Design method and metadevice of terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution metadevice

Abstract

The present invention discloses a terahertz circular polarization multiplexing long focal depth polarization state longitudinal evolution meta-structure device design method and meta-structure device, which belongs to the field of optical devices, and includes the following steps: S1, constructing a linear transmission Jones matrix; S2, setting a screening rule for a super unit structure based on the linear transmission Jones matrix; S3, designing the phase of the metasurface, the initial phase of the light field, and the initial surface phase distribution of the light field; S4, using all-silicon dielectric materials to construct a metasurface unit according to the designed phase of the metasurface, the initial phase of the light field, and the initial surface phase distribution of the light field; S5, based on the screening rule, screening the metasurface unit composition with a phase gradient of π / 8 and a unit library with full coverage from 0 to 2π; S6, based on the unit library, using an automatic layout algorithm to construct a metasurface structure. The present invention adopts the above-mentioned terahertz circular polarization multiplexing long focal depth polarization state longitudinal evolution meta-structure device design method and meta-structure device, and realizes that under the incidence of linear polarized terahertz light, the metasurface focuses the left and right hand components at different focal depths respectively, and realizes the generation of different polarization states through the principle of polarization superposition.

Description

Technical Field

[0001] The present invention relates to the technical field of optical devices, and in particular to a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device design method and a meta-device. Background Art

[0002] Polarization is the orderly movement of the direction of the electric field in an electromagnetic wave. As a fundamental property of light, polarization has been used in fields ranging from quantum optics and imaging to optical displays, light-matter interactions, and sensing. Traditional polarization control devices use optical devices such as polarizers and wave plates to independently change the orthogonal polarization of the incident light beam and convert it into the desired local polarization state distribution. It has been widely used in polarization gratings, holographic imaging, multifunctional polarization controllers, polarization imaging, vector beams, etc.

[0003] However, traditional polarization control schemes usually regard the polarization of light as a global static property, mainly focusing on the polarization state control in a single horizontal plane, and it is difficult to achieve dynamic changes in the longitudinal multi-plane spatial polarization state. Summary of the invention

[0004] To solve the above problems, the present invention provides a terahertz circular polarization multiplexing long focal depth polarization state longitudinal evolution metastructure device design method and metastructure device, which realizes that under the incidence of linearly polarized terahertz light, the metasurface focuses the left and right rotating components at different focal depths respectively, and realizes the generation of different polarization states through the principle of polarization superposition.

[0005] To achieve the above object, the present invention provides a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device design method, comprising the following steps:

[0006] S1. Construct a linear transmission Jones matrix to analyze the physical mechanism of metasurface regulation of terahertz light field;

[0007] S2, setting the screening rules of super unit structure based on linear transmission Jones matrix;

[0008] S3, design the metasurface phase, light field initial phase and light field initial surface phase distribution;

[0009] S4, constructing a metasurface unit using all-silicon dielectric materials according to the designed metasurface phase, light field initial phase, and light field initial surface phase distribution;

[0010] S5. Based on the screening rules, the metasurface unit composition is screened to have a unit library with a phase gradient of π / 8 and a full coverage of phases from 0 to 2π;

[0011] S6. Based on the cell library, the metasurface structure is constructed using the automatic layout algorithm.

[0012] Preferably, step S1 specifically includes the following steps:

[0013] S11. Construct a linear transmission Jones matrix of a supercell:

[0014]

[0015] Where, t xx represents the transmission coefficient of the electric field propagating along the x-axis, T xx , They represent the amplitude and phase of the electric field propagating along the x-axis respectively; t yy represents the transmission coefficient of the electric field propagating along the y-axis, T yy , They represent the amplitude and phase of the electric field propagating along the y-axis respectively;

[0016] S12, assuming the transmission amplitude T of the supercell xx =T yy =1, the rotation angle of the super unit structure is θ, and the linear Jones matrix J(θ) of the super surface is obtained as follows:

[0017] J(θ)=M(θ) T ×T×M(θ) (2)

