A flexible broadband terahertz waveband quarter-wave plate

Abstract

The application discloses a flexible broadband terahertz wave band quarter-wave plate, which comprises a flexible transparent film with a transmittance of more than 75% to terahertz waves, an array of sub-wavelength scale metal L-shaped structure units formed on the upper surface of the flexible transparent film, and a metal reflecting film formed on the lower surface of the flexible transparent film; when the terahertz wave band quarter-wave plate is in a planar state or a curved state with a curvature radius not less than a preset value, and linearly polarized terahertz waves with an incident angle in a preset angle range are irradiated onto the upper surface, the L-shaped structure unit array interacts with the terahertz waves, and broadband conversion of the terahertz waves from a linear polarization state to a right-handed circular polarization state or a left-handed circular polarization state can be realized.

Description

Technical Field

[0001] This invention belongs to the field of terahertz waveplate technology, specifically relating to a flexible broadband terahertz band quarter-wave plate. Background Technology

[0002] Terahertz wave communication technology is considered a revolutionary technology in sixth-generation (6G) mobile communication. In communication, the polarization information of electromagnetic waves is crucial. Linearly polarized waves often experience polarization deflection during propagation due to scattering, leading to polarization mismatch between the receiving electromagnetic wave and the receiving antenna. Circularly polarized wave communication can effectively mitigate such transmission interference. Furthermore, with the increasing prevalence of wearable smart devices, the performance changes of communication devices during bending processes need to be considered during development. Therefore, the research and development of flexible, broadband terahertz quarter-wave plates capable of stable circular polarization state modulation is becoming increasingly necessary.

[0003] Currently, commercially available terahertz sources typically emit linearly polarized terahertz waves. Using traditional natural materials to achieve polarization control of terahertz waves often faces drawbacks such as narrow bandwidth, large size, and resistance to bending and deformation. Metamaterials, with their superior electromagnetic wave manipulation capabilities, can overcome these limitations. In recent years, broadband control of the circular polarization state of terahertz waves has been achieved based on anisotropic or chiral metamaterials, laying a foundation for the development of terahertz communication technology. However, most current metamaterials are designed on rigid substrates, making them incompatible with wearable flexible wireless communication terminal devices. Although flexible terahertz circular polarization control devices with a certain degree of bending capability have begun to be designed based on metamaterials, their circular polarization performance often cannot be maintained after bending, thus failing to function under bending conditions. Therefore, it is essential to design a quarter-wave plate that can stably convert broadband linearly polarized terahertz waves into broadband left-hand circularly polarized terahertz waves (or broadband right-hand circularly polarized terahertz waves) under different bending conditions. Summary of the Invention

[0004] To address the aforementioned issues, this invention combines flexible materials and a subwavelength-scale chiral L-shaped unit structure to propose a flexible broadband terahertz quarter-wave plate that can stably achieve the conversion of broadband terahertz waves from linearly polarized to circularly polarized states even with varying degrees of bending.

[0005] The technical solution of the present invention is as follows:

[0006] A flexible terahertz quarter-wave plate mainly comprises: a flexible transparent film having a transmittance of over 75% for terahertz waves; an array of L-shaped metal structural units formed on the upper surface of the flexible transparent film, wherein the L-shaped structural units are subwavelength scale and have two arms connected end-to-end and arranged vertically; and a metal reflective film formed on the lower surface of the flexible transparent film. The terahertz quarter-wave plate is further configured such that, when the terahertz quarter-wave plate is in a planar or curved state, when a linearly polarized terahertz wave with an incident angle within a preset angle range irradiates the upper surface of the terahertz quarter-wave plate, the L-shaped structural unit array interacts with the terahertz wave, achieving a broadband conversion of the terahertz wave from a linearly polarized state to a right-hand circularly polarized state or a left-hand circularly polarized state; and when the terahertz quarter-wave plate is in a curved state, the radius of curvature is not less than a preset value.

