Method for generating subwavelength vortex beam array based on metasurface

Abstract

The application relates to a subwavelength vortex beam array generation method based on a metasurface and belongs to the technical field of polarization optics application. Specifically, the application relates to a two-way vortex beam metasurface design method based on phase control, which solves the problems of complex metasurface unit structure and symmetrical transmission limitation. Then, linearly polarized light is vertically irradiated on the metasurface structure in the positive and negative directions, the spatial symmetry in the electromagnetic wave propagation direction is broken, the full-range phase modulation is realized while the asymmetric transmission characteristic is obtained, the electromagnetic waves incident in the two directions can be independently controlled by changing the size parameters of the unit structure, and vortex beams carrying different topological charge numbers are respectively realized. Compared with other vortex beam generation methods, the method has the advantages of simple structure and easy integration.

Description

Technical Field

[0001] This invention relates to an asymmetric transmission wavefront modulation method, and more particularly to an asymmetric transmission wavefront modulation method based on a multilayer metasurface. It relates to the design technology of metasurfaces capable of simultaneously generating orbital angular momentum with different topological charges in both directions, and belongs to the application technology fields of holographic display, polarization optics, and information encryption. Background Technology

[0002] Vortex beams carrying orbital angular momentum (OAM) have shown great promise in optical communication. However, when relying on traditional helical waveplates to generate vortex beams, the large optical lenses are difficult to integrate into the entire system. Alternatively, when vortex beams are generated using holographic calculations, the imaging results are unclear and inefficient. To address this issue, ultrathin and ultralight metasurface structures have come into focus. With the increasing complexity of electromagnetic environments and the trend towards system intelligence and miniaturization, achieving efficient and flexible electromagnetic control within limited space using metasurfaces is currently a hot topic in electromagnetic wave research. Metasurfaces are artificially designed ultrathin two-dimensional metamaterials composed of subwavelength scatterers, capable of manipulating the phase, amplitude, and polarization of propagating waves. Due to their unique wavefront manipulation capabilities, many applications have been proposed based on metasurfaces, such as beam deflection, planar lenses, stealth, and holograms. Based on this, the bidirectional lens of this invention is a metasurface capable of independently controlling the transmission of linearly polarized electromagnetic waves from two directions at the same frequency, generating different vortex beams. This invention will provide a new mechanism and method for efficient and flexible electromagnetic wave manipulation, and is expected to be further applied to many electromagnetic wave manipulation-related fields such as low-cost, miniaturized communication and imaging. Summary of the Invention

[0003] The purpose of this invention is to design a subwavelength-sized metasurface-based bidirectional orbital angular momentum generator that enables cross-polarized wave transmission from the same polarization wave in two directions at the same frequency, and realizes two different vortex beams to provide a direction-selective asymmetric channel.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] The phenomenon of asymmetric propagation of electromagnetic waves refers to the different transmissive properties of electromagnetic waves propagating in a specific direction and those propagating in the opposite direction when passing through the same metasurface. To break the spatial symmetry of electromagnetic waves in the propagation direction and enable them to exhibit different wavefront modulation characteristics during forward and reverse propagation, it is first necessary to select a suitable unit structure to form a multilayer metasurface. This invention discloses an asymmetric wavefront modulation method based on a multilayer metasurface. It establishes the concept of a bidirectional orbital angular momentum generator on a two-layer metal patch and grating layer structure, effectively multiplexing the electromagnetic wave modulation function by utilizing the inherent characteristic of the propagation direction, thereby realizing a multifunctional electromagnetic device based on the propagation direction.

[0006] By adjusting the size and angle of the metal shape of the multilayer metasurface, the full phase modulation range of the orthogonal polarization transmission channel from 0 to 2π can be achieved. According to the phase distribution of the corresponding functions, the shape and size of all metal strips are determined one by one, that is, the phase information of vortex beams carrying different orbital angular momentum can be encoded in the multilayer metasurface. This allows vortex beams with different orbital angular momentum to be observed in the transmission channels corresponding to the positive and negative transmission directions of the linearly polarized wave.