[0018] Where M(θ) is the optical rotation matrix, and

[0019] S13. Combined with the analysis of the metasurface Jones matrix under circular polarization, the metasurface transmission Jones matrix when the supercell structure is rotated at an angle of θ is expressed as:

[0020]

[0021] And meet δ + ×E LCP =E RCP ; δ + ×E RCP =0δ - ×E RCP =E LCP ; δ - ×E LCP =0;

[0022] In the formula, is the phase difference between the x and y directions; δ + and δ - is the left-right polarized light conversion matrix; E LCP and E RCP are the Jones vectors of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

[0023] Preferably, step S2 specifically includes the following steps:

[0024] S21. Settings To suppress the transmission of the same polarization signal and improve the cross-polarization conversion rate; and set the same polarization channel amplitude of the metasurface to 0, and the cross-polarization channel amplitude to 1; at this time, the incident LCP is completely converted into RCP through the transmission field, and the phase of this transmission process is recorded as The incident RCP is completely converted into LCP through the transmission field, and the phase of this transmission process is recorded as

[0025] S22. Combine formula (3) to obtain the screening rule of super unit structure:

[0026]

[0027] Preferably, step S3 specifically includes the following steps:

[0028] S31, introduce telephoto deep focusing phase and design metasurface and Phase, and the relationship between the long focal depth convergence phase and the coordinate position of the metasurface superunit microstructure satisfies the following formula:

[0029]

[0030] In the formula, is the phase distribution function of the long focal depth convergence beam; k is the spatial wave vector, k = 2π / λ, λ is the wavelength of the incident light; r is the radius coordinate, f is the starting point of the focus of the metasurface transmission field; Δf is the focal depth range of the transmission field; R represents the numerical aperture of the metasurface, that is, the maximum radius of the metasurface;

[0031] S32, in order to realize the metasurface, the left-handed and right-handed components of the terahertz beam are focused to different focal depths of the optical axis, and the initial phase of the light field of the metasurface is And the initial surface phase distribution The design is as follows:

[0032]

[0033] Wherein, f2 is the starting point of the convergence depth of LCP, f2=4mm; f3 is the starting point of the convergence depth of RCP, f3=6mm; Δf=2.5mm.

[0034] Preferably, in step S4, the dielectric constant ε of the all-silicon dielectric is 11.9.

[0035] Preferably, in step S5, the screened 15 super unit structures form a unit library, so that when a 1.1 THz linearly polarized wave is incident, the transmittance of the super unit structure in the unit library along the x direction is selected to be greater than 0.6.

[0036] Preferably, in step S6, a supersurface structure of 80×80 units is constructed.

[0037] A meta-structure device, which is an all-silicon dielectric asymmetric meta-surface structure;

[0038] The all-silicon dielectric asymmetric metasurface structure includes a cross-shaped column at the top and a rectangular parallelepiped base at the bottom. A rectangular parallelepiped is also arranged on the convex surface of the cross-shaped column at the top. The length of the central axis of the cross section of the cross-shaped column at the top is 30μm-130μm, the cross-shaped width D of the cross section of the cross-shaped column at the top is 30μm, the height T of the cross-shaped column at the top is 200μm, and the step length is 1μm; the cross section length dw of the rectangular parallelepiped is 30-50μm, and the width s is 5μm. The step length is 1μm;

[0039] The upper and lower end faces of the bottom rectangular parallelepiped base are both squares, and the side length P of the square is 140 μm. The height H of the bottom rectangular parallelepiped base is 300 μm.

[0040] The present invention has the following beneficial effects:

[0041] 1. The left-handed and right-handed circularly polarized components of the incident linearly polarized terahertz beam are focused at positions within different focal depths of the optical axis, and the focal spots of the left-handed polarization and the right-handed polarization are partially overlapped, thereby realizing the longitudinal evolution of the polarization distribution at different longitudinal focal depths;

[0042] 2. Combine the dynamic phase and geometric phase, introduce the long focal depth convergence beam phase distribution function in the dynamic phase, and finally realize that when the 45° linearly polarized terahertz beam on the xoy plane is incident on the metasurface optical device, the SHEL spin-related left and right hand beams of the terahertz beam will be focused at different positions on the z-axis, forming a long focal depth spot with a focal depth of Δf;