[0007] As a preferred embodiment, the thickness of the flexible transparent film is 20-100 micrometers; the length l of the two arms in the L-shaped unit structure is 110-140 micrometers, and the width w is 25-45 micrometers.

[0008] As a preferred embodiment, the flexible transparent film is made of at least one of polyethylene terephthalate, polyimide, polymethyl methacrylate, polycarbonate, polypropylene, polyvinyl chloride, thermoplastic polyurethane elastomer rubber, polystyrene, polysulfone, and transparent nylon.

[0009] As a preferred embodiment, the thickness of the L-shaped structural unit array is 50-200 nanometers.

[0010] As a preferred embodiment, the L-shaped structural unit array has a period of 120-240 micrometers along the two orthogonal directions of the two arms.

[0011] As a preferred embodiment, the L-shaped structural unit array has the same period along the two orthogonal directions of the two arms; the two arms in the L-shaped unit structure have equal lengths.

[0012] As a preferred embodiment, the L-shaped structural unit is made of conductive metals such as gold, silver, copper, or aluminum, or alloys thereof.

[0013] As a preferred embodiment, the metal reflective film is made of conductive metals such as gold, silver, copper, or aluminum, or alloys thereof.

[0014] As a preferred embodiment, the thickness of the metal reflective film is 100-500 nanometers.

[0015] As a preferred embodiment, the flexible transparent film has a transmittance of over 90% for terahertz waves.

[0016] As a preferred embodiment, the metal reflective film has a broadband reflectivity of over 95% for terahertz waves.

[0017] As a preferred embodiment, the preset angle range is 0 to 60 degrees; the preset curvature radius of curvature is 0.1 meters or more.

[0018] The flexible broadband terahertz band quarter-wave plate disclosed in this invention has the following beneficial effects:

[0019] 1) It features stable operation under wide incident angles. During use, the electromagnetic wave can operate normally and stably within a preset angle range (e.g., 0–60 degrees), realizing the conversion of broadband terahertz waves from linear polarization to circular polarization. Furthermore, based on the flexible and bendable characteristics of the quarter-wave plate in the broadband terahertz band, the stability of the circular polarization characteristics of the emitted terahertz waves can be guaranteed when the radius of curvature is greater than or equal to a preset value (e.g., 0.1 meters).

[0020] 2) It has excellent broadband electromagnetic wave manipulation capability. By designing the appropriate geometric dimensions of the two arms in the L-shaped structural unit and the thickness of the intermediate dielectric layer (i.e., flexible transparent film), the time response of the electric dipoles in the two orthogonal directions can be effectively controlled, thereby controlling the amplitude ratio and phase difference between the electric field components in the two orthogonal directions of the emitted wave.

[0021] 3) It has a high polarization conversion efficiency. Theoretically, when the broadband terahertz band quarter-wave plate is not bent, it can achieve a right-hand circular polarization conversion efficiency of more than 95% (or a left-hand circular polarization conversion efficiency of more than 95%) by optimizing the measurement steps and improving the quality of the sample.

[0022] 4) The present invention is simple to implement and can produce large-area products with a scale of centimeters or larger through conventional processes such as photolithography. It has low production cost and is easy to promote. Attached Figure Description

[0023] Figure 1 The diagram shows a flexible broadband terahertz quarter-wave plate without bending and a sample photograph, wherein: (a) is a diagram of a flexible broadband terahertz quarter-wave plate without bending, and (b) is a sample photograph of a flexible broadband terahertz quarter-wave plate without bending.

[0024] Figure 2The measurement results of converting a linearly polarized terahertz wave with polarization along the y-axis into a right-hand circularly polarized terahertz wave are given, where: (a) is the ratio of the amplitudes of the electric field components in the y' and x' directions as a function of frequency; (b) is the phase difference between the electric field components in the y' and x' directions as a function of frequency; (c) is the ellipticity of the emitted circularly polarized terahertz wave; and (d) is the reflectivity of the emitted terahertz wave.

