Wavelength modulation longitudinal offset confocal vortex generation and topological charge structure and method

Abstract

The invention discloses a structure and a method for wavelength modulation longitudinal polarization confocal vortex generation and topological charge, and belongs to the technical field of scattered field regulation and control of a surface plasmon field, and the structure comprises a plurality of bowknot type slit element atoms which are etched on a substrate and have the same shape and size, and a circular grating, the bowknot type element atoms are uniformly arranged on the circular contour according to different rotation angles theta and share the same circle center with the circular grating. The structure constructed by the invention can realize generation, disappearance and topological charge of wavelength-controlled non-double-wave longitudinal polarization confocal vortex light spots, and can realize confocal of dual-wavelength longitudinal polarization hollow light spots. Based on the above properties, the device can be applied to a hybrid multi-wavelength far field system.

Description

Technical Field

[0001] The present invention belongs to the technical field of scattering field regulation of Surface Plasmon Polariton (SPP) field, and specifically relates to a structure and method for generating longitudinal polarization confocal vortices with wavelength modulation and topological charges. Background Art

[0002] An optical hollow spot is a special light field where the center of the spot is surrounded by a strong light field and the center of the spot is a dark spot, which can be applied to particle trapping, nano-lithography, dark-field microscopy, etc. The most classical method for generating a hollow spot is to use a high numerical aperture objective lens to focus an azimuthally polarized beam, thereby forming a hollow spot with a polarization singularity. To facilitate integration, a method of forming a hollow spot using a metalens has been developed. However, the polarization direction of the hollow spots generated by these above methods is all transverse. Since the longitudinal polarization electric field can be applied to fields such as particle acceleration, near-field microscopy, and high-resolution Raman spectroscopy, in order to expand the application scenarios of hollow spots, a method using an aperture modulation function has been proposed to generate a longitudinally polarized hollow vortex spot with a phase singularity in a tightly focused light field. Then, a plasmonic vortex lens is combined with a grating to form a longitudinally polarized hollow vortex spot generator designed based on the far-field scattering principle.

[0003] However, due to the dispersion characteristics of refractive, diffractive, or scattering optical elements, most hollow spot generators are used at a single wavelength. Even if focusing can be achieved at multiple wavelengths, it is very difficult to achieve confocal. Although some methods have achieved confocal for longitudinally polarized solid spots at different wavelengths or transversely polarized hollow spots with polarization or phase singularities, there is no device that can achieve confocal for double-wavelength longitudinally polarized hollow spots. Moreover, these multi-wavelength confocal devices do not have an optical switch function, that is, changing the polarization state or wavelength of the incident light cannot flexibly "turn on" or "turn off" one or more hollow spots, which is not conducive to applying the device to a hybrid multi-wavelength far-field system. Summary of the Invention

[0004] The present invention provides a structure and method for generating longitudinal polarization confocal vortices with wavelength modulation and topological charges to solve the problems raised in the above background art.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A structure for generating longitudinal polarization confocal vortices with wavelength regulation and non-harmonic frequency and topological charges includes a plurality of bowtie-shaped slit atoms with the same shape and size etched on a substrate and an annular grating, and the bowtie-shaped atoms are uniformly arranged in a circular contour according to different rotation angles θ and are concentric with the annular grating.

[0007] Preferably, the rotation angle θ of the bow-tie slit meta-atom is defined as the angle between the bottom edge of the bow-tie meta-atom and the horizontal direction, and the horizontal direction is the direction parallel to the substrate edge.

[0008] Preferably, the substrate is a metal thin film deposited on a glass substrate, and the thickness of the thin film is greater than the optical penetration depth to ensure that the incident light cannot directly pass through the metal thin film.

[0009] Preferably, the height of the bow-tie meta-atom is the same as the thickness of the metal thin film.

[0010] Preferably, the thickness of the circular grating is less than the thickness of the metal thin film, and the width of the circular grating is less than half of the incident light wavelength.

