Spin-decoupled visible light orbital angular momentum manipulation metasurface devices

Abstract

The application discloses a spin-decoupled visible light orbital angular momentum regulation super surface device, and relates to the technical field of nano-optics. The super surface is formed by arranging eight kinds of super atoms (the size of each kind of super atom is different), the super surface is equally angularly spaced and divided into 16 regions, the eight kinds of super atoms are arranged in a counterclockwise direction, and each region is distributed with the same kind of super atom; for left-handed light incidence, geometric phase differences are generated in each region of the super surface, combined with transmission phase differences -360° generated due to the size change of the super atom, so that the super surface forms a vortex phase gradient in the counterclockwise direction for the left-handed light in each region of the super surface, and the reflected light of the incident left-handed light is changed into right-handed vortex light with an orbital angular momentum of 2; for right-handed light incidence, geometric phase differences are generated in each region of the super surface, combined with transmission phase differences -360° generated due to the size change of the super atom, so that each region of the super surface has no phase gradient, and the reflected light of the incident right-handed light is changed into left-handed plane wave with an orbital angular momentum of 0.

Description

Technical Field

[0001] The present application relates to the field of nano-optical technology, and in particular to a spin-decoupled visible light orbital angular momentum control metasurface device. Background Art

[0002] The discovery of orbital angular momentum has changed the way people understand and apply light. Since vortex beams with different orbital angular momentum are orthogonal to each other, they have infinite degrees of freedom and can provide exponential information transmission capabilities. They show great potential in modern photonics, especially in the field of next-generation optical communications. Among them, the generation and control of vortex beams carrying orbital angular momentum are crucial to the application of orbital angular momentum. At present, great progress has been made in using metasurfaces to control the orbital angular momentum of light beams and generate vortex beams, but there are still challenges in realizing the control of chiral orbital angular momentum and the generation of chiral vortex beams in the visible light range.

[0003] An orbital angular momentum manipulation device is a device that can precisely control the orbital angular momentum of a light beam and generate a desired vortex beam. It can alter the orbital angular momentum of photons in a light field, including extraction, injection, conversion, and measurement. By manipulating the device's structure or material properties, the photon's orbital angular momentum can be manipulated, which has important applications in optical communications, laser processing, optical microscopy, and other fields.

[0004] Existing technology can prepare orbital angular momentum control devices through the following methods:

[0005] (1) Metasurface technology: Nanostructured metasurfaces can be used to control the orbital angular momentum of photons in the visible light range. By designing appropriate metasurface structures and periodicity, the phase and amplitude distribution of the incident light can be changed, thereby achieving control of the orbital angular momentum.

[0006] (2) Liquid crystal spatial light modulator: Liquid crystal spatial light modulator can change the phase distribution of incident light by adjusting the electric field of the liquid crystal layer, thereby achieving the control of the orbital angular momentum of photons.

[0007] (3) Optical wave plates: Optical wave plates can control the orbital angular momentum of photons by changing the phase difference and polarization state of the incident light.

[0008] (4) Polarization light modulator: The polarization light modulator can adjust the polarization state of the incident light, thereby achieving the control of the orbital angular momentum of the photon.

[0009] (5) Spin angular momentum converter: A spin angular momentum converter is a device that can convert spin angular momentum into orbital angular momentum or vice versa, thereby achieving the control of the orbital angular momentum of photons.

[0010] These technologies can be used to fabricate orbital angular momentum control devices, each with its own applicable scenarios and characteristics. Metasurfaces, with their subwavelength dimensions, large bandwidth adjustment range, and flexible control methods, demonstrate significant advantages in these areas, earning them continued attention in the scientific community.

[0011] When using traditional metasurface technology to fabricate orbital angular momentum manipulation devices, the manipulation of different circularly polarized electromagnetic waves by metasurfaces based solely on geometric phase is mirrored, significantly limiting practical applications. Furthermore, previous metasurfaces for orbital angular momentum manipulation have typically been designed for infrared or even microwave wavelengths, resulting in device sizes exceeding millimeters, which is inconsistent with the development needs of compact photonic devices. Therefore, developing orbital angular momentum manipulation devices capable of operating in the visible light band is a pressing technical challenge in this field. Summary of the Invention

[0012] The purpose of the present invention is to address the above-mentioned technical problems existing in the prior art and provide a spin-decoupled visible light orbital angular momentum control metasurface device, designing metaatoms with specific shapes and sizes, introducing specific phase gradients and / or polarization conversions; the metaatoms are arranged in a two-dimensional plane to form a metasurface that can operate in the visible light band.