[0025] Figure 3 The measurement results of converting a linearly polarized terahertz wave with polarization along the x-axis into a left-hand circularly polarized terahertz wave are given, where: (a) is the ratio of the amplitudes of the electric field components in the y' and x' directions as a function of frequency; (b) is the phase difference between the electric field components in the y' and x' directions as a function of frequency; (c) is the ellipticity of the emitted circularly polarized terahertz wave; and (d) is the reflectivity of the emitted terahertz wave.

[0026] Figure 4 The diagram shows a flexible broadband terahertz quarter-wave plate under bending conditions, and a photograph of the sample. (a) is a schematic diagram of the flexible broadband terahertz quarter-wave plate under bending conditions, and (b) is a photograph of the sample of the flexible broadband terahertz quarter-wave plate under bending conditions.

[0027] Figure 5 The amplitude ratios of the electric field components in the y' and x' directions of the emitted wave are given for a flexible broadband terahertz quarter-wave plate with different curvature radii. Among them, (a) represents the amplitude ratios of the electric field components in the y' and x' directions with different curvature radii, and (b) represents the relationship between the amplitude ratios of the electric field components in the y' and x' directions and the curvature radius at 0.54 THz.

[0028] Figure 6 The phase difference of the electric field components in the y' and x' directions of the emitted wave of a flexible broadband terahertz quarter-wave plate with different curvature radii is given, where: (a) represents the phase difference of the electric field components in the y' and x' directions with several curvature radii, and (b) represents the relationship between the phase difference of the electric field components in the y' and x' directions and the curvature radius at 0.54 THz. Detailed Implementation

[0029] In this invention, we construct a flexible and transparent terahertz waveplate using an L-shaped metal-dielectric-metal structure with broken spatial symmetry at the subwavelength scale as the basic unit. We utilize this flexible terahertz waveplate to achieve the conversion of broadband terahertz waves from linear polarization to circular polarization. We also demonstrate that the flexible terahertz waveplate can still stably achieve the conversion of broadband terahertz waves from linear polarization to circular polarization even with different degrees of bending.

[0030] The terahertz quarter-wave plate disclosed in this invention mainly comprises: a flexible transparent film, an L-shaped structural unit array, and a back metal reflective film. The flexible transparent film is a film with excellent flexibility and the ability to be bent and deformed significantly, and possesses broadband high transmittance for terahertz waves (terahertz wave absorption is negligible), typically with a transmittance greater than 75%, preferably above 90%. Specific materials that can be used for the terahertz flexible transparent film include flexible film materials with terahertz transparency such as polyethylene terephthalate, polyimide, polymethyl methacrylate, polycarbonate, polypropylene, polyvinyl chloride, thermoplastic polyurethane elastomer rubber, polystyrene, polysulfone, and transparent nylon, or composite materials formed from at least two of the above materials. The thickness of the intermediate flexible dielectric layer can affect the time response of the electric dipoles in the two orthogonal directions, thereby controlling the amplitude ratio and phase difference between the electric field components of the emitted wave in the two orthogonal directions. Typically, the thickness of the terahertz flexible transparent film can be designed to be 20-100 micrometers.

[0031] An L-shaped structural unit array is formed on the upper surface (i.e., the front side) of the flexible transparent film. It mainly consists of subwavelength-scale metallic L-shaped structural units arranged in an array. Each L-shaped structural unit has two arms connected end-to-end and arranged vertically. The lengths of the two arms can be equal or unequal. The length *l* can be designed to be 110-140 micrometers, the width *w* to be 25-45 micrometers, and the thickness to be approximately 50-200 nanometers. The length and width parameters of the L-shaped arms affect the time response of the electric dipoles in two orthogonal directions, thereby controlling the amplitude ratio and phase difference between the electric field components of the emitted wave in the two orthogonal directions. Preferably, the two arms are of equal length, allowing optimization of only one set of length and bandwidth parameters. If the lengths of the two arms are unequal, the relevant functions can still be achieved, but the length and width of each arm need to be optimized separately, resulting in a larger workload. The L-shaped structural unit array has the same period along the two orthogonal directions of the two arms, which can be designed to be 120-240 micrometers. This period mainly affects the operating band and center operating frequency of the quarter-wave plate. The metal material of the L-shaped structural unit is conductive metals such as gold, silver, copper, and aluminum, or an alloy of the above metals.