[0011] Preferably, the radius of the circular grating is determined by the focusing position of the vortex field.

[0012] Preferably, the circular grating satisfies that the radius r' is less than the radius r of the circular contour.

[0013] Preferably, the bow-tie meta-atoms are uniformly arranged such that the difference Δη in the angle between the line connecting the positions of any two adjacent bow-tie meta-atoms and the center of the circular contour and the horizontal direction is equal.

[0014] Preferably, the bow-tie slit meta-atom is composed of two isosceles triangles combined, and the two isosceles triangles satisfy the requirements that the apex angles overlap and the line connecting the in-centers is perpendicular to the bottom edges of the two triangles respectively.

[0015] Preferably, the length and width of the bow-tie meta-atom are respectively defined as the connection of the bottom endpoints of the two isosceles triangles and the bottom edge of the isosceles triangle, and the length and width of the bow-tie meta-atom and the base angle size of the isosceles triangle are determined by the wavelengths of the two incident polarized lights.

[0016] Preferably, when the rotation angle θ of all the bow-tie slit meta-atoms in the structure satisfies θ - Δη = -π / 4, a wavelength-modulated non-harmonic surface plasmon polariton (SPP) field vortex topological charge structure insensitive to the polarization direction of linearly polarized light is constructed.

[0017] The present invention also proposes a method for wavelength-modulated non-harmonic surface plasmon polariton vortex topological charge, specifically:

[0018] Select the wavelengths of two incident lights according to specific requirements;

[0019] Optimize and determine the length, width and base angle of the bow-tie meta-atom according to the selected wavelengths;

[0020] Optimize and determine the height and width of the circular grating according to the selected wavelengths;

[0021] Construct a structure for generating and controlling the topological charge of a longitudinally polarized confocal vortex spot with wavelength regulation and non-harmonic frequency doubling.

[0022] Vertically irradiate circularly polarized light from one side of the glass onto the structure to form a longitudinally polarized vortex spot above the metal film.

[0023] Change the wavelength of the incident circularly polarized light to control the generation and disappearance of the longitudinally polarized vortex spot and its topological charge.

[0024] Optimize the radius of the circular grating to achieve confocal of vortex light fields with different wavelengths.

[0025] Preferably, select the wavelengths of two incident lights to ensure that the generated SPP wavelengths do not satisfy the harmonic relationship. The specific method is as follows:

[0026] According to the calculation formula for the SPP wavelength:

[0027]

[0028] where λ is the wavelength of the incident linearly polarized light, ε'1 is the real part of the relative dielectric constant of gold, and ε2 is the relative dielectric constant of air. The wavelengths λ SPP1 and λ SPP2 of the SPPs excited by linearly polarized lights with wavelengths λ1 and λ2 can be calculated, ensuring that λ SPP1 and λ SPP2 do not satisfy the multiple relationship.

[0029] Preferably, the specific method for optimizing the length, width, and bottom angles of the bowtie-shaped meta-atom according to the selected wavelengths is as follows:

[0030] First, set the initial dimensions of the bowtie-shaped meta-atom. The basic requirement is that both the length and width of the bowtie-shaped meta-atom are less than half of the wavelength of the incident light.

[0031] Set the initial structural parameters of the bowtie-shaped meta-atom into the simulation software, and use the finite-difference time-domain method to optimize the structural dimensions. Ensure that when two wavelengths are incident, the intensities of the SPPs excited by the long side and the short side of the meta-atom are similar, and the phase of the SPP excited by the long side is π / 2 ahead of the phase of the SPP excited by the short side at one wavelength, and the phase of the SPP excited by the long side is π / 2 behind the phase of the SPP excited by the short side at the other wavelength.