[0013] In order to achieve the above object, the present invention is implemented in the following manner:

[0014] The present invention provides a spin-decoupled visible light orbital angular momentum control metasurface device. The metasurface is formed by 8 types of meta-atoms. The metasurface is divided into 16 regions at equal angular intervals with the geometric center of the metasurface as the origin. Starting from the junction of the second quadrant and the first quadrant, the 8 types of meta-atoms are arranged in a counterclockwise direction. The same type of meta-atom is distributed in each region. The cycle is repeated twice to form the metasurface. Each type of meta-atom has a different size. Due to the difference in the arrangement and size of the meta-atoms, a vortex phase gradient is generated. Specifically:

[0015] When left-handed light is incident, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of -22.5° compared to the metaatoms in the previous region, and the metaatoms in the last region produce a geometric phase difference of -360° compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size change of the metaatoms distributed in each region, each region of the metasurface forms a vortex phase gradient in the counterclockwise direction for left-handed light, thereby converting the reflected light of the incident left-handed light into right-handed vortex light with an orbital angular momentum of 2.

[0016] When right-handed light is incident, in adjacent areas of the metasurface, the metaatoms in the latter area produce a geometric phase difference of 22.5° compared with the metaatoms in the previous area, and the metaatoms in the last area produce a geometric phase difference of 360° compared with the metaatoms in the first area. Combined with the transmission phase difference of -360° caused by the size change of the metaatoms distributed in each area, there is no phase gradient in each area of ​​the metasurface, and the reflected light of the incident right-handed light is converted into a left-handed plane wave with an orbital angular momentum of 0.

[0017] In one possible implementation, each superatom includes an upper I-shaped nanostructure with a thickness of t1 = 190 nm; a middle SiO2 dielectric layer with a thickness of t2 = 30 nm; and a bottom metal substrate with a thickness of t3 = 100 nm.

[0018] In a possible implementation, the material of the I-shaped nanostructure is one of Au, Ag, Al, and Na.

[0019] In a possible implementation, the material of the I-shaped nanostructure is Au, and the corresponding Drude model is:

[0020]

[0021] Where ε(ω) is the frequency-dependent dielectric constant, ε ∞ is the limiting dielectric constant, which is 12, ω p is the plasma frequency, which is 1.37×10 16 rad / s, ω is the angular frequency of light, and γ is the electron collision frequency, which is 1.05×10 14 s -1 , i represents an imaginary number.

[0022] In one possible implementation, the I-shaped nanostructure includes a length l, a width w, and a bilateral spacing d, wherein the length l represents the length of the long side of the I-shaped nanostructure, the width w represents the width of each side of the I-shaped nanostructure, and the bilateral spacing d represents the interval between the two long sides of the I-shaped nanostructure. The length l varies between 60 and 360 nm, and the bilateral spacing d varies between 100 and 260 nm, varying with the change of the geometric phase of the superatom in which the I-shaped nanostructure is located. The width w is 60 nm.

[0023] In one possible implementation, the distance between the centers of two adjacent meta-atoms is defined as the period p, which is calculated using the generalized Snell's law:

[0024]

[0025] Among them, θ ris the reflection angle of the reflected light, θ i is the angle of incidence of the incident light, is the rotation angle between adjacent metaatoms, k0 is the free space wave vector, and p is the period of the metaatom.

[0026] In one possible implementation, the phase distribution expression of the vortex phase is:

[0027]

[0028] where x and y are the distances along the x-axis and y-axis of the metasurface, l is the topological charge number, and Φ is the azimuthal angle.

[0029] In one possible implementation, the phase of the meta-atom in each region of the metasurface under left-handed light is calculated by the following formula: and the phase under right-handed incident light

[0030]

[0031] in, is the transmission phase of the metaatom, and 2 is the geometric phase difference.

[0032] In a possible implementation, the central wavelength of the incident visible light is 632.8 nm, and the period of the superatom is 380 nm.

[0033] In a possible implementation, the total area S of the metasurface is ≥ (17λ) 2 , λ is the wavelength of incident visible light.