[0032] The back metal reflective film is formed on the lower surface (i.e., the back side) of the flexible transparent film and has a broadband reflectivity of over 95% for terahertz waves, making it suitable as a reflective layer in a quarter-wave plate. The metal reflective film can be made of conductive metals such as gold, silver, copper, or aluminum, or alloys thereof, with a thickness of approximately 100-500 nanometers.

[0033] When a linearly polarized terahertz wave is incident at an angle of 0 to 60 degrees (the angle of incidence in this application refers to the angle between the wave and the z-axis in the xz plane, i.e.) Figure 1When the terahertz wave (angle θ in (a)) strikes the surface of the quarter-wave plate, the interaction between the terahertz wave and the array of structural units generates a series of electric dipoles. Each L-shaped structural unit generates a first electric dipole along the direction of the incident electric field. This electric dipole then induces a second electric dipole perpendicular to the direction of the incident electric field (orthogonal to the direction of the incident electric field) by passing through the L-shaped structural unit. This allows for the separate manipulation of the electric fields in the two orthogonal directions, thereby changing the polarization of the incident terahertz wave and achieving a broadband conversion from a linearly polarized state to a right-handed or left-handed circularly polarized state. Furthermore, when the curvature of the quarter-wave plate is controlled to a radius of curvature of not less than 0.1 meters, the broadband terahertz wave conversion from a linearly polarized state to a circularly polarized state can still be stably achieved.

[0034] Based on the equivalent circuit model, we can explain the physical principle behind the broadband polarization conversion achieved by this flexible terahertz quarter-wave plate. When a linearly polarized terahertz wave with its polarization direction along the y-axis (or x-axis) is incident obliquely onto an L-shaped structural unit array at an incident angle θ, the terahertz wave interacts with the metamaterial, generating a series of electric dipoles. By changing the geometry of the L-shaped arms of the L-shaped structural unit and the thickness of the intermediate flexible dielectric layer, the time response of the electric dipoles in the two orthogonal directions can be effectively controlled, thereby controlling the amplitude ratio and phase difference between the electric field components of the outgoing wave in the two orthogonal directions. The total reflection field of the terahertz quarter-wave plate is mainly composed of... The decision, among which, For radiation field terms, Let d be the conjugate relation between the radiation of the L-shaped structural layer (i.e., the L-shaped structural unit array) and its image, d be the thickness of the intermediate flexible dielectric layer, i be the imaginary unit, k = (2π) / λ be the wave vector of the incident wave, and λ be the wavelength of the incident wave.

[0035] By selecting appropriate geometric structure parameters, the conjugate relation term... Dispersion with frequency can be related to radiation field terms within a certain range. The dispersions cancel each other out, thus enabling broadband polarization control.

[0036] Based on the equivalent circuit model, we can also explain the source of the broadband polarization conversion stability of this flexible terahertz quarter-wave plate under bending conditions. The terms related to the total reflection field and incident angle of the terahertz quarter-wave plate are mainly the conjugate relationship between the radiation of the L-shaped structural layer (i.e., the L-shaped structural unit array) and its mirror image. When the incident angle θ is not greater than 60 degrees, if d << λ, that is, the thickness of the intermediate flexible dielectric layer is much smaller than the wavelength, the absolute value of the conjugate relation term can be expressed as: It can be seen that when d << λ, the conjugate relation term The value of θ changes relatively little with the incident angle θ; therefore, when the incident angle is no greater than 60 degrees, the polarization of the reflected field can remain essentially unchanged. The superposition of the effects of terahertz waves irradiating the unbent sample at various incident angles can be equivalent to the interaction between the terahertz waves and the bent sample, thus explaining that the reflected field of the bent sample will also remain essentially unchanged with the change of the incident angle. The above analysis results theoretically explain the source of the polarization conversion stability of this flexible metamaterial under bending conditions.

