Method for realizing terahertz vortex Smith-Purcell radiation based on double-layer metasurface

Abstract

The invention relates to the technical field of electromagnetic radiation, in particular to a method for achieving terahertz vortex Smith-Purcell radiation based on a double-layer metasurface, and the method comprises the following steps: constructing a double-layer metasurface structure, the first layer is a resonance enhancement metasurface, the second layer is a geometric phase metasurface, the first layer and the second layer are arranged in the Z-axis direction, the first layer is located below the second layer, and the second layer is located below the second layer; setting a bunched electron beam to skim over the lower surface of the first layer of metasurface along the X direction of the XOY plane at a constant speed; regulating and controlling a unit structure of the first layer resonance enhancement metasurface, and converting a linear polarization state of Smith-Purcell radiation into a circular polarization state; and the converted circularly polarized radiation is incident to the second layer of geometric phase metasurface, and vortex Smith-Purcell radiation is generated. Polarization conversion of Smith-Purcell radiation is achieved through the first layer of resonance enhancement metasurface, accurate regulation and control of an OAM mode are achieved through the second layer of geometric phase metasurface, an external terahertz radiation source is not needed, and coherent vortex Smith-Purcell radiation at higher harmonic frequency can be achieved by matching with a bunched electron beam.

Description

A method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface Technical Field

[0001] This invention relates to the field of electromagnetic radiation technology, and specifically to a method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface. Background Technology

[0002] Currently, terahertz waves typically refer to electromagnetic radiation waves with wavelengths ranging from approximately 0.03 mm to 3 mm and frequencies between 0.1 THz and 10 THz. Because the terahertz band lies between microwaves and infrared waves in the electromagnetic frequency spectrum, it possesses advantages such as strong penetration into non-metallic / non-polar materials, safety without harming living organisms, high spatiotemporal resolution in imaging detection, and a wide operating frequency range. It has been widely applied in high-resolution imaging, non-destructive testing, life sciences, and terahertz communication. As a fundamental property of electromagnetic waves, orbital angular momentum (OAM) provides a new degree of phase freedom, attracting widespread interest. Terahertz waves carrying OAM are called terahertz vortex waves, and their spatial phase distribution is helical. Theoretically, vortex waves contain an infinite number of non-interfering orthogonal modes at any frequency, independent of amplitude, phase, and polarization characteristics. This allows each mode to transmit information independently, providing a unique dimension for modulation and multiplexing, with the potential to improve capacity and spectral efficiency, making it significant for terahertz communication. Therefore, generating reconfigurable, high-quality OAM beams in the terahertz band is a prerequisite for combining terahertz and orbital angular momentum technologies.

[0003] Currently, metasurfaces, as artificial composite structures composed of subwavelength units, have been widely used in the generation of terahertz vortex waves. Metasurfaces can achieve precise local control of the amplitude, phase, and polarization of electromagnetic waves on a two-dimensional plane at the subwavelength scale. Compared to three-dimensional metamaterials, they offer significant advantages such as simple structure, small size, low loss, and high integration, and are used to control the resonant frequency, radiation intensity, and direction of terahertz vortex waves. However, existing metasurface vortex wave generation schemes are highly dependent on plane wave excitation from external terahertz radiation sources. This external-source-dependent method suffers from significant problems such as high system complexity, poor integration and portability, and weak frequency tuning capability.

[0004] On the other hand, when charged particles glide across the surface of a periodic structure, they generate Smith-Purcell radiation, which provides a new approach for passively generating terahertz waves. However, conventional Smith-Purcell radiation produces linearly polarized waves and cannot directly form vortex waves carrying orbital angular momentum. How to directly convert radiation excited by charged particles into high-quality terahertz vortex radiation without an external terahertz source, and achieve continuous and efficient control from linear polarization to circular polarization and then to the vortex phase, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To overcome the shortcomings and deficiencies of existing technologies, the present invention aims to provide a method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface. The method achieves the polarization conversion of Smith-Purcell radiation through a first-layer resonant enhancement metasurface and the precise control of the OAM mode through a second-layer geometric phase metasurface. No external terahertz radiation source is required. Coherent vortex Smith-Purcell radiation at high harmonic frequencies can be achieved by combining it with a clustered electron beam.

[0006] This invention is achieved through the following technical solution:

[0007] A method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface includes the following steps: S100. Constructing a double-layer metasurface structure, wherein the first layer is a resonant-enhanced metasurface and the second layer is a geometric phase metasurface, both arranged along the Z-axis, with the first layer located below the second layer; setting a clustered electron beam to uniformly sweep across the lower surface of the first-layer metasurface along the X-direction of the XOY plane; S200. Adjusting the unit structure of the first-layer resonant-enhanced metasurface to give it equivalent quarter-wave plate characteristics, in order to generate... The phase difference converts the linearly polarized state of the Smith-Purcell radiation into a circularly polarized state; S300. The converted circularly polarized radiation is incident onto the second geometric phase metasurface, and vortex Smith-Purcell radiation is generated by the phase gradient modulation of the geometric phase metasurface.