[0032] Preferably, the height and width of the circular grating are optimized according to the selected wavelengths. The specific method is as follows:

[0033] First, set the initial dimensions of the bowtie-shaped meta-atom. The basic requirement is that the height of the grating is less than the thickness of the metal film, and the width is less than half of the wavelength of the incident light.

[0034] Set the initial structural parameters of the height and width of the circular grating into the simulation software, and optimize the structural dimensions using the finite-difference time-domain method to ensure that the intensities of the SPP outward-scattered light fields are similar when incident at two wavelengths.

[0035] Preferably, the specific method for constructing the structure is as follows:

[0036] Take the determined bow-tie meta-atoms and the circular grating as the basic units for constructing the structure, and construct a structure for generating a longitudinally polarized confocal vortex spot with wavelength regulation and non-doubled wave and topological charge.

[0037] Preferably, the specific method for generating an SPP vortex on the structure surface is as follows:

[0038] Irradiate the left-handed circularly polarized light with a wavelength of λ1 and a phase difference Δ = π / 2 and the right-handed circularly polarized light with a phase difference Δ = -π / 2 vertically onto the structure from the glass side. According to the principle of SPP excitation by the slit, any slit meta-atom can generate surface plasmon polaritons with a wave vector of kSPP on the metal thin film surface. And when there is no circular grating in the structure, when the SPP propagates to a point q(ρ, φ, z) near the center of the structure, the superposition field of the SPP can be expressed as:

[0039]

[0040] Therefore, when the left-handed circularly polarized light with a wavelength of λ1 is incident on the structure, an SPP vortex can be generated, and the topological charge l = 1, and the field intensity distribution conforms to the first-kind negative first-order Bessel function. When the incident circularly polarized light becomes right-handed circularly polarized light, no SPP vortex can be generated.

[0041] Preferably, the method for scattering the SPP vortex field into the far field to form a longitudinally polarized vortex light field is as follows:

[0042] When the left-handed circularly polarized light irradiates the structure, the generated SPP, when encountering the circular grating, the circular grating concentric with the circular contour can effectively scatter the SPP into a propagating wave in free space according to a certain angular spectrum distribution. Since the SPP that can originally form a vortex field is phase mismatched at the center of the PVL structure, the inwards-converging light with the same angle scattered by the grating interferes destructively after meeting on the optical axis; while the light propagating at different angles that meets certain conditions interferes constructively after meeting outside the optical axis, thus forming a focused vortex spot. And the scattered field can inherit the characteristic that the energy of the SPP is concentrated longitudinally, and finally a longitudinally polarized vortex light field is formed; when the right-handed circularly polarized light irradiates the device, no SPP vortex can be generated, so no longitudinally polarized vortex light field can be formed.

[0043] Preferably, the generation, disappearance and topological charge of the longitudinally polarized vortex spot are realized by changing the wavelength of the incident circularly polarized light, and the specific method is as follows:

[0044] Left-handed circularly polarized light with a wavelength of λ2 and a phase difference of Δ = π / 2 and right-handed circularly polarized light with a phase difference of Δ = -π / 2 are perpendicularly incident on the structure from one side of the glass. According to the principle of SPP excitation by slits, any slit element atom can generate surface plasmon polaritons with a wave vector of SPP on the surface of the metal film. And when there is no circular grating in the structure, when the SPP propagates to a point q(ρ, φ, z) near the center of the structure, the superposition field of the SPP can be expressed as:

[0045]

[0046] Therefore, when left-handed circularly polarized light with a wavelength of λ2 is incident on the structure, no SPP vortex can be generated. When the incident circularly polarized light becomes right-handed circularly polarized light, an SPP vortex can be generated, and the topological charge l = -1, and the field intensity distribution conforms to the first kind of negative first-order Bessel function. Therefore, by only changing the wavelength of the incident circularly polarized light, the generation and disappearance of the vortex light field and the change of the topological charge can be realized.