[0037] To provide a clearer understanding of the present invention, the invention will be further described below with reference to specific embodiments and accompanying drawings.

[0038] See Figure 1 As shown in the embodiment, a flexible broadband terahertz quarter-wave plate sample (hereinafter referred to as "sample") is disclosed, which can convert incident linearly polarized terahertz waves into right-hand circularly polarized terahertz waves or left-hand circularly polarized terahertz waves. Specifically, the sample mainly consists of a three-layer structure: a flexible transparent terahertz film 1, a subwavelength-scale metal L-shaped structure unit array 2 fabricated on the upper surface of the flexible transparent film, and a metal reflective film 3 deposited on the back of the flexible transparent film as a reflective layer.

[0039] When a linearly polarized terahertz wave with its polarization direction along the y-axis (or x-axis) illuminates the sample surface at an incident angle θ of 45 degrees, the sample will be able to achieve efficient conversion of the terahertz wave from a linearly polarized state to a right-handed circularly polarized state (or a left-handed circularly polarized state), such as... Figure 1 As shown in (a), the xyz coordinate system is established based on the geometry of the sample, and the x'y'z' coordinate system is established based on the direction of the emitted wave. The y and y' directions are consistent and both perpendicular to the xz plane (or x'z' plane). This structure is based on flexible polymer materials and is therefore suitable for applications under bending conditions. Experimentally, we first prepared a 38-micrometer-thick commercial polyethylene terephthalate film with negligible loss and a refractive index of 1.7. Then, through a series of processes including spin coating, photolithography, development, coating, and resist removal, a 50-nanometer-thick L-shaped gold structural unit array was fabricated on the surface of the polyethylene terephthalate film. The array has a period of 165 micrometers along both the x and y directions. The L-shaped structural unit has two arms arranged vertically end to end, with a length l of 125 micrometers and a width w of 35 micrometers for each arm. Subsequently, a 100-nanometer-thick gold film was deposited on the back of the polyethylene terephthalate film as a reflective layer. The front view of the prepared sample is shown in the image below. Figure 1 As shown in (b). The sample preparation mainly includes the following steps:

[0040] Step 1: First, apply a layer of photoresist to the cleaned polyethylene terephthalate film surface, and then allow the photoresist on the surface of the polyethylene terephthalate film to cure.

[0041] Step 2: Place the cured polyethylene terephthalate film in a maskless ultraviolet lithography machine and expose the film to ultraviolet light so that the photoresist at the exposed pattern is completely denatured.

[0042] Step 3: Remove the modified photoresist, leaving only L-shaped structural unit array photoresist grooves on the polyethylene terephthalate film;

[0043] Step 4: Deposit 50 nanometers of gold onto the surface of the developed polyethylene terephthalate film;

[0044] Step 5: Remove the non-exposed patterned area with photoresist remaining from the 50-nanometer gold-plated sample, leaving a 50-nanometer thick L-shaped unit structure array.

[0045] Step 6: Deposit a 100-nanometer-thick gold film as a reflective layer on the back side of the polyethylene terephthalate film after the gold L-shaped structural unit array has been prepared.

[0046] In the experiment, a terahertz time-domain spectrometer was used to measure the spectral and polarization characteristics of the sample. A mirror was used to obliquely incident the terahertz wave onto the sample surface at a 45-degree angle, and then the time-domain signal of the reflected wave was detected. By performing a Fourier transform on the measured time-domain signal and comparing it with a reference signal, the amplitude ratio and phase difference of the electric field components along the y' and x' directions of the sample as a function of frequency were obtained, thus allowing the determination of the polarization information of the emitted terahertz wave.