[0034] Compared to the prior art, the present invention utilizes metaatoms to form a plasmonic metasurface to design a spin-decoupled visible light orbital angular momentum manipulation metasurface device. The metaatom as a whole is equivalent to a half-wave plate, so the thickness of the middle SiO2 dielectric layer, t2, can be roughly determined to be 30nm. The thickness of the upper I-shaped nanostructure, t1, can be 190nm. The bottom metal substrate, with a thickness sufficient to ensure total internal reflection, can be as simple as t3 = 100nm. Using this principle, while ensuring a sufficiently high reflection amplitude, the transmission phase of the metaatom for left-handed and right-handed light can be dynamically altered by scanning parameters to change the length l, width w, and bilateral spacing d of the I-shaped nanostructure. By adjusting the width w to 60nm, the bilateral spacing d between 100 and 260nm, and the length l between 60 and 360nm, a series of metaatoms with transmission phases varying between 0° and 360° are obtained. Combined with the PB phase principle, by rotating each meta-atom with different transmission phases to obtain a geometric phase difference of 2, arbitrary LCP (left-handed light) and RCP (right-handed light) phase distributions can be achieved. Starting from the vortex phase distribution calculation formula, it is assumed that under normal incidence, the desired transformation from left-handed incident light to right-handed vortex light with an orbital angular momentum of 2, and from right-handed incident light to left-handed plane wave with an orbital angular momentum of 0, is achieved. By calculating the phase distribution required for LCP and RCP, the phase distribution at each position of the metasurface can be substituted into the corresponding formula to calculate the transmission phase. And the rotation angle α, further changing the length l of the I-shaped to obtain 8 metaatoms with different transmission phases, and then rotating the calculated angle, the required 8 types of metaatoms can be obtained. By arranging the metaatoms according to the phase (taking the geometric center of the metasurface as the origin, the metasurface is divided into 16 regions with equal angles, starting from the junction of the second quadrant and the first quadrant, the 8 types of metaatoms are arranged in a counterclockwise direction, with the same type of metaatom distributed in each region, and the cycle is repeated twice to form a metasurface with different sizes of each metaatom; for left-handed light incident, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of -22.5° compared to the metaatoms in the previous region, and the metaatoms in the last region produce a geometric phase difference of -360° compared to the metaatoms in the first region, combined with the transmission phase difference of -360° caused by the size change of the metaatoms distributed in each region, each region of the metasurface forms a counterclockwise phase difference for left-handed light. Vortex phase gradient, thereby converting the reflected light of the incident left-handed light into right-handed vortex light with an orbital angular momentum of 2; for right-handed light incident, in adjacent areas of the metasurface, the metaatoms in the latter area produce a 22.5° geometric phase difference compared to the metaatoms in the previous area, and the metaatoms in the last area produce a 360° geometric phase difference compared to the metaatoms in the first area. Combined with the transmission phase difference of -360° caused by the size change of the metaatoms distributed in each area, there is no phase gradient in each area of ​​the metasurface, thereby converting the reflected light of the incident right-handed light into a left-handed plane wave with an orbital angular momentum of 0. This can achieve the predetermined function, that is, after left-handed incidence, right-handed vortex light with an orbital angular momentum of 2 is obtained, and after right-handed incidence, left-handed plane wave with an orbital angular momentum of 0 is obtained. The spin-decoupled visible light orbital angular momentum control metasurface device designed in this way can operate in the visible light band, overcoming the defects of the existing technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the superatom structure.

[0036] Figure 2 Schematic diagram of the normalized reflection amplitude and phase after RCP and LCP are incident on different metaatoms.

[0037] Figure 3 Schematic diagram of 8 superatoms.

[0038] Figure 4 This is a scanning electron microscope (SEM) photograph of the meta-atom on the meta-surface in this embodiment.

[0039] Figure 5 Schematic diagram of incident light and reflected light in this embodiment.

[0040] Figure 6 is the phase distribution diagram of the metasurface.

[0041] Figure 7 These are the far-field measurement results when the incident wavelength is 632.8 nm and left-handed and right-handed light are incident vertically.

[0042] Figure 8 These are the experimental results for incident light at a wide wavelength. DETAILED DESCRIPTION

[0043] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application.

[0044] The present invention provides a special metasurface device that changes the orbital angular momentum of reflected light by imparting a spiral phase to incident light. The metasurface comprises a metasurface formed by an arrangement of eight metaatoms, distributed on the metasurface in a specific order and angle. Specifically, the metasurface is divided into 16 regions at equal angular intervals, with the geometric center of the metasurface as the origin. The 16 regions are organized into two circles, each containing eight regions, corresponding to four quadrants. Starting at the junction of the second and first quadrants, the eight metaatoms are arranged counterclockwise, with each metaatom occupying one region. The same metaatom is distributed in each region, and the cycle repeats twice to form the metasurface. Each metaatom has a different size, and when light passes through, a different phase delay is generated due to the size difference. Each metaatom has a different size, and when light passes through these metaatoms, a different phase delay is generated due to the size difference.