[0008] In step S100, the double-layer metasurface is disposed on the XOY plane along the Z-axis direction, the distance between the first and second metasurfaces is set to D, and the clustered electron beam passes parallel to the lower surface of the metasurface at a distance d below the first metasurface and moves at a constant speed along the X direction.

[0009] The dual-layer metasurface is composed of several groups of basic structural units arranged periodically. The first and second layers use different unit patterns. All structural units are arranged in the XOY plane along the X and Y directions at a fixed period. arrangement.

[0010] The two-layer metasurface is composed of 24×24 structural units arranged periodically; the distance between the first and second metasurfaces is D=0.1mm, and the clustered electron beam passes parallel to the first metasurface at a distance of d=0.01mm below it.

[0011] The unit structure of the first layer of resonant-enhanced metasurface includes: a unit period p = 0.66 mm, and a dielectric substrate thickness of... =0.16mm, metal layer parameters: =0.63mm, =0.2mm, =0.06mm, =0.15mm, fixed rotation angle φ=64°; the unit structure of the second layer geometric phase metasurface includes: two dielectric substrates with a thickness of... =0.12mm, metal layer parameters: =0.38mm, =0.36mm, =0.38mm, =0.08mm, =0.06mm, =0.02mm, rotation angle range θ is 0°~157.5°.

[0012] The second layer of geometric phase metasurface is based on the Pancharatnam-Berry phase principle, achieving phase modulation range coverage under the operating wavelength. .

[0013] The operation of the geometric phase metasurface depends on the incident circularly polarized wave, when the anisotropic unit rotates by an angle of... At that time, a circularly polarized incident wave produces The phase abruptly changes, and the polarization chirality of the outgoing wave is opposite to that of the incident wave.

[0014] The double-layer metasurface is placed in a uniform static magnetic field, with the magnetic field direction along the X-axis, the direction of movement of the clustered electron beam, in order to maintain the clustering stability of the clustered electron beam and prevent divergence.

[0015] Both the resonant enhancement metasurface and the geometric phase metasurface are made of a copper metal layer and a polytetrafluoroethylene fiberglass cloth dielectric substrate.

[0016] Specifically, by adjusting the phase gradient distribution of the second-layer geometric phase metasurface, vortex Smith-Purcell radiation of the target OAM mode order l is generated: when the target OAM mode order... At that time, a central phase singularity is generated, and the phase change is... When the target OAM mode order At that time, a central phase singularity is generated, and the phase change is... .

[0017] The beneficial effects of this invention are as follows: This invention provides a method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface. By setting up two metasurfaces with different functions—a first-layer resonant enhancement metasurface and a second-layer geometric phase metasurface—and combining them with direct excitation by a clustered electron beam, it solves the problem that traditional metasurface vortex wave generation schemes heavily rely on external, large terahertz radiation sources. The first-layer metasurface accurately achieves the in-situ conversion from linear to circular polarization, while the second layer performs gradient modulation of complex phases on this basis. The two layers work together to make the entire radiation system structure extremely compact. This scheme realizes the generation of terahertz vortex radiation under passive excitation, significantly improving the integration and portability of the device. Attached Figure Description

[0018] The present invention will be further described with reference to the accompanying drawings, but the embodiments in the drawings do not constitute any limitation on the present invention. For those skilled in the art, other drawings can be obtained based on the following drawings without creative effort.

[0019] Figure 1 is a schematic diagram of the structure for realizing vortex Smith-Purcell radiation in an embodiment of the present invention.

[0020] Figure 2 is a schematic diagram of the structure of the first layer of the resonant enhanced metasurface of the present invention, wherein (a) is a top view of the xoy plane and (b) is a three-dimensional structural schematic diagram of the resonant enhanced metasurface unit.

[0021] Figure 3 is a schematic diagram of the structure of the second layer geometric phase metasurface of the present invention, wherein (a) is a top view of the xoy plane and (b) is a three-dimensional structural schematic diagram of the geometric phase metasurface unit.

[0022] Figure 4 is a schematic diagram of the mechanism of clustered electron beam excitation of coherent terahertz radiation in an embodiment of the present invention.

[0023] Figure 5 shows the transmission characteristics of the geometric phase metasurface unit in the embodiment of the present invention, where (a) represents the amplitude and (b) represents the phase.