[0043] 3. Based on the geometric design and array arrangement of anisotropic all-silicon dielectric micro-nano units, the construction of a metasurface with longitudinal evolution of polarization states within a long focal depth range is realized. Under the incidence of linearly polarized terahertz light, the metasurface focuses the left and right components at different focal depths respectively. Through the principle of polarization superposition, different polarization states are generated. That is, at the overlap of the focal spots of the two polarization states, the superposition of left and right circularly polarized light with different amplitudes and phases can generate any type of polarized light, which is suitable for optical communications, optical imaging, photonic devices, photonic crystals, and biomedical imaging.

[0044] 4. By precisely controlling the phase and amplitude of the initial surface of the metasurface light field, dynamic control of beam polarization is achieved and the needs of specific applications can be met.

[0045] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A flow chart of the method for designing a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device of the present invention;

[0047] Figure 2 It is a schematic diagram of the structure of the meta-structure device of the present invention;

[0048] Figure 3 is a phase distribution diagram of the meta-device of the present invention;

[0049] Figure 4 Amplitude and phase information diagram of 15 structures in the unit library of simulation experiments;

[0050] Figure 5 is the electric field distribution diagram of the metasurface transmission field in the simulation experiment; Figure 5 (a) Figure 5 (b) Figure 5 (c) The distribution diagrams of the right-handed wave component, left-handed wave component and total electric field intensity of the transmitted field SHEL, respectively; Figure 5 (d) Figure 5 (e) Figure 5 (f) The normalized intensity results of SHEL right-handed wave component, left-handed wave component and total electric field are shown respectively;

[0051] Figure 6 is the polarization distribution diagram of the focal position of the simulation experiment; Figure 6 (a) is the polarization distribution diagram of the transmission field xoz plane; Figure 6 (b) Figure 6 (c) Ex and Ey intensity distribution diagrams at the focus f4, respectively; Figure 6 (d) is the normalized intensity result of the electric field at the focus f4; Figure 6 (e) Figure 6 (f) Phase distribution diagrams in x and y directions at the focus f4, respectively;

[0052] Figure 7 The distribution diagram of the Stokes parameter S3 of the left-right hand component intensity and the diffraction center point in different z planes of the simulation experiment; Figure 7 (a) is the diagram of the change of left-right polarization of the transmitted field along the propagation direction, and Figure 7 In (a), the first and second rows of pictures show the electric field intensity distribution of the SHEL right-handed and left-handed components, respectively, and the third row of pictures shows the distribution of the Stokes parameter S3 in the transmission field; Figure 7 (b) is the parameter variation curve of the transmission field along the propagation direction S3; Figure 7 (c) is the trajectory of the polarization state of the transmitted field along the propagation direction on the surface of the Poincare sphere

[0053] Figure 8 is the local polarization state distribution diagram of different focal depths in the transmission field of the simulation experiment, where Figure 8 (a) is a schematic diagram of the polarization change of the terahertz beam metasurface optical device with long focal depth and arbitrary polarization state. Figure 8 In (a), the first row of images shows different z-plane electric field directions, and the second row of images shows different z-plane polarization states as polarization ellipses; Figure 8 (b) is a schematic diagram of the dynamic change of the polarization state of the transmitted field along the propagation direction.

[0054] Among them, 1. The bottom is a rectangular base; 2. The top is a cross-shaped column; 3. The rectangular block. DETAILED DESCRIPTION

[0055] In order to make the purpose, technical scheme and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions.

[0056] It should be noted that the terms "include", "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or devices.

[0057] Like reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0058] In the description of the present invention, it should be noted that the terms "upper", "lower", "inside", "outside", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, or are directions or positional relationships in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0059] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "setting", "installation" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0060] The spin-related component of SHEL (optical spin Hall effect) is itself an orthogonal circularly polarized light. Combined with the metasurface's ability to finely control phase, amplitude, and transmittance, it provides new possibilities for solving the problem of dynamic control of light beam polarization.