[0047] Preferably, the radius of the circular grating is optimized to achieve the confocal of vortex light fields with different wavelengths. The specific method is as follows:

[0048] When the radius of the circular grating is different, the angular spectrum distribution of the SPP scattered outward will also change. By optimizing the radius of the circular grating, the focusing positions of the scattered fields with different wavelengths can be made the same;

[0049] First, set the initial value of the radius of the circular grating, set the initial value of the radius of the circular grating into the simulation software, and use the finite-difference time-domain method to optimize the structure size to ensure that the focusing positions of the scattered fields are the same when two wavelengths are incident.

[0050] Therefore, the constructed structure can realize the generation of a non-harmonic longitudinal polarization confocal vortex spot and the topological charge by wavelength regulation.

[0051] Compared with the prior art, the beneficial effects of the present invention are:

[0052] The structure constructed by the present invention can realize the generation of a non-harmonic longitudinal polarization confocal vortex spot and the topological charge by wavelength regulation, can realize the confocal of double-wavelength longitudinal polarization hollow spots, and can change the polarization state or wavelength of the incident light, which is beneficial to applying the device to a hybrid multi-wavelength far-field system.

[0053] The above description is only an overview of the technical solution of the present invention. In order to be able to understand the technical means of the present invention more clearly and implement it in accordance with the content of the specification, the following uses the preferred embodiments of the present invention and the accompanying drawings to describe in detail as follows. The specific implementation manners of the present invention are given in detail by the following embodiments and their accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The accompanying drawings described herein are used to provide a further understanding of the present invention and form a part of this application. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0055] Figure 1 It is a top view of the structure for generating a wavelength-regulated non-doubled longitudinal polarization confocal vortex spot and topological charge, and a schematic diagram of the details of a single bow-tie meta-atom and related structure parameters.

[0056] Figure 2 It is a schematic diagram of the instantaneous electric field and phase distribution of SPP excited by a single bow-tie meta-atom under linearly polarized light of different wavelengths.

[0057] Figure 3 It is a schematic diagram of the intensity and phase distribution of the vortex field generated when circularly polarized light of different rotation directions with a wavelength of λ1 irradiates the structure.

[0058] Figure 4 It is a schematic diagram of the intensity and phase distribution of the vortex field generated when circularly polarized light of different rotation directions with a wavelength of λ2 irradiates the structure.

[0059] Figure 5 It is the light intensity and phase distribution when circularly polarized light of two different wavelengths and different rotation directions irradiates simultaneously to form vortex spots after the radius of the circular grating is optimized, and the light intensity distribution along the z-axis.

[0060] Figure 6 It is the proportion of the polarization components in each direction of the vortex light fields excited by circularly polarized light of two different wavelengths. Specific Embodiments

[0061] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention. In the following paragraphs, the present invention is described more specifically by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all in a very simplified form and use non-precise scales, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0062] See Figure 1, A top view of the structure for generating a non-harmonic longitudinal polarization confocal vortex spot and its topological charge, details of a single bowtie-shaped meta-atom, and a schematic diagram of relevant structure parameters. This structure includes multiple bowtie-shaped slit meta-atoms with the same shape and size etched on a substrate, and an annular grating. The substrate is a metal film deposited on a glass substrate, and the thickness of the film is greater than the optical penetration depth to ensure that the incident light cannot directly pass through the metal film. By changing the wavelength of the incident linearly polarized light, the change of the topological charge of the non-harmonic SPP vortex field is realized. Specifically, linearly polarized lights with wavelengths of 570 nm and 670 nm are used to generate SPP vortices, the metal is gold, and the thickness of the film is 200 nm. According to the calculation formula of the SPP wavelength:

[0063]

[0064] where ε'1 = -4.28 is the real part of the relative dielectric constant of gold, and ε2 = 1 is the relative dielectric constant of air. It can be calculated that when the wavelengths of the incident light are 570 nm and 670 nm respectively, the wavelengths of the SPP are 526 nm and 644 nm respectively.