[0024] Figure 6 shows the real distribution of the radiation field Ey of the entire double-layer metasurface system on the XOZ plane in an embodiment of the present invention.

[0025] Figure 7 shows the vortex Smith-Purcell radiation results of two OAM modes implemented in the embodiments of the present invention, where (a) and (b) are the real part and phase distribution of the electric field component Ey when the OAM mode order l=+1, respectively, and (c) and (d) are the real part and phase distribution of the electric field component Ey when l=+2, respectively.

[0026] The attached diagram is labeled with the following symbols: resonant-enhanced metasurface--1, geometric phase metasurface--2, clustered electron beam--3. Detailed Implementation

[0027] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0028] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0030] Please refer to Figures 1 to 7. A method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface, in this embodiment, includes the following steps: S100. Constructing a double-layer metasurface structure, wherein the first layer is a resonant enhancement metasurface 1, and the second layer is a geometric phase metasurface 2, both arranged along the Z-axis, with the first layer located below the second layer. A clustered electron beam 3 is set to uniformly sweep across the lower surface of the first layer metasurface along the X-direction of the XOY plane; S200. Adjusting the unit structure of the first layer resonant enhancement metasurface 1 to make it have equivalent quarter-wave plate characteristics to generate The phase difference converts the linearly polarized state of the Smith-Purcell radiation into a circularly polarized state; S300. The converted circularly polarized radiation is incident on the second geometric phase metasurface 2, and vortex Smith-Purcell radiation is generated by the phase gradient modulation of the geometric phase metasurface 2.

[0031] In this embodiment, the double-layer metasurface is composed of 24×24 cells (structural units) arranged periodically. The first layer and the second layer use different unit patterns, as shown in Figure 2(a) and Figure 3(a) respectively. All cells are arranged in the XOY plane along the X and Y directions with a fixed period of p=0.66mm (0.48 times the working wavelength of 1.36mm).

[0032] The unit cell structure of the first resonant-enhanced metasurface 1 includes: a unit cell period p = 0.66 mm, and a dielectric substrate thickness of... =0.16mm, metal layer parameters: =0.63mm, =0.2mm, =0.06mm, =0.15mm, fixed rotation angle φ=64°; the unit structure of the second layer geometric phase metasurface 2 includes: two dielectric plates with a thickness of 0.15mm and a fixed rotation angle φ=64°; =0.12mm, metal layer parameters: =0.38mm, =0.36mm, =0.38mm, =0.08mm, =0.06mm, =0.02mm, rotation angle range θ is 0°~157.5°.

[0033] When the electron beam passes through the periodic structure, it excites surface waves (local fields), which decay rapidly as they move away from the metasurface. In this embodiment, the fundamental frequency (surface wave frequency) of the resonant-enhanced metasurface 1 is fb = 0.11 THz. As shown in Figure 4, the electron beam line with diffraction order m = 0 intersects the metasurface dispersion curve. This interaction point means that the phase velocity of the surface wave matches the velocity of the electron beam, determining the surface wave frequency to be 0.11 THz. At this frequency, the fundamental energy cannot couple to the radiation mode, thus Smith-Purcell radiation cannot be generated. However, the clustered electron beam 3 contains higher harmonic components, with the second harmonic frequency fh = 2fb = 0.22 THz (corresponding to the designed operating wavelength λ = 1.36 mm), which is located within the Smith-Purcell radiation region (0.148 THz ~ 0.425 THz). Therefore, it can be efficiently converted into coherent Smith-Purcell radiation.

[0034] The resonant-enhanced metasurface 1 has an equivalent quarter-wave plate function, which is used to convert the linear polarization state of Smith-Purcell radiation into a circular polarization state, providing the necessary conditions for the subsequent generation of vortex radiation from the geometric phase metasurface 2.

[0035] The geometric phase metasurface 2 is based on the Pancharatnam-Berry phase principle, achieving a phase modulation range covering 2π at the operating wavelength. As shown in Figure 5(a), the cross-polarization transmittance of units 1 to 8 (with rotation angles θ of 0°, 22.5°, 45°, 67.5°, 90°, 112.5°, 135°, and 157.5° respectively) is greater than 0.85; as shown in Figure 5(b), the phase difference between units 1 and 8 is an integer multiple of 45° (Δφ = 2θ), where θ is the unit rotation angle.

[0036] The electron beam velocity ve = 0.484c (where c is the speed of light in vacuum) is derived from the Smith-Purcell dispersion relation. The bilayer metasurface is placed in a uniform static magnetic field of 0.5T, with the magnetic field direction along the direction of electron beam movement (X-axis) to maintain the stability of electron beam aggregation and prevent divergence.

[0037] Figure 6 shows the real distribution of the radiation field Ey of the entire bilayer metasurface system on the XOZ plane in this embodiment, with the electron beam operating voltage being 73 keV.