[0061] Based on the above analysis, the present invention is designed as follows: Figure 1 As shown, the design method of a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device includes the following steps:

[0062] S1. Construct a linear transmission Jones matrix to analyze the physical mechanism of metasurface regulation of terahertz light field;

[0063] Step S1 specifically includes the following steps:

[0064] S11. Construct a linear transmission Jones matrix of a supercell:

[0065]

[0066] Where, t xx represents the transmission coefficient of the electric field propagating along the x-axis, T xx , They represent the amplitude and phase of the electric field propagating along the x-axis respectively; t yy represents the transmission coefficient of the electric field propagating along the y-axis, T yy , They represent the amplitude and phase of the electric field propagating along the y-axis respectively;

[0067] S12, assuming the transmission amplitude T of the supercell xx =Tyy =1, the rotation angle of the super unit structure is θ, and the linear Jones matrix J(θ) of the super surface is obtained as follows:

[0068] J(θ)=M(θ) T ×T×M(θ) (2)

[0069] Where M(θ) is the optical rotation matrix, and

[0070] S13. Combined with the analysis of the metasurface Jones matrix under circular polarization, the metasurface transmission Jones matrix when the supercell structure is rotated at an angle of θ is expressed as:

[0071]

[0072] And meet δ + ×E LCP =E RCP ; δ + ×E RCP =0δ - ×E RCP =E LCP ; δ - ×E LCP =0;

[0073] In the formula, is the phase difference between the x and y directions; δ + and δ - is the left-right polarized light conversion matrix; E LCP and E RCP are the Jones vectors of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

[0074] At this time, the transmission characteristics of the metasurface are divided into two parts. The first part represents the transmission characteristics of the same polarization channel, and the second part represents the transmission characteristics of the cross-polarization channel. and By choosing θ, the energy distribution of the two channels can be achieved. By changing θ, the phase of the cross-polarization channel can be regulated.

[0075] S2, setting the screening rules of super unit structure based on linear transmission Jones matrix;

[0076] Step S2 specifically includes the following steps:

[0077] S21. Settings To suppress the transmission of the same polarization signal and improve the cross-polarization conversion rate; and set the same polarization channel amplitude of the metasurface to 0, and the cross-polarization channel amplitude to 1; at this time, the incident LCP (left-handed polarized light) is completely converted into RCP (right-handed polarized light) through the transmission field, and the phase of this transmission process is recorded as The incident RCP is completely converted into LCP through the transmission field, and the phase of this transmission process is recorded as and The first subscript of represents the transmitted polarization state, and the second subscript represents the incident polarization state.

[0078] S22. Combine formula (3) to obtain the screening rule of super unit structure:

[0079]

[0080] S3, design the metasurface phase, light field initial phase and light field initial surface phase distribution;

[0081] Step S3 specifically includes the following steps:

[0082] S31, introduce telephoto deep focusing phase and design metasurface and Phase, and the relationship between the long focal depth convergence phase and the coordinate position of the metasurface superunit microstructure satisfies the following formula:

[0083]

[0084] In the formula, is the phase distribution function of the long focal depth convergence beam; k is the spatial wave vector, k = 2π / λ, λ is the wavelength of the incident light; r is the radius coordinate, f is the starting point of the focus of the metasurface transmission field; Δf is the focal depth range of the transmission field; R represents the numerical aperture of the metasurface, that is, the maximum radius of the metasurface;

[0085] S32, in order to realize the metasurface, the left-handed and right-handed components of the terahertz beam are focused to different focal depths of the optical axis, and the initial phase of the light field of the metasurface is And the initial surface phase distribution The design is as follows:

[0086]

[0087] Wherein, f2 is the starting point of the LCP convergence depth, f2 = 4 mm; f3 is the starting point of the RCP convergence depth, f3 = 6 mm; Δf = 2.5 mm.

[0088] The super-standard surface designed according to this phase formula can focus the RCP and LCP at 4mm-6.5mm and 6mm-8.5mm of the optical axis respectively when 45° linearly polarized light is incident.