[0007] A vortex beam carrying orbital angular momentum possesses [something] in a cross-section perpendicular to the direction of electromagnetic wave propagation. The phase distribution, where l is the orbital angular momentum number, whose value is an integer. Let be the azimuth angle within the cross-section perpendicular to the propagation direction of the vortex beam. To ensure that the cross-polarized transmitted wave passing through the lens carries an orbital angular momentum number of l, the distribution of the positive and negative phase abrupt changes introduced by the unit structure at position coordinates (x, y) during electromagnetic wave propagation satisfies the following formula:

[0008] φ f (x,y)=l1 arctan(y / x)

[0009] φ b (x,y)=l2arctan(y / x)

[0010] Among them l i The topological charge represents a vortex beam with orbital angular momentum.

[0011] The multilayer metasurface can realize asymmetric transmission of vortex beams with different topological charges. The unit structure is characterized by a metal layer A, a dielectric layer, a grating layer B, a dielectric layer and a metal layer C; the metal sheet C array is arranged at the bottom layer, the grating layer B is located between the two dielectric layers, and the top layer is a metal patch layer A. The angles between the centers of the metal patch layers A and C and the positive x-axis are θ1 and θ2.

[0012] The multilayer metasurface can realize asymmetric transmission of vortex beams with different topological charges. The feature is that the unit structure is arranged periodically, including m×n periodically arranged phase change units, where m and n are both positive integers; and both m and n are 51.

[0013] The multilayer metasurface can realize asymmetric transmission of vortex beams with different topological charges, characterized in that: the metal layer A, the grating layer B and the metal layer C are all copper layers and have the same thickness of 0.018 mm.

[0014] The multilayer metasurface can realize asymmetric transmission of vortex beams with different topological charges. It is characterized in that: the unit structure is a square with a side length at the subwavelength level and a length of p = 6 mm, the thickness of the dielectric layer is 1 mm, and the dielectric constant is 2.65.

[0015] The multilayer metasurface can realize asymmetric transmission of vortex beams with orbital angular momentum of different topological charges. The feature is that linearly polarized waves are incident perpendicularly on the metasurface in two directions. The transmitted cross-polarized wave incident in the forward direction generates a vortex beam with a topological charge number l1 = 2, and the transmitted cross-polarized wave incident in the reverse direction generates a vortex beam with a topological charge number l1 = 3.

[0016] Beneficial effects:

[0017] This invention is based on an asymmetric propagation wavefront modulation method using multilayer metasurfaces. It employs multilayer metal patches and grating layers as basic structural units, breaking the spatial symmetry of the electromagnetic wave propagation direction. This allows the generation of vortex beams with different topological charges during forward and reverse propagation. Full-phase modulation from 0 to 2π is achieved by adjusting the size of the metal patches, thus enabling the encoding of corresponding phases of two vortex beams. Combining the encoded phase distribution map with the asymmetric propagation characteristics of this cascaded multilayer metasurface allows for the generation of different vortex beams that are coupled with the propagation direction. The phases of the vortex beams are directly integrated into the metasurface, offering advantages such as small size, simple structure, and ease of integration. Attached Figure Description

[0018] Figure 1 This is a unit structure diagram of the vortex beam with orbital angular momentum and different topological charges based on a multilayer metasurface, which enables asymmetric transmission according to the present invention. The fixed parameter values ​​of the unit structure in the figure are p = 6 mm, b = 0.6 mm, s = 1.2 mm, w = 0.4 mm, and l = 5 mm. Among them, (a) is a 3D view of the unit structure. (b) Metal patch A layer of the unit structure. (c) Grating B layer of the unit structure. (d) Metal patch C layer of the unit structure.

[0019] Figure 2The 16 different basic building blocks selected for the multilayer metasurface of the present invention, which enables asymmetric transmission of vortex beams with orbital angular momentum of different topological charges, show the amplitude and phase modulation effects of each channel.

[0020] Figure 3 The theoretical phase distribution in the xoy plane is given by the multilayer metasurface of the present invention, which enables asymmetric transport and has topological charge orbital angular momentum l = 2.

[0021] Figure 4 The present invention provides a simulated phase distribution in the xoy plane for the multilayer metasurface to achieve asymmetric transport and possess topological charge orbital angular momentum l = 2.