[0045] When left-handed light is incident on this metasurface, the metaatoms in adjacent regions of the metasurface produce a -22.5° geometric phase difference compared to the metaatoms in the previous region. The metaatoms in the final region, on the other hand, produce a -360° geometric phase difference compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size variation of the metaatoms distributed in each region, each region of the metasurface forms a counterclockwise vortex phase gradient for left-handed light. This, in turn, converts the reflected light of the incident left-handed light into right-handed vortex light with an orbital angular momentum of 2.

[0046] When right-handed light is incident on this metasurface, the metaatoms in adjacent regions of the metasurface produce a 22.5° geometric phase difference compared to the metaatoms in the previous region. The metaatoms in the final region produce a 360° geometric phase difference compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size variation of the metaatoms distributed in each region, the metasurface has no phase gradient across regions. Consequently, the reflected light of the incident right-handed light is converted into a left-handed plane wave with zero orbital angular momentum.

[0047] See also Figure 1 , Figure 1Schematic diagram of the superatom structure.

[0048] In this embodiment, meta-atoms are used to form a plasma metasurface to design a spin-decoupled visible light orbital angular momentum control metasurface device. The meta-atom as a whole is equivalent to a half-wave plate. The thickness of the middle SiO2 dielectric layer is determined to be t2 = 30nm, the thickness of the upper I-shaped nanostructure is t1 = 190nm, and the bottom is a metal substrate with a thickness that satisfies total reflection. The thickness of the metal substrate is t3 = 100nm, and the period of the meta-atom is 380nm.

[0049] The I-shaped nanostructure comprises a length l, a width w and a bilateral spacing d, wherein the length l represents the length of the long side of the I-shaped nanostructure, the width w represents the width of each side of the I-shaped nanostructure, and the bilateral spacing d represents the interval between the two long sides of the I-shaped nanostructure.

[0050] The material of the I-shaped nanostructure is one of Au, Ag, Al, and Na. When the material of the I-shaped nanostructure is Au, the corresponding Drude model is:

[0051]

[0052] Where ε(ω) is the frequency-dependent dielectric constant, ε ∞ is the limiting dielectric constant, which is 12, ω p is the plasma frequency, which is 1.37×10 16 rad / s, ω is the angular frequency of light, and γ is the electron collision frequency, which is 1.05×10 14 s -1 , i represents an imaginary number.

[0053] Based on this principle, while ensuring sufficiently high reflection amplitude, the I-shaped nanostructure's length l, width w, and bilateral spacing d can be scanned to dynamically change the meta-atom's transmission phase for left-handed and right-handed light. By adjusting the width w to 60nm, the bilateral spacing d between 100 and 260nm, and the length l between 60 and 360nm, a series of meta-atoms with transmission phases varying between 0° and 360° were obtained.

[0054] Combining the PB phase principle, by rotating each metaatom with different transmission phase to obtain a geometric phase difference of 2, arbitrary LCP (left-handed rotating light) and RCP (right-handed rotating light) phase distributions can be achieved.

[0055] Specifically, in order to impose independent phase distribution on orthogonal circularly polarized channels, it is necessary to combine the transmission phase and geometric phase to decouple the relationship between LCP waves and RCP waves. Based on the Jones matrix analysis, when the meta-atom with half-wave plate effect During rotation, the circular polarization phase is described by the following formula:

[0056]

[0057] in, is the phase of the meta-atom in any region of the metasurface under left-handed light incidence, is the phase of the meta-atom in any region of the metasurface under right-handed light incidence, is the transmission phase of the metaatom, 2 is the geometric phase difference, and α refers to the rotation angle.

[0058] Combining the above equations, we can get any phase distribution and the required On this basis, the calculation formulas for transmission phase and rotation angle are obtained:

[0059]

[0060] Based on this, any phase distribution of LCP and RCP can be achieved.