[0089] S4, constructing a metasurface unit using all-silicon dielectric materials according to the designed metasurface phase, light field initial phase, and light field initial surface phase distribution;

[0090] In step S4 , the dielectric constant ε of the all-silicon dielectric is 11.9.

[0091] S5. Based on the screening rules, the metasurface unit composition is screened to have a unit library with a phase gradient of π / 8 and a full coverage of phases from 0 to 2π;

[0092] In step S5, the 15 selected super unit structures form a unit library, so that when a 1.1 THz linearly polarized wave is incident, the transmittance of the super unit structure in the unit library along the x direction is selected to be greater than 0.6.

[0093] S6. Based on the cell library, the metasurface structure is constructed using the automatic layout algorithm.

[0094] In step S6, a supersurface structure of 80×80 units is constructed.

[0095] like Figure 2 and Figure 3 As shown, a meta-structure device, the meta-structure device is an all-silicon dielectric asymmetric meta-surface structure;

[0096] The all-silicon dielectric asymmetric metasurface structure includes a cross-shaped column 2 at the top and a rectangular parallelepiped base 1 at the bottom. A rectangular parallelepiped 3 is also arranged on the convex surface of the cross-shaped column 2 at the top, and the length of the central axis of the cross-section of the cross-shaped column 2 at the top is 30μm-130μm, the cross-shaped width D of the cross-section of the cross-shaped column 2 at the top is 30μm, the height T of the cross-shaped column 2 at the top is 200μm, and the step length is 1μm; the cross-section length dw of the rectangular parallelepiped 3 is 30-50μm, and the width s is 5μm. The step length is 1μm; the upper and lower end faces of the bottom rectangular parallelepiped base 1 are both squares, and the side length P of the square is 140μm, and the height H of the bottom rectangular parallelepiped base 1 is 300μm. The focusing range of the metastructure device is RCP: 4-6.5mm, LCP: 6-8.5mm, and the focal depth is 2.5mm.

[0097] Simulation experiment

[0098] The simulation software was used to perform time domain simulation of the metasurface with a simulation frequency of 1.1 THz. The boundary ranges in the x, y, and z directions were set to -5.6 mm to 5.6 μm, -5.6 μm to 5.6 μm, and -0.3 mm to 10 mm, respectively, and the boundary conditions were set to open (addspace).

[0099] Results Analysis

[0100] from Figure 5 In (a), an obvious focal spot can be seen. The starting position of the focal spot is around 3.5 mm, the focal spot length extends to around 6.5 mm, and the focal depth length is about 3 mm. This focal spot is the electric field intensity distribution of the transmitted field of the right-handed component of SHEL. Figure 5 In (b), we can see that the focal spot of the electric field intensity of the SHEL left-handed component transmission field starts at about 5.4 mm, the focal spot length extends to about 8.5 mm, and the focal depth length is about 0.31 mm; Figure 5 (c) shows the total field intensity distribution of the transmission field, the starting position of the focal spot is 3.5mm, extending to 0.85mm, and the focal depth is about 5mm. From the focus position, it can be seen that compared with the theoretical design of right-handed polarization and left-handed polarization focusing on the optical axis in the range of 4-6.5mm and 6-8.5mm respectively, the focus range and focal depth have slight deviations. This is because the long focal depth convergent light field will have an electric field extending from the focus center to the far field, and the small error is within an acceptable range, which proves the feasibility of the design theory of this embodiment.

[0101] Depend on Figure 5 (d)- Figure 5 (f) From the above polarization distribution, the polarization state distribution of the transmission field LCP and RCP is almost consistent with the theoretical design, which can also prove the reliability of the design method of this embodiment.

[0102] The Stokes parameter S3 value is used to represent the polarization distribution at the focal position. Figure 6 (a) shows the xoy plane polarization distribution of the transmitted field. It can be seen that the distribution positions of SHEL's RCP and LCP are 3.5-5.5mm and 5.5-8.5mm, respectively. Since the intensity of the right-handed polarization distribution at 5.5-6.5mm is much smaller than that of the left-handed polarization distribution, the polarization distribution and intensity distribution positions are slightly different.

