Structure and method for wavelength modulation of surface plasmon vortex topological charge

Abstract

The invention relates to a structure and a method for wavelength modulation of surface plasmon polariton (SPP) vortex topological charge, and belongs to the technical field of surface plasmon polariton field regulation, the structure comprises bowknot type slit element atoms and a substrate, the element atoms are arranged on the contour in a circular shape, the substrate is arranged on the surface plasmon polariton field, and the surface plasmon polariton field is arranged on the surface plasmon polariton field. The rotation angle of the bowknot type slit element atom is defined as the included angle between the bottom edge of the bowknot type slit element atom and the horizontal direction. In the invention, based on the principle of slit excitation and regulation and control of SPP, bowknot type slit element atoms with different wavelength response characteristics are introduced to solve the problem that the topological charge of a non-double-wave SPP vortex field cannot be changed by directly changing the wavelength of linearly polarized light in the prior art and structure; according to the invention, a slit is formed on the surface plasmon surface plasmon topological charge, the structure of the wavelength modulation non-double-wave surface plasmon vortex topological charge is constructed by using the slit, the non-double-wave SPP vortex topological charge is regulated and controlled by the wavelength of incident light, a new method is introduced for designing a wavelength multiplexing structure, and a bridge is established between spatial modulation and frequency domain modulation of an SPP vortex field.

Description

Technical Field

[0001] The present invention relates to the technical field of surface plasmon field regulation, and particularly to a structure and method for modulating the topological charge of surface plasmon vortices by wavelength. Background Art

[0002] Surface Plasmon Polariton (SPP) is a surface electromagnetic mode formed by the resonance of an external optical field and metal surface plasmons, with near-field localization and near-field enhancement characteristics. It has been applied in fields such as sub-wavelength lithography, super-resolution imaging, laser physics, photon integration, biochemical sensing, surface-enhanced Raman scattering spectroscopy, etc. Further introducing the orbital angular momentum of the vortex optical field into SPP generates SPP vortices that are confined to the metal surface, have a uniform annular field strength distribution, and a continuous helical phase. The emergence of SPP vortices not only meets the requirements of device integration and miniaturization but also can utilize the near-field confinement and enhancement characteristics of SPP and the characteristics of the vortex field carrying orbital angular momentum to ultimately realize functions such as particle manipulation, quantum communication, and super-resolution imaging on more integrated devices. Therefore, how to simply and effectively excite and flexibly regulate SPP vortex fields with different properties has become a research hotspot for a while.

[0003] Compared with methods for exciting SPP such as prism coupling and grating coupling, the method of using metal nano-slits to excite SPP is more flexible because the size, shape, rotation direction, arrangement method, etc. of the slits will all affect the properties of SPP. Therefore, although the method of using slits to excite SPP has a lower coupling efficiency compared to other methods such as prism coupling, because it can not only excite SPP from the backside incidence to avoid the interference noise of the incident light, but also has excellent integration characteristics and can flexibly regulate SPP, it has unique advantages in generating and regulating complex SPP fields. Each point on the edge of the slit etched on the metal film surface can be regarded as an SPP point source. By changing the relevant parameters or arrangement method of the slit, a structure that can generate SPP vortices can be obtained, and this structure is called a Plasmonic Vortex Lens (PVL), while the basic nano-slit unit for constructing the PVL is called a slit atom.

[0004] The most classic PVL is a simple circular slit. After being irradiated by vector vortex light, the orbital angular momentum of the incident light is transferred to the SPP field, and finally an SPP vortex is formed. In order to simplify the excitation requirements, some PVLs constructed by different slits have been designed. Using circularly polarized light as the excitation source, the spin angular momentum of the incident light can be converted into the orbital angular momentum of the SPP, and finally an SPP vortex is formed. However, most of these PVLs are for a single wavelength. Some structures using nano-circular holes arranged in a spiral profile have been proven to be able to excite SPP vortices in a wide wavelength band, but the topological charge of the SPP vortex field generated by this structure does not change with the wavelength. Although the structure constructed by rectangular slit meta-atoms has been proven to be able to achieve the topological charge of the SPP vortex field by changing the wavelength of the incident light under circularly polarized light irradiation, it cannot break the condition that the wavelength of the SPP field needs to satisfy the harmonic relationship. After that, in order to further simplify the incident conditions, although some PVLs that directly generate SPP vortices using linearly polarized light have been designed, these devices also cannot achieve the change of the topological charge of the non-harmonic SPP vortex field by changing the wavelength of the incident light. This limitation makes the SPP vortex field unable to be dynamically modulated in the frequency domain, restricting the application of SPP vortices in the multi-wavelength field.