[0022] Figure 5 The multilayer metasurface of the present invention enables asymmetric transport and provides a test phase distribution in the xoy plane when the orbital angular momentum l = 2 with topological charge.

[0023] Figure 6 The present invention provides a simulated amplitude distribution in the xoz plane for the multilayer metasurface to achieve asymmetric transport and have orbital angular momentum l = 2 with topological charge.

[0024] Figure 7 The multilayer metasurface of the present invention enables asymmetric transport and has an orbital angular momentum l = 2 with topological charge, and the measured amplitude distribution in the xoz plane is shown.

[0025] Figure 8 The theoretical phase distribution in the xoy plane is given by the multilayer metasurface of the present invention, which enables asymmetric transport and has orbital angular momentum l = 3 with topological charge.

[0026] Figure 9 The present invention provides a simulated phase distribution in the xoy plane for the multilayer metasurface to achieve asymmetric transport and possess topological charge orbital angular momentum l = 3.

[0027] Figure 10 The test phase distribution in the xoy plane is shown for the multilayer metasurface of the present invention, which enables asymmetric transport and has orbital angular momentum l = 3 with topological charge.

[0028] Figure 11 The present invention provides a simulated amplitude distribution in the xoz plane for the multilayer metasurface to achieve asymmetric transport and possess topological charge orbital angular momentum l = 3.

[0029] Figure 12 The multilayer metasurface of the present invention enables asymmetric transport and has a topological charge orbital angular momentum l = 3, and the measured amplitude distribution in the xoz plane is obtained. Detailed Implementation

[0030] This invention is designed through the following steps.

[0031] Specific implementation method one: Combining Figure 1 This embodiment will be described in detail below. The unit structure used in the metasurface for generating vortex beams carrying different topological charges described in this embodiment is as follows: Figure 1 As shown, the thickness of the intermediate dielectric substrate is h = 3 mm, and the dielectric constant is 2.65. Metal patches (labeled A and C) are used in the top and bottom layers respectively, and the intermediate layer consists of equally spaced metal gratings (labeled B). The angles between the centers of metal patch layers A and C and the positive x-axis are θ1 and θ2, respectively. The unit structure has the following fixed parameters: p = 6 mm, b = 0.6 mm, s = 1.2 mm, w = 0.4 mm, l = 5 mm.

[0032] Specific Implementation Method Two: Combining Figure 2 Specifically, to achieve bidirectional functionality, the phase change amount for each unit structure is set to π / 2. We further achieve phase changes for the corresponding 16 unit structures by altering the size and angle of the unit structures. The corresponding dimensions are shown in Table 1. Figure 2 This represents the amplitude spectrum of the transmitted x-polarized wave in all 16 coding units under forward and reverse incident y-polarized waves. Figure 2 This indicates the transmission phase of all 16 coding elements used for the transmission cross-polarization components (x-polarized waves) of y-polarized waves incident in both the forward and reverse directions.

[0033] Table 1. Structural parameters (mm) of the 16 coding units of the metasurface

[0034]

[0035] Specific implementation method three: The center frequency of the functional structure for generating vortex beam metasurfaces carrying different topological charges is 15 GHz.

[0036] Specific Implementation Method Four: The aforementioned metasurface for generating vortex beams carrying different topological charges is characterized by comprising m×n periodically arranged phase abrupt change units, where m and n are both positive integers; and both m and n are 51. That is, the metasurface is composed of 51×51 subatomic atoms, with a total area of ​​306×306 mm². 2 .

[0037] Specific Implementation Method Five: Combining Figures 3-7 In this embodiment, the metasurface can generate a vortex beam with orbital angular momentum l = 2 and topological charge for the transmission channel of a normally incident electromagnetic wave. The distribution of the positive phase abrupt change introduced by the unit structure at position coordinates (x, y) during electromagnetic wave propagation satisfies the following formula:

[0038] φ f (x,y)=l1arctan(y / x)

[0039] Where l1 = 2 represents the topological charge of the vortex beam with orbital angular momentum.

[0040] When a positively y-polarized wave is incident on the metasurface, the transmitted cross-polarized wave vector precisely realizes a vortex beam with l1=2 along the theoretically predicted target.

