Terahertz vortex beam metasurface structure with self-focusing function

Abstract

The application discloses a terahertz vortex beam metasurface structure with a self-focusing function, and belongs to the technical field of terahertz functional devices. The structure is a three-layer structure, which is composed of a polyimide medium layer and a first metal pattern layer and a second metal pattern layer symmetrically arranged on the upper and lower surfaces of the polyimide medium layer. The metal pattern layer adopts a double-C-ring-to-opened-elliptical-ring metasurface unit, which is arranged according to a preset rotation angle, and the rotation angle satisfies a superposition formula of a vortex phase term and a focusing phase compensation term. The application can convert an incident circularly polarized terahertz wave into a transmission-type terahertz vortex beam carrying an orbital angular momentum and focused at a preset focal length. The transmittance reaches 0.65 at a working frequency, and the purity of the orbital angular momentum mode is 86%. Without an additional focusing element, the application has the advantages of compact structure, high integration, easy planar processing and the like.

Description

Technical Field

[0001] This invention belongs to the field of terahertz functional device technology, specifically relating to a terahertz vortex beam metasurface structure with self-focusing function. Background Technology

[0002] Terahertz waves have significant applications in communication, imaging, non-destructive testing, and security inspection. In terahertz systems, it is often necessary to control the intensity, phase, polarization, and propagation direction of terahertz waves. However, traditional bulk optical devices suffer from problems such as large size, low integration, and complex fabrication and assembly, making it difficult to meet the application requirements of miniaturized terahertz functional devices.

[0003] Terahertz vortex beams carry orbital angular momentum, and their phase is spirally distributed along the azimuth direction. A phase singularity exists at the beam center, making them suitable for applications such as communication multiplexing, imaging, particle manipulation, and wavefront modulation. Existing terahertz vortex beam generating devices mostly rely on spiral phase plates, spatial light modulators, or multi-stage optical systems, resulting in complex device structures that are not conducive to integration with terahertz systems.

[0004] Metasurfaces are ultrathin planar devices composed of subwavelength artificial structural units that can control the amplitude, phase, and polarization state of electromagnetic waves by adjusting the geometric parameters, rotation angles, or arrangement of these units. Introducing metasurfaces into the terahertz band can provide a planarized and integrated technical approach for the generation and wavefront modulation of terahertz vortex beams.

[0005] Existing terahertz vortex metasurfaces primarily rely on single vortex phase control. To achieve energy focusing in the propagation direction, additional lenses or focusing elements are typically required, increasing system thickness and assembly complexity. Therefore, providing a terahertz metasurface structure capable of simultaneously achieving vortex phase control and focusing phase compensation within the same metasurface structure is a pressing technical problem for those skilled in the art. Summary of the Invention

[0006] To address the problems of existing terahertz vortex beam generating devices being complex in structure, having low integration, and being unable to simultaneously achieve vortex modulation and focusing modulation, this invention provides a terahertz vortex beam metasurface structure with self-focusing function. This structure can convert incident circularly polarized terahertz waves into a transmission-type terahertz vortex beam carrying orbital angular momentum and focused at a preset focal length. It features a compact structure, ease of planar fabrication, and the ability to simultaneously achieve vortex modulation and focusing modulation.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A terahertz vortex beam metasurface structure with self-focusing function is a three-layer structure, consisting of a dielectric layer and a first metal pattern layer and a second metal pattern layer symmetrically arranged on the upper and lower surfaces of the dielectric layer. The first metal pattern layer is disposed on the incident side surface of the dielectric layer, and the second metal pattern layer is disposed on the exit side surface of the dielectric layer, with the positions of the first and second metal pattern layers corresponding to each other. Both the first and second metal pattern layers include multiple metasurface units, and the multiple metasurface units are arranged according to a preset rotation angle. The arrangement of the metasurface units in the second metal pattern layer corresponds to the arrangement of the metasurface units in the first metal pattern layer, so that the cross-polarization transmission component of the incident circularly polarized terahertz wave obtains a target phase distribution formed by the superposition of the vortex phase term and the focusing phase compensation term, and forms a focused terahertz vortex beam carrying orbital angular momentum at a preset propagation distance.

[0009] Furthermore, both the first and second metal pattern layers are made of copper, and the dielectric layer is made of polyimide.

[0010] Furthermore, the metasurface unit is a double C-ring open elliptical ring structure, consisting of two oppositely arranged C-shaped open elliptical metal rings, with the openings of the two C-shaped open elliptical metal rings being arranged opposite each other along the minor axis direction.

[0011] Furthermore, the minor axis parameter of the double C-ring relative to the open elliptical ring structure , Major axis parameters , Opening gap .

[0012] Furthermore, the array period of the metasurface unit .

[0013] Furthermore, the thickness of both the first metal pattern layer and the second metal pattern layer is... The thickness of the dielectric layer is The dielectric constant is The loss tangent is .

[0014] Furthermore, located in polar coordinates The rotation angle of the metasurface unit at that location satisfy:

[0015]

[0016] in, The incident terahertz wavelength; For designing the focal length; The topological charge of the target terahertz vortex beam is given by the sign, which corresponds to the direction of rotation of the incident circularly polarized terahertz wave.