[0038] By adjusting the phase gradient distribution of the second-layer geometric phase metasurface 2, vortex Smith-Purcell radiation of the target OAM mode order l can be generated. Figures 7(a) and (b) show that when the target OAM mode order l=+1, the electric field component Ey in the XOY plane 20 mm above the metasurface (along the +z direction) exhibits a central phase singularity with a phase change of 2π, verifying the generation of l=+1 vortex radiation; Figures 7(c) and (d) show that when the target is l=+2, the distribution of Ey in the XOY plane 20 mm above the metasurface (along the +z direction) also exhibits a central phase singularity with a phase change of 4π, verifying the generation of l=+2 vortex radiation.

[0039] Both the resonant enhancement metasurface 1 and the geometric phase metasurface 2 employ a copper metal layer and a polytetrafluoroethylene fiberglass cloth (F4B) dielectric substrate to ensure phase modulation consistency.

[0040] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art, within the technical scope disclosed in the present invention, may make equivalent changes based on the technical solutions and improved concepts of the present invention, which shall still fall within the scope of protection covered by the present invention.

Claims

1. A method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface, characterized in that, Includes the following steps: S100. Construct a two-layer metasurface structure, wherein the first layer is a resonant-enhanced metasurface and the second layer is a geometric phase metasurface, both arranged along the Z-axis, with the first layer located below the second layer. A clustered electron beam is positioned to uniformly sweep across the lower surface of the first metasurface along the X-axis of the XOY plane. S200. Adjust the unit cell structure of the first resonant-enhanced metasurface to give it equivalent quarter-wave plate characteristics, thereby generating… The phase difference converts the linearly polarized state of the Smith-Purcell radiation into a circularly polarized state; S300. The converted circularly polarized radiation is incident onto the second geometric phase metasurface, and vortex Smith-Purcell radiation is generated by the phase gradient modulation of the geometric phase metasurface.

2. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1, characterized in that, In step S100, the double-layer metasurface is disposed on the XOY plane along the Z-axis direction, the distance between the first and second metasurfaces is set to D, and the clustered electron beam passes parallel to the lower surface of the metasurface at a distance d below the first metasurface and moves at a constant speed along the X direction.

3. A method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1 or 2, characterized in that, The bilayer metasurfaces are each composed of several groups of basic structural units arranged periodically. The first and second layers use different unit patterns. All structural units are arranged in the XOY plane along the X and Y directions at a fixed period. arrangement.

4. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 3, characterized in that, The two-layer metasurface is composed of 24×24 structural units arranged periodically; the distance between the first and second metasurface layers is D=0.1mm, and the clustered electron beam passes parallel to the first metasurface layer at a distance of d=0.01mm below it.

5. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 3, characterized in that, The unit cell structure of the first resonant-enhanced metasurface includes: a unit cell period p = 0.66 mm, and a dielectric substrate thickness of... =0.16mm, metal layer parameters: =0.63mm =0.2mm, =0.06mm, =0.15mm, fixed rotation angle φ=64°; the unit structure of the second layer geometric phase metasurface includes: two dielectric substrates with a thickness of... =0.12mm, metal layer parameters: =0.38mm, =0.36mm, =0.38mm, =0.08mm, =0.06mm, =0.02mm, rotation angle range θ is 0°~157.5°.

6. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1, characterized in that, The second-layer geometric phase metasurface, based on the Pancharatnam-Berry phase principle, achieves phase modulation range coverage at the operating wavelength. 。 7. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 6, characterized in that, The operation of the geometric phase metasurface depends on the incident circularly polarized wave, when the anisotropic unit rotates by an angle of . At that time, a circularly polarized incident wave produces The phase abruptly changes, and the polarization chirality of the outgoing wave is opposite to that of the incident wave.

8. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1, characterized in that, The double-layer metasurface is placed in a uniform static magnetic field, with the magnetic field direction along the X-axis, the direction of movement of the clustered electron beam, in order to maintain the clustering stability of the clustered electron beam and prevent divergence.

9. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1, characterized in that, Both the resonant enhancement metasurface and the geometric phase metasurface are made of a copper metal layer and a polytetrafluoroethylene fiberglass cloth dielectric substrate.

10. The method for realizing terahertz vortex Smith-Purcell radiation based on a double-layer metasurface according to claim 1, characterized in that, By adjusting the phase gradient distribution of the second-layer geometric phase metasurface, vortex Smith-Purcell radiation of the target OAM mode order l is generated: when the target OAM mode order l... At that time, a central phase singularity is generated, and the phase change is... When the target OAM mode order At that time, a central phase singularity is generated, and the phase change is... 。

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