[0005] Therefore, a structure and method for wavelength modulating the topological charge of surface plasmon polariton vortices are proposed. Based on the principle of exciting and regulating SPPs by slits, aiming at the problem that the previous technologies and structures cannot directly change the topological charge of the non-harmonic SPP vortex field by changing the wavelength of linearly polarized light, bowtie-shaped slit meta-atoms with different wavelength response characteristics are introduced, and a structure for wavelength modulating the topological charge of non-harmonic surface plasmon polariton vortices is constructed using this slit, realizing the regulation of the topological charge of non-harmonic SPP vortices by the wavelength of the incident light, introducing a new method for designing wavelength multiplexing PVLs, and establishing a bridge between the spatial modulation and frequency domain modulation of the SPP vortex field. Summary of the Invention

[0006] The present invention provides a structure and method for wavelength modulating the topological charge of surface plasmon polariton vortices, solving the problems raised in the above-mentioned background art. Aiming at the problem that the previous technologies and structures cannot directly change the topological charge of the non-harmonic SPP vortex field by changing the wavelength of linearly polarized light, the topological charge of non-harmonic SPP vortices is regulated by the wavelength of the incident light.

[0007] The solution of the present invention to the above technical problems is as follows: A structure for modulating the topological charge of surface plasmon vortices with wavelengths includes bowtie slit meta-atoms and a substrate. A plurality of bowtie slit meta-atoms are provided, and the bowtie slit meta-atoms are arranged on the substrate in a circular profile. The rotation angle of the bowtie slit meta-atom is defined as the included angle between the bottom edge of the bowtie slit meta-atom and the horizontal direction, and the horizontal direction is the direction parallel to the edge of the substrate. The substrate is composed of a metal thin film deposited on a glass substrate, and the height of the bowtie slit meta-atom is the same as the thickness of the metal thin film;

[0008] The usage method includes the following steps:

[0009] S1: Select the wavelengths of two incident lights according to specific requirements to ensure that the generated SPP wavelengths do not satisfy the harmonic relationship. The specific method is as follows:

[0010]

[0011] 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 ε 2 is the relative dielectric constant of air. The wavelengths λ 1 and λ 2 of the SPPs excited by linearly polarized lights can be calculated, and the wavelengths λ SPP1 and λ SPP2 are ensured not to satisfy the multiple relationship; SPP1 SPP2

[0012] S2: Optimize and determine the length, width, and bottom angle of the bowtie slit meta-atom according to the selected wavelengths. The specific method is as follows: First, set the initial dimensions of the bowtie slit meta-atom. The basic requirement is that both the length and width of the bowtie slit meta-atom are less than half of the incident light wavelength;

[0013] Set the initial structural parameters of the length, width, and bottom angle of the bowtie slit meta-atom into the simulation software, and use the finite-difference time-domain method to optimize the structural dimensions to 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 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;

[0014] ​​S3: Construct a structural vortex topological charge structure for wavelength-modulated non-harmonic surface plasmon vortices. The specific method is as follows: Use the bowtie-shaped slit element atoms determined in S2 as the basic units for constructing the structure. Etch multiple bowtie-shaped slit element atoms with the same shape and size on the substrate, and arrange them regularly on a circular contour according to different rotation angles θ. When the rotation angles θ of all bowtie-shaped slit element atoms satisfy θ - α = π / 4 or θ - α = -π / 4, a wavelength-modulated non-harmonic surface plasmon vortex topological charge structure insensitive to the polarization direction of linearly polarized light is constructed. Define the structure satisfying θ - α = -π / 4 as PVL1 and the structure satisfying θ - α = π / 4 as PVL2;