[0103] In addition, since the electric field will extend beyond the designed focal depth when the metasurface is designed with a long focal depth, the polarization distribution in the entire transmission field focal depth of this embodiment should be elliptical polarization, and it is difficult to observe a complete left-handed or right-handed circular polarization distribution.

[0104] like Figure 6 (b) Figure 6 As shown in (c), in this embodiment, the focus position is selected as f4 = 6.25mm in the transmission field of the metasurface structure to observe its electric field strength and phase distribution. The x and y polarization electric field strengths at the focus are not much different and are evenly distributed in a point shape. The electric field is strongest at the focus center and gradually decreases toward the surroundings (as shown in FIG. Figure 6 (d)), and the phase change of the focus position is small (as shown in Figure 6 (e) Figure 6 (f) shows an obvious elliptical polarization distribution, which is consistent with the theoretical analysis.

[0105] In order to reflect the function of the longitudinal evolution of the polarization of the metasurface transmission field, such as Figure 7As shown in the figure, the analysis is performed using the intensity of the left-right hand component in different z planes and the distribution of the Stokes parameter S3 at the diffraction center. Figure 7 (a) It can be seen that within the focal depth of the transmission field, the left-handed and right-handed polarization field intensities of the SHEL at different spatial positions along the propagation direction are all distributed in a point-like manner and show a trend of first increasing and then gradually decreasing. The value of S3 at the diffraction center point of the transmission field from focal plane B to L changes from +0.8 to -0.5. The more detailed S3 parameter changes are shown in Figure 7 (b) It can be seen that the polarization distribution within the focal depth of 3.5-8.5mm gradually changes from right-handed elliptical polarization to left-handed elliptical polarization. The polarization state at different focal positions is then represented using the Poincare sphere. Figure 7 (c) shows that the polarization state changes counterclockwise around the Poincare sphere surface from a focal depth of 3.5 mm to 8.5 mm. This shows that the metasurface can obtain different polarization states along the propagation direction of the light beam.

[0106] In addition, polarization ellipses are used to represent the local polarization characteristics at different focal planes. Figure 8 (a) depicts the polarization states of the transmitted field simulated at different z planes when the incident terahertz light is 1.1THz45° linearly polarized light. Figure 8 In (a), the first row of pictures shows the change of the electric field direction in different z planes; the second row of pictures shows the local polarization state in different z planes in the form of polarization state ellipses; and for different elliptical polarization states, the angle Ψ between the polarization state ellipse and the principal axis and the elliptical polarization ellipticity are used. The transmission fields of the B, C, E, I, and L planes are respectively (Ψ=45°, ) of right-handed elliptical polarization, vertical polarization, horizontal polarization, (Ψ=-30°, ) of left-handed elliptical polarization and (Ψ=-60°, ) of left-handed elliptical polarization.

[0107] Figure 8 (b) is a schematic diagram of the polarization change of the terahertz beam metasurface optical device with long focal depth and arbitrary polarization state. In this figure, the black arrow on the optical axis indicates the main axis direction of the polarization ellipse. It can be seen that the polarization state of the diffraction center point of the metasurface transmission field light beam changes with the change of the metasurface spatial position along the z-axis, thereby confirming the feasibility of this embodiment.