[0065] The rotation angle θ of the bowtie-shaped slit meta-atom is defined as the angle between the bottom edge of the bowtie-shaped meta-atom and the horizontal direction, and the horizontal direction is the direction parallel to the edge of the substrate. The angle between the line connecting the position of the uniformly arranged bowtie-shaped meta-atoms and the center of the circular contour and the horizontal direction is represented by α. The length and width of the bowtie-shaped meta-atom and the base angle size of the isosceles triangle are represented by L, D, and β respectively.

[0066] According to the selected wavelength of the incident light, the size of the bowtie-shaped meta-atom is optimized. Specifically, when the wavelengths of the incident light are selected to be 570 nm and 670 nm respectively, the parameters of the bowtie-shaped meta-atom optimized by the finite-difference time-domain method are: L = 160 nm, D = 230 nm, β = π / 3. When linearly polarized lights with the same polarization direction and different wavelengths are irradiated on this meta-atom, the instantaneous electric field and phase distribution of the generated SPP are as Figure 2As shown. From the electric field results, it can be found that the electric field intensity at the edge of the meta-atom is very large, indicating that SPP can be generated at the edge of the meta-atom. And the SPP field is spirally distributed with the meta-atom as the center, and the pitch is equivalent to the wavelength of SPP (526 nm and 644 nm), further indicating that the meta-atom can generate SPP and the SPP can propagate outward along the metal surface. From the phase results, it can be found that when the wavelengths of the incident light are 570 nm and 670 nm respectively, the meta-atoms satisfying the above parameters can emit SPP fields with uniform intensity distribution in all directions in the plane and the phase changing uniformly with the spatial angle α. This indicates that at this time, the meta-atom can be regarded as the superposition of two oscillating dipole fields with perpendicular oscillation directions, the same oscillation amplitude, and a phase difference δ of |π / 2|. And when the wavelength of the incident light is 570 nm, the phase of the SPP at each point in space changes counterclockwise by 2π, indicating that the phase difference δ between the two dipoles is π / 2; when the wavelength changes from 570 nm to 670 nm, the phase of the SPP at each point in space changes counterclockwise by -2π, and the phase difference between the two dipoles changes from δ = π / 2 to δ = -π / 2. The above verifies that the optimized bow-tie meta-atom meets the requirements for constructing a wavelength-modulated non-harmonic surface plasmon vortex topological charge structure.

[0067] According to the selected wavelength, the height and width of the circular grating are further optimized and determined. Specifically, when the wavelengths of the incident light are selected to be 570 nm and 670 nm respectively, the height h and width w of the circular grating are both 100 nm optimized by the finite-difference time-domain method.

[0068] The optimized meta-atoms are uniformly arranged on a circular contour with a radius of r, combined with a circular grating with a radius of r', to construct a structure for generating a wavelength-regulated non-harmonic longitudinal polarization confocal vortex spot and topological charge. Specifically, 30 optimized meta-atoms are arranged on a circular contour with a radius r = 3 um, and the radius of the circular grating r' = 1.37 um.

[0069] When circularly polarized light with different rotation directions is perpendicularly incident on the structure from the glass side, a vortex light field is generated above the structure surface. Specifically, when circularly polarized light with different rotation directions and a wavelength of 570 nm is incident on the structure satisfying θ-α = -π / 4, the intensity and phase distributions of the generated scattering field are as Figure 3 shown. From the intensity distribution, it can be found that when left-handed circularly polarized light is incident, the scattering field shows a uniform ring distribution and approximately conforms to the first-kind first-order Bessel function. From the phase results, it can be found that the phase of the scattering field changes by 2π as the azimuth angle changes counterclockwise, so a vortex with a topological charge l = 1 and a uniform ring distribution of intensity is formed; when right-handed circularly polarized light is incident, no SPP vortex can be generated.