[0047] We first measured the polarization conversion performance of the sample when it was not bent. When a linearly polarized terahertz wave with a polarization direction along the y-axis was incident on the sample surface at a 45-degree angle... Figure 2 (a) and Figure 2 (b) The measured amplitude ratio and phase difference of the electric field components in the y' and x' directions as a function of frequency are given. It can be seen that in the range of 0.46-0.62 THz, the amplitude ratio of the electric field components in the y' and x' directions as a function of frequency remains basically around 1, and the phase difference of the electric field components in the y' and x' directions as a function of frequency remains basically around -0.5π. This indicates that the emitted terahertz wave is in a right-hand circularly polarized state, which shows that the sample has realized the conversion of the terahertz wave from a linearly polarized state to a right-hand circularly polarized state. Figure 2 (c) is the ellipticity angle of the emitted circularly polarized light. It can be seen that it is close to 45 degrees, indicating that the emitted terahertz wave is a relatively good broadband right-hand circularly polarized terahertz wave. Figure 2 (d) is the reflectivity of the emitted terahertz wave obtained by measurement, indicating that the efficiency of the quarter-wave plate actually obtained by the sample is about 70%. The results of numerical simulation calculation show that when the sample is not bent, the right-hand circular polarization conversion efficiency of the quarter-wave plate can be achieved by optimizing the measurement steps and improving the quality of the sample.

[0048] When a linearly polarized terahertz wave with its polarization direction along the x-axis is incident on the sample surface at a 45-degree angle... Figure 3 (a) and Figure 3 (b) The measured amplitude ratio and phase difference of the electric field components in the y' and x' directions as a function of frequency are given. It can be seen that in the range of 0.46-0.62 THz, the amplitude ratio of the electric field components in the y' and x' directions as a function of frequency remains basically around 1, and the phase difference of the electric field components in the y' and x' directions as a function of frequency remains basically around 0.5π. This indicates that the emitted terahertz wave is in a left-handed circularly polarized state, which shows that the sample has realized the conversion of the terahertz wave from a linearly polarized state to a left-handed circularly polarized state. Figure 3 (c) is the ellipticity angle of the emitted circularly polarized light. It can be seen that it is broadband and close to -45 degrees, indicating that the emitted terahertz wave is a relatively good broadband left-handed circularly polarized terahertz wave. Figure 3 (d) is the reflectivity of the emitted terahertz wave obtained by measurement, indicating that the efficiency of the quarter-wave plate actually obtained by the sample is about 60%. The results of numerical simulation calculation show that when the sample is not bent, the left-hand circular polarization conversion efficiency of the quarter-wave plate can be achieved by optimizing the measurement steps and improving the quality of the sample.

[0049] Next, we measured the polarization conversion performance of the sample under different degrees of curvature. Figure 4 (a) A schematic diagram of the operation of a flexible broadband terahertz quarter-wave plate under bending is given. Figure 4 (b) shows a photograph of the sample under bending conditions, demonstrating its excellent flexibility and ability to bend almost arbitrarily. For ease of description, we use the radius of curvature *r* to represent the degree of bending. The relationship between the radius of curvature *r* and relevant structural parameters is: *h* = *r* – *r* cos[s / (2r)], where the arc length *s* is the total length of the sample and *h* is the arc height. In the experiment, the polarization state of the emitted terahertz wave was measured when a linearly polarized terahertz wave with a polarization direction along the y-axis was incident on the sample surface at a 45-degree angle, with sample curvature radii *r* of 0.70 m, 0.38 m, 0.24 m, 0.17 m, and 0.14 m, respectively. Figure 5 The measured amplitude ratios of the electric field components in the y' and x' directions for a flexible broadband terahertz quarter-wave plate with different radii of curvature are given, where: Figure 5 (a)-(c) show the amplitude ratio of the electric field components in the y' and x' directions with three different radii of curvature (0.70m, 0.38m, and 0.17m). It can be seen that in the range of 0.46-0.62THz, the amplitude ratio of the electric field components in the y' and x' directions with frequency remains basically around 1. Figure 5(d) shows the relationship between the amplitude ratio of the electric field components in the y' and x' directions at 0.54 THz and the radius of curvature during bending. It can be seen that the amplitude ratio of the electric field components in the y' and x' directions remains basically unchanged, indicating that the amplitude ratio of the electric field components remains stable during the bending process of the sample.