[0108] Therefore, the present invention adopts the above-mentioned terahertz circular polarization multiplexing long focal depth polarization state longitudinal evolution metastructure device design method and metastructure device, and realizes that under the incidence of linearly polarized terahertz light, the metasurface focuses the left and right hand components at different focal depths respectively, and realizes the generation of different polarization states through the principle of polarization superposition.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device design method, characterized by: The following steps are involved: S1. Construct a linear transmission Jones matrix to analyze the physical mechanism of metasurface regulation of terahertz light field; S2, setting the screening rules of super unit structure based on linear transmission Jones matrix; Step S2 specifically includes the following steps: S21. Settings , in order to suppress the transmission of the same polarization signal and improve the cross-polarization conversion rate; and set the same polarization channel amplitude of the metasurface to 0, and the cross-polarization channel amplitude to 1; at this time, the incident LCP is completely converted into RCP through the transmission field, and the phase of this transmission process is recorded as ; and the incident RCP is completely converted into LCP through the transmission field, and the phase of this transmission process is recorded as ; S22. Screening rules for obtaining superunit structures: （4）； S3, design the metasurface phase, light field initial phase and light field initial surface phase distribution; Step S3 specifically includes the following steps: S31, introduce telephoto deep focusing phase and design metasurface and Phase, and the relationship between the long focal depth convergence phase and the coordinate position of the metasurface superunit microstructure satisfies the following formula: （5） In the formula, is the phase distribution function of the long focal depth focusing beam; is the spatial wave vector, , is the wavelength of incident light; is the radius coordinate, ; is the starting point of the focus of the metasurface transmission field; is the focal depth range of the transmission field; represents the numerical aperture of the metasurface, i.e., the maximum radius of the metasurface; S32, in order to realize the metasurface, the left-handed and right-handed components of the terahertz beam are focused to different focal depths of the optical axis, and the initial phase of the light field of the metasurface is , And the initial surface phase distribution The design is as follows: （6） In the formula, is the starting point of the convergence depth of LCP, ; It is the starting point of the convergence and deepening of RCP. ; ; S4, constructing a metasurface unit using all-silicon dielectric materials according to the designed metasurface phase, light field initial phase, and light field initial surface phase distribution; S5. Based on the screening rules, the metasurface unit composition is screened to have a unit library with a phase gradient of π / 8 and a full coverage of phases from 0 to 2π; S6. Based on the cell library, the metasurface structure is constructed using the automatic layout algorithm.

2. The method for designing a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device according to claim 1, characterized in that: Step S1 specifically includes the following steps: S11. Construct a linear transmission Jones matrix of a supercell: （1） In the formula, Indicates that the electric field The transmission coefficient for axial propagation is , , Respectively represent the electric field along Amplitude and phase of axial propagation; Indicates that the electric field The transmission coefficient for axial propagation is , , Respectively represent the electric field along Amplitude and phase of axial propagation; S12, Transmission Amplitude of Hypothetical Supercell , the rotation angle of the super unit structure is , obtain the linear Jones matrix of the hypersurface as follows: （2） In the formula, is the optical rotation matrix, and ; S13. Combined with the analysis of the metasurface Jones matrix under circular polarization, the supercell structure is rotated by an angle of The metasurface transmission Jones matrix is ​​expressed as: （3） And meet ; ; ; ; ; In the formula, is the phase difference between the x and y directions; and It is the left-right polarized light conversion matrix; and are the Jones vectors of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

3. The method for designing a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device according to claim 2, characterized in that: In step S4 , the dielectric constant ε of the all-silicon dielectric is 11.

9.

4. The method for designing a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device according to claim 3, characterized in that: In step S5, the 15 selected super unit structures form a unit library, so that when a 1.1 THz linearly polarized wave is incident, the transmittance of the super unit structure in the unit library along the x direction is selected to be greater than 0.

6.

5. The method for designing a terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device according to claim 4, characterized in that: In step S6, a supersurface structure of 80×80 units is constructed.

6. A meta-device, designed based on the terahertz circular polarization multiplexing long-focus deep polarization state longitudinal evolution meta-device design method according to any one of claims 1 to 5, characterized in that: The meta-device is an all-silicon dielectric asymmetric meta-surface structure; The all-silicon dielectric asymmetric metasurface structure includes a cross-shaped column at the top and a rectangular parallelepiped base at the bottom. A rectangular parallelepiped is also arranged on the convex surface of the cross-shaped column at the top. The length of the central axis of the cross section of the cross-shaped column at the top is 30μm-130μm, the cross-shaped width D=30μm of the cross section of the cross-shaped column at the top, the height T=200μm of the cross-shaped column at the top, and the step length is 1μm; the cross section length dw=μm30-50μm, the width s=5μm; the step length is 1μm; The upper and lower end faces of the bottom rectangular parallelepiped base are both squares, and the side length of the square is P=140 μm. The height of the bottom rectangular parallelepiped base is H=300 μm.

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