[0017] The present invention has the following beneficial effects:

[0018] This invention utilizes the modulation capability of metasurface devices for terahertz waves, simultaneously introducing vortex phase terms and focusing phase compensation terms in the same planar structure, achieving self-focusing of terahertz vortex beams without the need for additional focusing elements. The invention employs a transmissive metasurface unit composed of a double-layer metal patterned layer and a polyimide dielectric layer, which has advantages such as small size, thin thickness, easy processing, high integration, and flexible control of the terahertz wavefront. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the parameters of a terahertz vortex beam metasurface structure with self-focusing function according to an embodiment of the present invention.

[0020] Figure 2 This is a schematic diagram of a double-layer structure of a terahertz vortex beam metasurface structure with self-focusing function, as described in an embodiment of the present invention.

[0021] Figure 3 The image shows the S-parameters of a terahertz vortex beam metasurface structure with self-focusing function as described in an embodiment of the present invention.

[0022] Figure 4 This is a projection phase diagram of a terahertz vortex beam metasurface structure with self-focusing function as described in an embodiment of the present invention.

[0023] Figure 5 This is a diagram showing the array arrangement of metasurface units in a terahertz vortex beam metasurface structure with self-focusing function, as described in an embodiment of the present invention.

[0024] Figure 6 The image shows the electric field distribution of a terahertz vortex beam metasurface structure with self-focusing function under 1.2 THz plane wave irradiation, as described in an embodiment of the present invention.

[0025] Figure 7 The image shows the metasurface phase distribution of a terahertz vortex beam metasurface structure with self-focusing function under 1.2 THz plane wave illumination, as described in an embodiment of the present invention.

[0026] Figure 8 This is the OAM purity spectrum of a terahertz vortex beam metasurface structure with self-focusing function as described in an embodiment of the present invention.

[0027] Figure 9 This is an isometric schematic diagram of a terahertz vortex beam metasurface structure with self-focusing function according to an embodiment of the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 , Figure 2 , Figure 5 , Figure 9 As shown in the figure, an embodiment of the present invention discloses a terahertz vortex beam metasurface structure with self-focusing function, used to convert incident circularly polarized terahertz waves into a transmission-type terahertz vortex beam carrying orbital angular momentum and focused at a preset focal length. The terahertz vortex beam metasurface structure is a three-layer structure, consisting of a dielectric layer and a first metal pattern layer and a second metal pattern layer symmetrically arranged on the upper and lower surfaces of the dielectric layer; the first metal pattern layer is disposed on the incident side surface of the dielectric layer, and the second metal pattern layer is disposed on the exit side surface of the dielectric layer, with the positions of the first metal pattern layer and the second metal pattern layer corresponding to each other; both the first and second metal pattern layers include multiple metasurface units, and the multiple metasurface units are arranged according to a preset rotation angle. The arrangement of the metasurface units in the second metal pattern layer corresponds to the arrangement of the metasurface units in the first metal pattern layer, so that the cross-polarization transmission component of the incident circularly polarized terahertz wave obtains a target phase distribution formed by the superposition of the vortex phase term and the focusing phase compensation term, and forms a focused terahertz vortex beam carrying orbital angular momentum at a preset propagation distance.

[0030] Furthermore, both the first and second metal pattern layers are made of copper, and the dielectric layer is made of polyimide.

[0031] Furthermore, such as Figure 1 As shown, the metasurface unit is a double C-ring-to-open elliptical ring structure, which consists of two oppositely arranged C-shaped open elliptical metal rings, with the openings of the two C-shaped open elliptical metal rings being opposite each other along the minor axis direction.

[0032] In this embodiment, the minor axis parameter of the double C-ring relative to the open elliptical ring structure , Major axis parameters , Opening gap ; Array period of metasurface units .

[0033] In this embodiment, the thickness of both the first metal pattern layer and the second metal pattern layer is The thickness of the dielectric layer is dielectric constant Loss tangent .

[0034] Furthermore, this embodiment employs the Pancharatnam-Berry geometric phase principle to achieve phase modulation. When a circularly polarized terahertz wave is incident on a rotation angle of... When the anisotropic metasurface unit is used, the cross-polarized transmission component acquires a geometric phase related to the rotation angle, and its phase change It can be represented as:

[0035]

[0036] in, The rotation direction of the incident circularly polarized state is indicated by setting a corresponding rotation angle for each metasurface unit in the metasurface array. This allows you to obtain the preset spatial phase distribution.

[0037] Furthermore, the preset rotation angle arrangement of the multiple metasurface units simultaneously satisfies both the vortex phase distribution and the focusing phase compensation distribution. In this embodiment, to simultaneously achieve vortex beam generation and self-focusing functions, the metasurface unit structure is rotated and arranged to form a metasurface array with a composite phase distribution, so that the incident circularly polarized terahertz wave, after transmission, forms a terahertz vortex beam with self-focusing function. The composite phase distribution includes a vortex phase term varying along the azimuth angle and a focusing phase compensation term varying along the radial direction. The vortex phase term imparts orbital angular momentum to the transmitted terahertz wave, and the focusing phase compensation term compensates for the optical path difference of the transmitted wave reaching the preset focal point at different radial positions, thereby enabling the transmitted vortex beam to achieve energy convergence at the preset focal length.