[0015] S4: Fix the polarization direction and wavelength of the incident linearly polarized light, and irradiate it vertically onto the structure from one side of the glass to generate SPP vortices on the surface of the structure. The specific method is as follows:

[0016] First, irradiate a linearly polarized light with a wavelength of λ and a polarization direction of vertically onto the structure from one side of the glass. According to the principle of SPP excitation by slits, any slit element atom can generate surface plasmons with a wave vector of k SPP on the surface of the metal film. When the SPP propagates to the center O point of the structure, the SPP field can be expressed as:

[0017]

[0018] All slit element atoms excite SPPs towards point O according to the excitation characteristics of formula (1) and achieve superposition near point O. The superposition field of SPPs at any point q(ρ, φ, z) near point O can be expressed as:

[0019]

[0020] Therefore, when linearly polarized light is incident on PVL1, an SPP vortex with a topological charge l = 1 and a field strength distribution conforming to the first kind of first-order Bessel function is formed; when linearly polarized light is incident on PVL2, an SPP vortex with a topological charge l = -1 and a field strength distribution conforming to the first kind of negative first-order Bessel function is formed;

[0021] S5: Fix the wavelength of the incident linearly polarized light and change its polarization direction to ensure that the SPP vortex field is insensitive to the polarization direction of the incident light. The specific method is as follows:

[0022] Fix the wavelength λ of the incident linearly polarized light and change its polarization direction When changing for the m-th time, the polarization direction of the incident light is At this time, the superposition field of SPPs at any point q(ρ, φ, z) near point O can be expressed as:

[0023]

[0024] According to formula (3), it can be seen that changing the polarization direction of the incident light only affects the phase of the SPP vortex field, but does not affect the topological charge and intensity of the SPP vortex field. Therefore, the SPP vortex field is insensitive to the polarization direction of the incident light;

[0025] S6: Fix the polarization direction of the incident linearly polarized light and change its wavelength to achieve wavelength modulation of the non-harmonic SPP vortex topological charge. The specific method is as follows:

[0026] Fix the polarization direction of the incident linearly polarized light and change its wavelength λ. At this time, at two different wavelengths λ 1 and λ 2 When the SPP generated by any one slit element atom propagates to the center O point of the structure, the SPP field can be expressed as:

[0027]

[0028] Based on the above technical solutions, the present invention can also be improved as follows.

[0029] Further, the thickness of the metal thin film is greater than the optical penetration depth to ensure that the incident light cannot directly pass through the metal thin film.

[0030] Further, the radius of the circular contour is greater than the wavelength of the incident light.

[0031] Further, the bow-tie slit element atoms are uniformly arranged such that the difference Δη in the angles between the lines connecting the positions of any two adjacent bow-tie slit element atoms to the center of the circular contour and the horizontal direction is equal.

[0032] Further, the bow-tie slit element atom is composed of two isosceles triangles combined, and the two isosceles triangles meet the requirements that the apex angles overlap and the line connecting the in-centers is perpendicular to the bases of the two triangles respectively.

[0033] Further, the length and width of the bow-tie slit element atom are respectively defined as the connection line of the bottom endpoints of the two isosceles triangles and the base of the isosceles triangle, and the length and width of the bow-tie slit element atom and the base angle size of the isosceles triangle are determined by the wavelengths of two incident linearly polarized lights.

[0034] Further, it can be found from formula (4) that when the linearly polarized light with wavelength λ 1 is incident on PVL1, an SPP vortex with topological charge l = 1 and field intensity distribution conforming to the first kind of first-order Bessel function is formed. When the wavelength can become λ 2 , the topological charge becomes l = -1; when the wavelength is λ 1When linearly polarized light is incident on PVL2, an SPP vortex with a topological charge l = -1 and an electric field distribution conforming to the first kind of negative first-order Bessel function is formed. When the wavelength can be changed to λ 2 the topological charge becomes l = 1. Therefore, both the constructed PVL1 and PVL2 can achieve wavelength modulation of the topological charge of non-harmonic SPP vortices.

