isotropic structures with polarization-independent responses.

[0116] Fig.10 The schematic diagram shows the amplitude transmittance spectra of four terahertz filters according to the embodiments of the present disclosure obtained by experimental measurement when the terahertz wave is incident at different oblique angles.

[0117] like Fig.10 As shown, the angle between the incident direction of the terahertz electromagnetic wave and the normal direction of the filter sample is defined as , respectively measured The amplitude transmittance spectrum of each filter is shown below. Among them, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.22 THz is as follows Fig.10 In a, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.35 THz is as follows Fig.10 In b, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.22 THz is shown in Fig.10 In c, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.35 THz is as follows Fig.10 From the test results, we can see that as Gradually increasing to 30°, changing the incident angle of the terahertz electromagnetic wave has no significant effect on the spectrum of the filter, which confirms that the terahertz filter disclosed in the present invention is robust under different electromagnetic wave incident angles.

[0118] Fig.11 The schematic diagram shows the amplitude transmittance spectra obtained by experimental measurement of four terahertz filters in normal and bent states according to the embodiments of the present disclosure.

[0119] like Fig.11 As shown in FIG. 1 , the filter sample is slightly bent to a certain degree, and the amplitude transmittance spectrum of each filter is measured under normal incidence of the terahertz electromagnetic wave. Among them, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.22 THz is shown in FIG. Fig.11 In a, the amplitude transmittance spectrum of the center point of the low-pass filter with a cutoff frequency of 0.35 THz is as follows Fig.11 In b, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.22 THz is shown in Fig.11 In c, the amplitude transmittance spectrum of the center point of the bandpass filter with a center frequency of 0.35 THz is as follows Fig.11 From the test result graph, it can be found that compared with the case where the filter sample is not bent, the amplitude transmittance measured when the filter sample is bent on the surface will be slightly reduced, but the overall spectral response does not change much, which confirms that the filter disclosed in the present invention is robust under slight bending.

[0120] The flowcharts and block diagrams in the accompanying drawings illustrate the possible implementation architecture, functions and operations of the systems, methods and computer program products according to various embodiments of the present disclosure. In this regard, each box in the flowchart or block diagram may represent a module, a program segment, or a part of a code, and the above-mentioned module, program segment, or a part of the code contains one or more executable instructions for implementing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box may also occur in an order different from that marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should also be noted that each box in the block diagram or flowchart, and the combination of boxes in the block diagram or flowchart, can be implemented with a dedicated hardware-based system that performs a specified function or operation, or can be implemented with a combination of dedicated hardware and computer instructions. It can be understood by those skilled in the art that the features recorded in the various embodiments of the present disclosure can be combined and / or combined in a variety of ways, even if such a combination or combination is not explicitly recorded in the present disclosure. In particular, without departing from the spirit and teaching of the present disclosure, the features described in the various embodiments of the present disclosure may be combined and / or combined in a variety of ways. All of these combinations and / or combinations fall within the scope of the present disclosure.

[0121] The embodiments of the present disclosure are described above. However, these embodiments are only for illustrative purposes and are not intended to limit the scope of the present disclosure. Although the embodiments are described above, this does not mean that the measures in the various embodiments cannot be used in combination to advantage. Without departing from the scope of the present disclosure, those skilled in the art may make a variety of substitutions and modifications, which should all fall within the scope of the present disclosure.

Claims

1. A terahertz filter based on a multilayer metasurface, comprising: M layers of composite supersurface and M-1 layers of cycloolefin copolymer film, wherein each layer of the M-1 layers of cycloolefin copolymer film is located between two adjacent layers of the composite supersurface, each layer of the composite supersurface includes a first layer of polyimide film, a supersurface and a second layer of polyimide film, the supersurface is composed of a plurality of metal microstructures, the plurality of metal microstructures are periodically and uniformly arranged, the supersurface is located between the first layer of polyimide film and the second layer of polyimide film, and M is a positive integer greater than 1.

2. The filter according to claim 1, wherein The filters include low-pass filters and band-pass filters.

3. The filter according to claim 2, wherein: When the filter is a low-pass filter, the projection shape of the supersurface in each layer of the composite supersurface in the first layer of polyimide film is a square array, and the projection shape of the metal microstructure in the first layer of polyimide film is a square in the square array.

4. The filter according to claim 2, wherein: When the filter is a bandpass filter, the projection shape of the supersurface in each layer of the composite supersurface in the first layer of polyimide film is an array of squares and square rings, and the projection shape of the metal microstructure in the first layer of polyimide film is a square and a square ring in the array of squares and square rings.

5. The filter according to any one of claims 1 to 4, wherein: The material of the metal microstructure in the super surface is aluminum.

6. The filter according to any one of claims 1 to 4, wherein: The frequency band of the filter is in the range of 0.1-1 THz.

7. The filter according to any one of claims 1 to 4, wherein: In the composite supersurface, the thickness of the metal microstructure in the supersurface is 195~205 nanometers, the thickness of the first layer of polyimide film is 9~11 microns or 15~17 microns, and the thickness of the second layer of polyimide film is 9~11 microns or 15~17 microns.

8. The filter according to any one of claims 1 to 4, wherein: The thickness of the cycloolefin copolymer film is 49-51 microns or 79-81 microns.

9. A method for processing a filter as claimed in any one of claims 1 to 8, comprising: A first layer of polyimide film, a super surface and a second layer of polyimide film are sequentially formed on a substrate to obtain a composite super surface, wherein the super surface is composed of a plurality of metal micro structures, and the plurality of metal micro structures are periodically and uniformly arranged; The M layers of the composite metasurface and the M-1 layers of cyclic olefin copolymer films are hot pressed at a preset temperature to obtain a filter as described in any one of claims 1 to 8, wherein each layer of the M-1 layers of cyclic olefin copolymer films is located between two adjacent layers of the composite metasurface, and M is a positive integer greater than 1.

10. The method according to claim 9, wherein: The method of sequentially forming a first layer of polyimide film, a super surface and a second layer of polyimide film on a substrate to obtain a composite super surface comprises: Evenly coating the polyimide gel on the substrate to form the first layer of polyimide film; forming a metal film on the first polyimide film by vacuum thermal evaporation; uniformly coating photoresist on the metal film; Removing the metal area other than the metal microstructure on the metal film by exposure, development and etching, and then washing the surface of the metal film with acetone solution, isopropanol and deionized water to obtain the super surface; The polyimide gel is evenly coated on the super surface to form the second layer of polyimide film to obtain the composite super surface.

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