[0038] In this embodiment, the target phase is set as a vortex phase term that varies along the azimuth angle. With radially varying focusing phase compensation term The superposition of these elements will be located in polar coordinates. Target phase at satisfy:

[0039]

[0040] Therefore, it is located in polar coordinate position Metasurface unit rotation angle at the location satisfy:

[0041]

[0042] in, The incident terahertz wavelength; For designing the focal length; The topological charge of the target terahertz vortex beam is given by the sign, which corresponds to the direction of rotation of the incident circularly polarized terahertz wave.

[0043] In this embodiment, the focal length is designed. Target topological load After rotating and arranging the metasurface units according to the above phase distribution, the incident circularly polarized terahertz wave is simultaneously endowed with a helical phase and a focusing phase during transmission, thereby forming a self-focusing terahertz vortex beam near a preset propagation distance.

[0044] like Figure 3 As shown in Figure 3, the S-parameter simulation results of the metasurface unit at different frequencies are described. Figure 3 shows that at the designed operating frequency... Nearby, the metasurface unit has a corresponding transmission response, wherein The amplitude can be used to characterize the transmission modulation capability of metasurface units to incident terahertz waves.

[0045] like Figure 4 As shown, the transmission phase results of double C-ring paired with open elliptical ring structures under circularly polarized terahertz wave incident conditions are presented with different rotation angles. Metasurface units with different rotation angles can form a covering near the design frequency. The transmission phase change, which is consistent with the geometric phase principle, provides the necessary phase gradient for the generation of terahertz vortex beams and focusing phase compensation.

[0046] like Figure 5 As shown, based on the desired composite phase distribution, multiple metasurface units are rotated and arranged to form a metasurface array with the target topological charge and target focal length. The rotation angle of each metasurface unit in the metasurface array is jointly determined by the vortex phase term and the focusing phase compensation term corresponding to its position.

[0047] like Figure 6 , Figure 7 As shown, the field intensity and phase distribution of a self-focusing vortex beam generated under terahertz plane wave incidence conditions are illustrated. In the field intensity distribution diagram, a low field intensity region forms at the beam's center, while a ring-shaped distribution appears on the outer periphery. The phase distribution diagram exhibits a spiral variation, indicating that the transmitted beam carries orbital angular momentum and interacts with the target topological charge. Correspondingly.

[0048] Figure 8 shows the orbital angular momentum mode purity spectrum of the generated terahertz vortex beam. In one embodiment, the metasurface structure in Transmittance at operating frequency The orbital angular momentum mode purity of the generated terahertz vortex beam is .

[0049] The simulation results above demonstrate that the metasurface structure can achieve transmission phase modulation, vortex phase modulation, and focusing phase compensation within the same planar structure, thereby generating a terahertz vortex beam with self-focusing capability. It will be readily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A terahertz vortex beam metasurface structure with self-focusing function, characterized in that, It has a three-layer structure, consisting of a dielectric layer and a first metal pattern layer and a second metal pattern layer symmetrically arranged on the upper and lower surfaces of the dielectric layer; the first metal pattern layer is arranged on the incident side surface of the dielectric layer, and the second metal pattern layer is arranged on the exit side surface of the dielectric layer, with the positions of the first metal pattern layer and the second metal pattern layer corresponding to each other. Both the first and second metal pattern layers include multiple metasurface units, and the multiple metasurface units are arranged according to a preset rotation angle. The arrangement of the metasurface units in the second metal pattern layer corresponds to the arrangement of the metasurface units in the first metal pattern layer, so that the cross-polarization transmission component of the incident circularly polarized terahertz wave obtains the target phase distribution formed by the superposition of the vortex phase term and the focusing phase compensation term, and forms a focused terahertz vortex beam carrying orbital angular momentum at a preset propagation distance.

2. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 1, characterized in that, Both the first and second metal pattern layers are made of copper, and the dielectric layer is made of polyimide.

3. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 1, characterized in that, The metasurface unit is double The ring-to-open elliptical ring structure consists of two oppositely arranged... It consists of two elliptical metal rings with open shapes. The openings of the elliptical metal rings are positioned opposite each other along their minor axes.

4. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 3, characterized in that, The dual Minor axis parameters of the ring-to-open elliptical ring structure , Major axis parameters , Opening gap .

5. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 1, characterized in that, The array period of the metasurface unit is .

6. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 1, characterized in that, The thickness of both the first metal pattern layer and the second metal pattern layer is The thickness of the dielectric layer is The dielectric constant is The loss tangent is .

7. The terahertz vortex beam metasurface structure with self-focusing function as described in claim 1, characterized in that, Located in polar coordinates The rotation angle of the metasurface unit at that location satisfy: ； in, The incident terahertz wavelength; For designing the focal length; The topological charge of the target terahertz vortex beam is given by the sign, which corresponds to the direction of rotation of the incident circularly polarized terahertz wave.

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