[0035] The beneficial effects of the present invention are as follows: The present invention provides a structure and method for wavelength modulation of the topological charge of surface plasmon polariton vortices, having the following advantages:

[0036] Based on the principle of slit excitation and regulation of SPP, aiming at the problem that the previous technologies and structures cannot directly change the topological charge of the non-harmonic SPP vortex field by changing the wavelength of linearly polarized light, bowtie slit meta-atoms with different wavelength response characteristics are introduced, and a structure for wavelength modulation of the topological charge of non-harmonic surface plasmon polariton vortices is constructed using this slit, realizing the regulation of the topological charge of non-harmonic SPP vortices by the wavelength of incident light, introducing a new method for designing wavelength multiplexing PVL, and establishing a bridge between the spatial modulation and frequency domain modulation of the SPP vortex field.

[0037] 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 takes the preferred embodiments of the present invention and describes them in detail in conjunction with the accompanying drawings. 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

[0038] The accompanying drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and the schematic embodiments and descriptions of the present invention are used to explain the present invention without unduly limiting the present invention. In the drawings:

[0039] Figure 1 Schematic diagram of manipulating SPP vortices by a structure for wavelength modulation of the topological charge of non-harmonic surface plasmon polariton vortices.

[0040] Figure 2 Top view of a structure for wavelength modulation of the topological charge of non-harmonic surface plasmon polariton vortices and details of a single bowtie slit meta-atom and related structural parameters.

[0041] Figure 3 Schematic diagram of the instantaneous electric field and phase distribution of SPP excited by a single bowtie slit meta-atom under linearly polarized light with different wavelengths.

[0042] Figure 4 Schematic diagram of the electric field strength of the SPP field generated after linearly polarized light with a fixed polarization direction and wavelength is incident on PVL1 and PVL2 and the phase distribution of the SPP field within 1 um of the center of the structure.

[0043] Figure 5 Schematic diagram of the field strength of the SPP field generated after linearly polarized light with a fixed wavelength and a changing polarization direction irradiates PVL1 and the phase distribution of the SPP field within 1 μm of the structure center.

[0044] Figure 6 Schematic diagram of the field strength of the SPP field generated after linearly polarized light with a fixed polarization direction and a changing wavelength irradiates PVL1 and the phase distribution of the SPP field within 1 μm of the structure center. Detailed implementation manners

[0045] The principles and features of the present invention will be described below in conjunction with the attached Figure 1-6 The principles and features of the present invention will be described below, and the examples given are only used to explain the present invention and are not intended to limit the scope of the present invention. In the following paragraphs, the present invention will be described more specifically by way of example with reference to the accompanying drawings. The advantages and features of the present invention will be clearer according to the following description and the claims. It should be noted that the attached drawings are all in a very simplified form and use non-precise scales, and are only used to facilitate and clearly assist in explaining the purpose of the embodiments of the present invention.

[0046] It should be noted that when a component is referred to as being "fixed to" another component, it can be directly on the other component or there can also be an intermediate component. When a component is considered to be "connected to" another component, it can be directly connected to the other component or there may be an intermediate component at the same time. When a component is considered to be "disposed on" another component, it can be directly disposed on the other component or there may be an intermediate component at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are only for the purpose of illustration.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0048] See Figure 1, A schematic diagram of a structure for manipulating the topological charge of a non - harmonic surface plasmon polariton (SPP) vortex by wavelength modulation. The structure includes multiple bow - tie - shaped slit meta - atoms with the same shape and size etched on a substrate. They are uniformly arranged according to different rotation angles on a circular contour. 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. 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 thin film is 200 nm. According to the calculation formula of the SPP wavelength:

[0049]

[0050] where ε' 1 =-4.28 is the real part of the relative permittivity of gold, and ε 2 = 1 is the relative permittivity 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.

[0051] See Figure 2 , the rotation angle θ of the bow - tie - shaped slit meta - atom is defined as the angle between the bottom edge of the bow - tie - shaped slit meta - atom and the horizontal direction. The horizontal direction is the direction parallel to the substrate edge. The angle between the line connecting the position of the uniformly arranged bow - tie - shaped slit meta - atoms and the center of the circular contour and the horizontal direction is represented by α. The length, width of the bow - tie - shaped slit meta - atom and the base angle size of the isosceles triangle are represented by L, D, and β respectively.