[0050] In addition to obtaining the amplitude ratio of the electric field components in the y' and x' directions, we can also obtain the relationship between the phase difference of the electric field components in the y' and x' directions of the emitted wave and the frequency when the sample is bent with different radii of curvature. Figure 6 (a)-(c) show the relationship between the phase difference of the electric field components in the y' and x' directions and the frequency when there are three different radii of curvature (0.70m, 0.38m, and 0.17m). It can be seen that in the range of 0.46-0.62THz, the phase difference of the electric field components in the y' and x' directions is basically maintained at around -0.5π. Figure 6 (d) shows the relationship between the phase difference of the electric field components in the y' and x' directions at 0.54 THz and the radius of curvature. It can be seen that the phase difference of the electric field components in the y' and x' directions remains essentially constant, proving that the phase difference of the electric field components maintains good stability during sample bending. Furthermore, combined with... Figure 5 The stability result, in which the amplitude ratio of the electric field components in the y' and x' directions remains basically around 1, clearly shows that the sample can still stably maintain the right-hand circularly polarized state of the emitted terahertz wave under bending conditions.

[0051] Finally, it should be noted that this invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and guiding, not restrictive. Those skilled in the art, guided by this specification, can make many other forms without departing from the scope of protection of the claims, and all of these are within the scope of protection of this invention.

Claims

1. A flexible terahertz quarter-wave plate, characterized in that, include: A flexible transparent film having a transmittance of more than 75% for terahertz waves; The metal L-shaped structural unit array formed on the upper surface of the flexible transparent film, wherein the L-shaped structural unit is subwavelength scale and has two arms connected end to end and arranged vertically; A metal reflective film formed on the lower surface of the flexible transparent film; The terahertz band quarter-wave plate is also constructed such that when the terahertz band quarter-wave plate is in a planar state or a curved state with a radius of curvature not less than a preset value, when a linearly polarized terahertz wave with an incident angle within a preset angle range irradiates its upper surface, the L-shaped structural unit array interacts with the terahertz wave to realize the broadband conversion of the terahertz wave from a linearly polarized state to a right-hand circularly polarized state or a left-hand circularly polarized state. The length of the two arms in the L-shaped structural unit l It is 110-140 micrometers wide. w It is 25-45 micrometers; The period of the L-shaped structural unit array along the two orthogonal directions of the two arms is 120-240 micrometers; The preset angle range is 0 to 60 degrees; the preset value is 0.1 meters.

2. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The thickness of the flexible transparent film is 20-100 micrometers.

3. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The flexible transparent film is made of at least one of polyethylene terephthalate, polyimide, polymethyl methacrylate, polycarbonate, polypropylene, polyvinyl chloride, thermoplastic polyurethane elastomer rubber, polystyrene, polysulfone, and transparent nylon.

4. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The thickness of the L-shaped structural unit array is 50-200 nanometers.

5. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The L-shaped structural unit array has the same period along the two orthogonal directions of the two arms; the two arms of the L-shaped structural unit have equal lengths.

6. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The L-shaped structural unit is made of conductive metals such as gold, silver, copper, and aluminum, or alloys thereof.

7. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The metal reflective film is made of conductive metals such as gold, silver, copper, and aluminum, or alloys thereof.

8. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The thickness of the metal reflective film is 100-500 nanometers.

9. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The flexible transparent film has a transmittance of over 90% for terahertz waves.

10. The terahertz band quarter-wave plate as described in claim 1, characterized in that, The metal reflective film has a broadband reflectivity of over 95% for terahertz waves.

Images