[0070] When the wavelength is changed to 670 nm, a vortex optical field can also be generated above the surface of the structure. However, the generation of the vortex field and the topological charge are both opposite to those in the case of 570 nm. Specifically, when circularly polarized light with different rotation directions at a wavelength of 670 nm is incident on the structure satisfying θ-α=-π / 4, the intensity and phase distributions of the generated scattering field are as follows Figure 4 shown. The rotation direction of the incident light is shown in the lower left corner of the result. It can be found from the intensity distribution that when left-handed circularly polarized light is incident, an SPP vortex cannot be generated. When right-handed circularly polarized light is incident, the scattering field shows a uniform annular distribution and approximately conforms to the first-kind first-order Bessel function. From the phase result, it can be found that the phase of the scattering field changes by 2π as the azimuth angle changes clockwise, so a vortex with a uniform annular intensity distribution with a topological charge l=-1 is formed.

[0071] The radius of the circular grating is further optimized to make the focusing positions of the two vortex optical fields the same. Specifically, when the wavelengths of the incident light are selected to be 570 nm and 670 nm respectively, the radius r'=1.05 μm of the circular grating optimized by the finite-difference time-domain method. After circularly polarized light with wavelengths of 570 nm and 670 nm is incident on the structure, the light intensity distribution of the formed vortex spot along the z-axis is as follows Figure 5 (a) shown. The light intensity and phase distributions of the vortex field in the case of 570 nm are as follows Figure 5 (b) shown, and the light intensity and phase distributions of the vortex field in the case of 670 nm are as follows Figure 5 (c) shown. The rotation direction of the incident light is shown in the lower left corner of the result. The results show that when r'=1.05 μm, confocal of the vortex field is achieved.

[0072] The total light intensity and the light intensity distributions of each polarization component on the focal plane when the wavelengths of the incident light are 570 nm and 670 nm are as follows Figure 6 (a) and 6(b) shown. The results show that most of the energy in the vortex optical field is longitudinally polarized. Therefore, this structure can realize the generation of non-harmonic longitudinally polarized confocal vortex spots and topological charges.

[0073] The above are only the preferred embodiments of the present invention and do not impose any form of limitation on the present invention; any ordinary technician in the industry can smoothly implement the present invention according to the illustrations in the specification and the above description; however, any equivalent changes such as slight modifications, decorations, and evolutions made by those skilled in the art without departing from the technical solution of the present invention by using the technical content disclosed above are equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A structure for generating a longitudinally polarized confocal vortex with wavelength modulation and topological charge, characterized in that, It includes multiple bow - tie - shaped slit meta - atoms with the same shape and size etched on a substrate and an annular grating. The bow - tie - shaped meta - atoms are uniformly arranged according to different rotation angles θ on a circular contour and are concentric with the annular grating. The rotation angle θ of the bow - tie - shaped slit meta - atom is defined as the angle between the bottom side of the bow - tie - shaped meta - atom and the horizontal direction, and the horizontal direction is the direction parallel to the edge of the substrate. The substrate is a metal thin film deposited on a glass substrate, and the thickness of the thin film is greater than the optical penetration depth to ensure that the incident light cannot directly pass through the metal thin film. The height of the bow - tie - shaped meta - atom is the same as the thickness of the metal thin film. The thickness of the annular grating is less than the thickness of the metal thin film, and the width of the annular grating is less than half of the incident light wavelength. The radius of the annular grating is determined by the focusing position of the vortex field. The annular grating satisfies that the radius r' is less than the radius r of the circular contour. The bow - tie - shaped meta - atoms are uniformly arranged such that the difference Δη between the angles between the lines connecting the positions of any two adjacent bow - tie - shaped meta - atoms and the center of the circular contour and the horizontal direction is equal. The bow - tie - shaped slit meta - atom is composed of two isosceles triangles combined, and the two isosceles triangles meet the requirements that the vertex angles overlap and the line connecting the in - centers is perpendicular to the bottom sides of the two triangles respectively. The length and width of the bow - tie - shaped meta - atom are respectively defined as the connection line of the bottom - end points of the two isosceles triangles and the bottom side of the isosceles triangle. The length, width of the bow - tie - shaped meta - atom and the base - angle size of the isosceles triangle are determined by the wavelengths of two incident polarized lights. When the rotation angles θ of all bow - tie - shaped slit meta - atoms satisfy θ - α=-π / 4, a wavelength - modulation non - harmonic - generation surface plasmon vortex topological charge structure insensitive to the polarization direction of linearly polarized light is constructed.

2. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and its topological charge, which is applied to the structure for generating a wavelength-modulated longitudinal polarization confocal vortex and its topological charge described in claim 1, and is characterized in that, Specifically: Step 1: Select the wavelengths of two incident lights according to specific requirements to ensure that the generated SPP wavelengths do not satisfy the harmonic - wave relationship. Step 2: Optimize and determine the length, width, and base - angle of the bow - tie - shaped meta - atom according to the selected wavelengths. Step 3: Optimize and determine the height and width of the annular grating according to the selected wavelengths. Step 4: Construct the structure described in claim 1. Step 5: Irradiate the structure with circularly polarized light to form a longitudinally polarized vortex light spot above the metal thin film. Step 6: Change the wavelength of the incident circularly polarized light to control the generation and disappearance of the longitudinally polarized vortex light spot and the topological charge. Step 7: Optimize the radius of the annular grating to achieve confocal of vortex light fields with different wavelengths.

3. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that In step 1, select the wavelengths of two incident lights according to specific requirements to ensure that the generated SPP wavelengths do not satisfy the harmonic - wave relationship. The specific method is: According to the calculation formula of the SPP wavelength: where λ is the wavelength of the incident linearly polarized light, ε'1 is the real part of the relative permittivity of gold, and ε2 is the relative permittivity of air. The wavelengths λ of the SPPs excited by linearly polarized lights with wavelengths λ1 and λ2 can be calculated SPP1 and λ SPP2 , ensuring that λ SPP1 and λ SPP2 do not satisfy a multiple relationship.

4. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that In step 2, optimize and determine the length, width, and base - angle of the bow - tie - shaped meta - atom according to the selected wavelengths. The specific method is: First, set the initial size of the bow - tie - shaped meta - atom. The basic requirement is that both the length and width of the bow - tie - shaped meta - atom are less than half of the incident light wavelength. Subsequently, the initial structural parameters of the length, width, and base angles of the bow-tie-shaped meta-atom are set into the simulation software, and the finite-difference time-domain method is used to optimize the structural dimensions to ensure that when two wavelengths are incident, the intensities of the surface plasmon polaritons (SPPs) excited by the long side and the short side of the meta-atom are similar, and the phase difference δ between the SPP excited by the long side and the SPP excited by the short side is π / 2 at one wavelength, and the phase difference δ between the SPP excited by the long side and the SPP excited by the short side is -π / 2 at the other wavelength.

5. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that, In step 3, the height and width of the circular grating are optimized and determined according to the selected wavelength. The specific method is as follows: First, set the initial dimensions of the bow-tie-shaped meta-atom. The basic requirement is that the height of the grating is less than the thickness of the metal film, and the width is less than half of the incident light wavelength. The initial structural parameters of the height and width of the circular grating are set into the simulation software, and the finite-difference time-domain method is used to optimize the structural dimensions to ensure that when two wavelengths are incident, the intensities of the SPP scattered light fields are similar.

6. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that, In step 4, the specific method for constructing the structure is as follows: The bow-tie-shaped meta-atom and the circular grating determined in claim 1 are used as the basic units for constructing the structure, and according to claim 1, a structure for generating a wavelength-tuned non-harmonic longitudinal polarization confocal vortex spot and topological charge is constructed.

7. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that In step 5, the structure is irradiated with circularly polarized light to form a longitudinal polarization vortex spot above the metal film. The specific method is as follows: Left-handed circularly polarized light with a wavelength of λ1 and a phase difference of Δ = π / 2 and right-handed circularly polarized light with a phase difference of Δ = -π / 2 are perpendicularly incident on the structure from one side of the glass. According to the principle of SPP excitation by slits, any slit element atom can generate surface plasmon polaritons with a wave vector of k SPP on the surface of the metal film. And when there is no circular grating in the structure, when the SPP propagates to a point q(ρ, φ, z) near the center of the structure, the superposition field of the SPP can be expressed as: Therefore, when left-handed circularly polarized light with a wavelength of λ1 is incident on the structure, an SPP vortex can be generated, and the topological charge l = 1. The field intensity distribution conforms to the first kind of negative first-order Bessel function. Finally, the SPP vortex field is scattered into the far field to form a longitudinal polarization vortex light field. When the incident circularly polarized light becomes right-handed circularly polarized light, no SPP vortex can be generated.

8. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 7, characterized in that The method for scattering the SPP vortex field into the far field to form a longitudinal polarization vortex light field is as follows: When the left-handed circularly polarized light irradiates the structure, the generated SPP, when encountering the circular grating, the circular grating concentric with the circular contour can effectively scatter the SPP into a propagating wave in free space according to a certain angular spectrum distribution. Since the SPP that can originally form a vortex field is phase mismatched at the center of the PVL structure, the in-ward converging light of the same angle scattered by the grating interferes destructively after meeting on the optical axis; while the light propagating at different angles that meets certain conditions interferes constructively after meeting outside the optical axis, thus forming a focused vortex spot, and the scattered field can inherit the characteristic that the energy of the SPP is concentrated longitudinally, and finally a longitudinal polarization vortex light field is formed; when the right-handed circularly polarized light irradiates the device, no SPP vortex can be generated, so no longitudinal polarization vortex light field can be formed.

9. A method for generating a wavelength-modulated longitudinal polarization confocal vortex and topological charge according to claim 2, characterized in that, In step 6, the generation and disappearance of the longitudinal polarization vortex spot and the topological charge are realized by changing the wavelength of the incident circularly polarized light. The specific method is as follows: Left-handed circularly polarized light with a wavelength of λ2 and a phase difference of Δ = π / 2 and right-handed circularly polarized light with a phase difference of Δ = -π / 2 are perpendicularly incident on the structure from one side of the glass. According to the principle of SPP excitation by slits, any slit element atom can generate surface plasmon polaritons with a wave vector of k SPP on the surface of the metal film. And when there is no circular grating in the structure, when the SPP propagates to a point q(ρ, φ, z) near the center of the structure, the superposition field of the SPP can be expressed as: Therefore, when left-handed circularly polarized light with a wavelength of λ2 is incident on the structure, SPP vortices cannot be generated; when the incident circularly polarized light becomes right-handed circularly polarized light, SPP vortices can be generated, and the topological charge l = -1, and the field intensity distribution conforms to the first kind of negative first-order Bessel function; therefore, by only changing the wavelength of the incident circularly polarized light, the generation and disappearance of the vortex optical field and the change of the topological charge can be realized.

10. A method for generating longitudinal polarization confocal vortices and topological charges by wavelength modulation according to claim 2, in step 7, optimizing the radius of the circular grating to achieve confocal of vortex optical fields with different wavelengths, the specific method is: When the radius of the circular grating is different, the angular spectrum distribution of SPP scattered outward will also change. By optimizing the radius of the circular grating, the focusing positions of scattered fields with different wavelengths can be made the same; First, set the initial value of the radius of the circular grating, set the initial value of the radius of the circular grating into the simulation software, and use the finite-difference time-domain method to optimize the structure size to ensure that the focusing positions of the scattered fields are the same when incident with two wavelengths.

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