[0052] According to the selected wavelength of the incident light, the size of the bow - tie - shaped slit meta - atom is optimized. Specifically, when the wavelengths of the incident light are 570 nm and 670 nm respectively, the parameters of the bow - tie - shaped slit 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 but different wavelengths irradiate the meta - atom, the instantaneous electric field and phase distribution of the generated SPP are as Figure 3As shown in the figure, from the electric field results, it can be found that the electric field intensity at the edge of the meta-atom is very large, which indicates that the edge of the meta-atom can generate SPP, and the SPP field is spirally distributed with the meta-atom as the center, and the pitch is equivalent to the wavelength of the SPP (526nm and 644nm), which further shows 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 wavelength of the incident light is 570nm and 670nm respectively, the meta-atoms that meet the above parameters can emit SPP fields with uniform intensity distribution in all directions in the plane and the phase changes uniformly with the spatial angle α. This shows 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 amplitudes and a phase difference of |π / 2|, and when the wavelength of the incident light is 570nm, the phase of the SPP at each point in space changes counterclockwise by 2π, which indicates that the phase difference between the two dipoles δ=π / 2; and when the wavelength changes from 570nm to 670nm, 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-type slit meta-atom meets the requirements for constructing a wavelength-modulated non-wavedoubling surface plasmon vortex topological charge structure.

[0053] The optimized meta-atoms are evenly arranged on a circular contour with a radius of r to construct a structure of wavelength-modulated non-wave-doubling surface plasmon vortex topological charge. Specifically, 30 optimized meta-atoms are arranged on a circular contour with a radius of r = 3 um.

[0054] When linearly polarized light with fixed polarization direction and wavelength is irradiated vertically onto PVL1 and PVL2 from one side of the glass, SPP vortices are generated on the surface of the structure. Specifically, when the wavelength is 570nm and the polarization direction is When the linearly polarized light is irradiated onto PVL1 or PVL2 satisfying θ-α=-π / 4, the intensity and phase distribution of the generated SPP field are as follows: Figure 4 As shown in the figure, it can be found from the field intensity distribution that the SPP field presents a uniform annular distribution and approximately conforms to the first-order Bessel function of the first kind. From the phase results, it can be found that the phase of the SPP field excited by PVL1 changes by 2π with the change of the azimuth angle in the counterclockwise direction, so PVL1 forms an SPP vortex with a uniform annular distribution of topological charge l=1; the phase of the SPP field excited by PVL2 changes by 2π with the change of the azimuth angle in the clockwise direction, so PVL2 forms an SPP vortex with a uniform annular distribution of topological charge l=-1. Therefore, both PVL1 and PVL2 have a polarization direction of SPP vortices are formed under the irradiation of linearly polarized light with a wavelength of 570nm.

[0055] Since both PVL1 and PVL2 can generate SPP vortices under linearly polarized light irradiation with a certain fixed wavelength and polarization direction, only one structure is selected for discussion in the subsequent analysis. Specifically, PVL1 with θ-α=-π / 4 is selected for subsequent research.

[0056] When linearly polarized light with a fixed wavelength and a changed polarization direction irradiates PVL1, SPP vortices with an invariant topological charge are generated on the surface of the structure. Specifically, when linearly polarized light with wavelengths of 570 nm and polarization directions of π / 4, π / 2, and 3π / 4 irradiates PVL1, the field strength and phase distributions of the generated SPP field are as shown in Figure 5 (Ⅰ)–5(Ⅳ). It can be found from the field strength distribution that in all four cases, the SPP fields show a uniform annular distribution and are all approximately in line with the first-kind first-order Bessel function. From the phase results, it can be found that in all four cases, the phase of the SPP field changes by 2π as the azimuth angle changes counterclockwise. Therefore, PVL1 forms an SPP vortex with a uniform annular intensity distribution and a topological charge l = 1. Therefore, the change in the polarization direction of the incident light does not cause a change in the topological charge of the SPP vortex field, that is, PVL1 is a linearly polarized-insensitive device.