[0041] Furthermore, Figure 3 , Figure 4 and Figure 5 The theoretical, simulation, and measurement results of the phase distribution in the xoy plane are presented. It can be seen that in the xoy plane, the phase distribution of the beam conforms to l*2π. Furthermore, Figures 6-7 Simulation and test results of amplitude distribution on the xoz surface are presented, showing that for electromagnetic waves incident on the metasurface in the forward direction, the transmitted cross-polarized wave vector accurately realizes a vortex beam with l1=2 along the theoretically predicted target.

[0042] Specific Implementation Method Five: Combining Figures 8-12 In this embodiment, the metasurface can generate a vortex beam with orbital angular momentum l = 3 and topological charge for the transmission channel of a back-incident electromagnetic wave. The distribution of the positive phase abrupt change introduced by the unit structure at position coordinates (x, y) during electromagnetic wave propagation satisfies the following formula:

[0043] φ b (x,y)=l2arctan(y / x)

[0044] Where l2 = 3 represents the topological charge of the vortex beam with orbital angular momentum.

[0045] When a reverse y-polarized wave is incident on the metasurface, the transmitted cross-polarized wave vector precisely realizes a vortex beam with l2=3 along the theoretically predicted target.

[0046] Furthermore, Figure 8 , Figure 9 and Figure 10 The theoretical, simulation, and measurement results of the phase distribution in the xoy plane are presented. It can be seen that in the xoy plane, the phase distribution of the beam conforms to l*2π. Furthermore, Figures 11-12 Simulation and test results of amplitude distribution on the xoz surface are presented, showing that for electromagnetic waves incident on the metasurface in the forward direction, the transmitted cross-polarized wave vector accurately realizes a vortex beam with l2=3 along the theoretically predicted target.

Claims

1. A method for generating a subwavelength vortex beam array based on a metasurface, characterized in that: By changing the size and angle of the metal shape of the metasurface, the full phase modulation range of the orthogonal polarization transmission channel from 0 to 2π can be achieved; according to the phase distribution of the corresponding functions, the shape and size of all metal strips are determined one by one, that is, the phase information of different functions can be encoded in the metasurface; vortex beams with different orbital angular momentum can be observed in the transmission channels corresponding to the positive and negative transmission directions of the linearly polarized wave. A vortex beam carrying orbital angular momentum possesses [something] in a cross-section perpendicular to the direction of electromagnetic wave propagation. The phase distribution, where l is the orbital angular momentum number (an integer value), and i refers to the imaginary unit. Let be the azimuth angle within a cross-section perpendicular to the propagation direction of the vortex beam. To ensure that the cross-polarized transmitted wave passing through the metasurface carries an orbital angular momentum with an orbital angular momentum number of l, the position coordinates are... The distribution of positive and negative phase abrupt changes introduced by the unit structure at a given location during electromagnetic wave propagation satisfies the following formula: ； ； in The subscript 'i' in the text represents the two states of electromagnetic wave incidence: forward and reverse. When referring to forward incidence, When the light is incident in the opposite direction, , The distribution function representing the phase abrupt change in the forward propagation direction. The distribution function representing the phase abrupt change in the reverse propagation direction; The unit structure includes a metal layer A, a dielectric layer, a grating layer B, another dielectric layer, and a metal layer C. The metal layer C array is arranged at the bottom layer, the grating layer B is located between two dielectric layers, and the top layer is metal layer A. The angle between the centers of metal layers A and C and the positive x-axis is [value missing]. and The metal layer A, grating layer B, and metal layer C are all copper layers and have the same thickness of 0.018 mm. The location coordinates are The unit structure at that location exhibits a periodic arrangement, including... The structure consists of periodically arranged phase transition units, where m and n are both positive integers; and m and n are both 51 units. The unit structure is square with a side length at the subwavelength level and a length of p=6mm. The thickness of the dielectric layer is 1mm and the dielectric constant is 2.

65.

2. The method for generating a subwavelength vortex beam array based on a metasurface according to claim 1, characterized in that: When linearly polarized waves are incident perpendicularly to the metasurface in two directions, the transmitted cross-polarized waves incident in the forward direction generate a topological charge number. The vortex beam, the cross-polarized wave transmitted by the reverse incident generates the topological charge number of the vortex beam.

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