[0057] When linearly polarized light with a fixed polarization direction and a changed wavelength irradiates PVL1, the generated SPP vortices on the surface of the structure change with the change of the wavelength. Specifically, when linearly polarized light with wavelengths of 570 nm and 670 nm and a polarization direction of irradiates PVL1, the field strength and phase distributions of the generated SPP field are as shown in Figure 6 It can be found from the field strength distribution that in both cases, the SPP fields show a uniform annular distribution and are all approximately in line with the first-kind first-order Bessel function. From the phase results, it can be found that when the wavelength of the incident light is 570 nm, the phase of the SPP field changes by 2π as the azimuth angle changes counterclockwise. At this time, PVL1 forms an SPP vortex with a topological charge l = 1 and a wavelength of 526 nm and a uniform annular intensity distribution; when the wavelength becomes 670 nm, the phase of the SPP field changes by 2π as the azimuth angle changes clockwise. At this time, PVL1 forms an SPP vortex with a topological charge l = -1 and a wavelength of 644 nm and a uniform annular intensity distribution. Therefore, PVL1 can realize the regulation of the topological charge of the non-harmonic SPP vortex by the wavelength of the incident light.

[0058] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art.

[0059] As described above, it is only the preferred embodiment of the present invention, and there is no restriction in any form to the present invention; any ordinary technician in this industry can smoothly implement the present invention according to what is shown in the accompanying drawings of the specification and what is described above; however, any equivalent changes such as slight modifications, decorations and evolutions made by those skilled in this profession without departing from the technical solution of the present invention by using the technical content disclosed above are all equivalent embodiments of the present invention; at the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essence of the present invention still fall within the protection scope of the technical solution of the present invention.

Claims

1. A wavelength modulated surface plasmon vortex topological charge structure, comprising a bow-tie slit meta-atom and a substrate, characterized in that: The bow-tie slit meta-atom is provided in plurality, and the bow-tie slit meta-atom is arranged on the substrate in a circular outline, and the rotation angle of the bow-tie slit meta-atom is defined as the angle between the bottom edge of the bow-tie slit meta-atom and the horizontal direction, and the horizontal direction is a direction parallel to the edge of the substrate, and the substrate is formed by coating a layer of metal film on a glass substrate, and the height of the bow-tie slit meta-atom is the same as the thickness of the metal film; The method of use includes the following steps: S1: Select two wavelengths of incident light according to specific requirements to ensure that the generated SPP wavelength does not satisfy the double-wavelength relationship. The specific method is: Where λ is the wavelength of the incident linear polarization, ε'1 is the real part of the relative dielectric constant of gold, and ε2 is the relative dielectric constant of air. The wavelength λ of the SPP excited by linear polarization with wavelengths of λ1 and λ2 can be calculated. SPP1 and λ SPP2 , ensure λ SPP1 and λ SPP2 The multiple relationship is not satisfied; S2: Optimize and determine the length, width and bottom angle of the bow-tie slit meta-atom according to the selected wavelength. The specific method is as follows: first, set the initial size of the bow-tie slit meta-atom. The basic requirement is that the length and width of the bow-tie slit meta-atom are both less than half of the wavelength of the incident light. The initial structural parameters of the length, width and bottom angle of the bow-tie slit meta-atom are set into the simulation software, and the structural dimensions are optimized using the finite difference time domain method to ensure that the intensities of the SPPs excited by the long and short sides of the meta-atom are similar when two wavelengths are incident, and the phase difference between the SPP excited by the long side and the SPP excited by the short side at one wavelength is δ=π / 2, and the phase difference between the SPP excited by the long side and the SPP excited by the short side at another wavelength is δ=-π / 2; S3: constructing a structure of wavelength modulated non-doubled-wave surface plasmon vortex topological charge, the specific method is: according to the bow-tie slit meta-atom determined in S2 as the basic unit for constructing the structure, a plurality of bow-tie slit meta-atoms of the same shape and size are etched on the substrate, and they are evenly and regularly arranged on the circular contour according to different rotation angles θ, and when the rotation angles θ of all bow-tie slit meta-atoms satisfy θ-α=π / 4 or θ-α=-π / 4, a wavelength modulated non-doubled-wave surface plasmon vortex topological charge structure that is insensitive to the polarization direction of linearly polarized light is constructed, and the structure satisfying θ-α=-π / 4 is defined as PVL1, and the structure satisfying θ-α=π / 4 is defined as PVL2; S4: Fix the polarization direction and wavelength of the incident linear polarized light, and irradiate it vertically from one side of the glass to the structure to generate SPP vortices on the surface of the structure. The specific method is as follows: First, let the wavelength be λ and the polarization direction be Linearly polarized light is irradiated vertically onto the structure from one side of the glass. According to the principle of slit-induced SPP, any slit atom can generate a wave vector k on the surface of the metal film. SPP When the SPP propagates to the center of the structure, the SPP field can be expressed as: All slit atoms excite SPP toward point O according to the excitation characteristics of formula (1) and achieve superposition near point O. The superposition field of SPP at any point q(ρ,φ,z) near point O can be expressed as: Therefore, when linearly polarized light is incident on PVL1, an SPP vortex with topological charge l=1 and field intensity distribution conforming to the first-order Bessel function of the first kind is formed; when linearly polarized light is incident on PVL2, an SPP vortex with topological charge l=-1 and field intensity distribution conforming to the first-order negative Bessel function of the first kind is formed. S5: Fix the wavelength of the incident linearly polarized light and change its polarization direction to ensure that the SPP vortex field is insensitive to the polarization direction of the incident light. The specific method is as follows: The wavelength λ of the incident linear polarized light is fixed while its polarization direction is changed When the polarization direction of the incident light is changed for the mth time, At this time, the superposition field of SPP at any point q(ρ,φ,z) near point O can be expressed as: According to formula (3), changing the polarization direction of the incident light will only affect the phase of the SPP vortex field, but will not affect the topological charge and intensity of the SPP vortex field. Therefore, the SPP vortex field is insensitive to the polarization direction of the incident light. S6: Fix the polarization direction of the incident linearly polarized light and change its wavelength to achieve wavelength modulation of the non-wave-doubling SPP vortex topological charge. The specific method is as follows: Fix the polarization direction of incident linear polarized light And change its wavelength λ. At this time, under two different wavelengths λ1 and λ2, when the SPP generated by any slit atom propagates to the center point O of the structure, the SPP field can be expressed as:

2. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: The thickness of the metal film is greater than the optical penetration depth, ensuring that incident light cannot directly pass through the metal film.

3. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: The circular profile radius is larger than the wavelength of the incident light.

4. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: The bow-tie slit meta-atoms are evenly arranged so that the difference Δα between the angle α between the line connecting the positions of any two adjacent bow-tie slit meta-atoms and the center of the circular contour and the horizontal direction is equal.

5. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: The bow-tie-shaped slit element atom is composed of two isosceles triangles, and the two isosceles triangles meet the requirements that the apex angles overlap and the line connecting the incenters is perpendicular to the bases of the two triangles.

6. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: The length and width of the bow-tie slit meta-atom are defined as the line connecting the endpoints of the base of two isosceles triangles and the base of the isosceles triangle, respectively. The length and width of the bow-tie slit meta-atom and the base angle of the isosceles triangle are determined by the wavelengths of the two incident polarized rays.

7. The structure of wavelength modulated surface plasmon vortex topological charge according to claim 1, characterized in that: From formula (4), it can be found that when linearly polarized light with a wavelength of λ1 is incident on PVL1, an SPP vortex with a topological charge of l=1 and a field intensity distribution that conforms to the first-order Bessel function of the first kind is formed. When the wavelength is changed to λ2, the topological charge becomes l=-1. When linearly polarized light with a wavelength of λ1 is incident on PVL2, an SPP vortex with a topological charge of l=-1 and a field intensity distribution that conforms to the first-order negative Bessel function of the first kind is formed. When the wavelength is changed to λ2, the topological charge becomes l=1. Therefore, the constructed PVL1 or PVL2 can realize the wavelength-modulated non-doubled-wave SPP vortex topological charge.

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