[0061] The period p of a superatom (the distance between the centers of two adjacent superatoms is defined as the period p) can be calculated using the generalized Snell's law:

[0062]

[0063] Among them, θ r is the reflection angle of the reflected light, θ i is the angle of incidence of the incident light, is the rotation angle between adjacent metaatoms, k0 is the free space wave vector, and p is the period of the metaatom.

[0064] Starting from the calculation formula of the vortex phase distribution:

[0065]

[0066] where x and y are the distances along the x-axis and y-axis of the metasurface, l is the topological charge number, and Φ is the azimuthal angle.

[0067] If we want to achieve a left-handed vortex light with an orbital angular momentum of 2 after the incident left-handed light, and a left-handed plane wave with an orbital angular momentum of 0 after the incident right-handed light, then the phase distribution required for LCP and RCP is calculated as follows: Figure 2 shown. Figure 2 Schematic diagram of the normalized reflection amplitude and phase after RCP and LCP incident on different meta-atoms, where Figure 2Figures a and b show the normalized reflection amplitude and phase of different meta-atoms after RCP light is incident on them. It can be seen that the amplitude of all meta-atoms is above 0.8 when RCP light is incident, indicating a relatively good reflection efficiency. The phase exhibits a gradient characteristic, which can be used to control orbital angular momentum. Figure 2 Figures c and d are schematic diagrams of the normalized reflection amplitude and phase after LCP light is incident on different metaatoms. It can be seen that the amplitude of LCP light is also above 0.8, which is not much different from that of RCP light. However, the phase remains unchanged, so no additional orbital angular momentum is provided under the spiral phase distribution.

[0068] Substituting the phase distribution at each position of the metasurface into formula (4) and formula (5), the transmission phase is calculated and the rotation angle α, and further changing the length l and bilateral spacing d of the superatomic I-shaped nanostructure to obtain 8 different transmission phases The 8 super atoms required can be determined, such as Figure 3 As shown in the superatoms 1 to 8 in the embodiment (in this embodiment, the parameters of the 8 types of superatoms 1 to 8 of the I-shaped nanostructures obtained are as follows: superatom 1, the I-shaped nanostructure length l is 360nm, the width w is 60nm, and the bilateral spacing d is 100nm; superatom 2, the I-shaped nanostructure length l is 190nm, the width w is 60nm, and the bilateral spacing d is 220nm; superatom 3, the I-shaped nanostructure length l is 190nm, the width w is 60nm, and the bilateral spacing d is 260nm; superatom 4, the I-shaped nanostructure length l is 190nm , width w is 60nm, and bilateral spacing d is 180nm; superatom 5, I-shaped nanostructure length l is 190nm, width w is 60nm, and bilateral spacing d is 180nm; superatom 6, I-shaped nanostructure length l is 190nm, width w is 60nm, and bilateral spacing d is 200nm; superatom 7, I-shaped nanostructure length l is 85nm, width w is 60nm, and bilateral spacing d is 260nm; superatom 8, I-shaped nanostructure length l is 65nm, width w is 60nm, and bilateral spacing d is 180nm). Then, the superatoms are arranged by corresponding phases (such as Figure 4 ), the predetermined function can be achieved, that is, after the left-handed light is incident, the right-handed vortex light with an orbital angular momentum of 2 is obtained, and after the right-handed light is incident, the left-handed plane wave with an orbital angular momentum of 0 is obtained.

[0069] The specific arrangement is designed as follows: taking the geometric center of the metasurface as the origin, the metasurface is divided into 16 regions at equal angles, starting from the junction of the second quadrant and the first quadrant, and the eight types of metaatoms are arranged in a counterclockwise direction. The same type of metaatom is distributed in each region, and the cycle is repeated twice to form a metasurface.

[0070] For left-handed light, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of -22.5° compared to the metaatoms in the former region, and the metaatoms in the last region produce a geometric phase difference of -360° compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size change of the metaatoms distributed in each region, each region of the metasurface forms a vortex phase gradient in the counterclockwise direction for left-handed light, thereby converting the reflected light of the incident left-handed light into right-handed vortex light with an orbital angular momentum of 2. For right-handed light incident on the metasurface, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of 22.5° compared to the metaatoms in the former region, and the metaatoms in the last region produce a geometric phase difference of 360° compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size change of the metaatoms distributed in each region, there is no phase gradient in each region of the metasurface, and the reflected light of the incident right-handed light is converted into a left-handed plane wave with an orbital angular momentum of 0.

[0071] Based on this, a spin-decoupled visible light orbital angular momentum control metasurface device was constructed, such as Figure 4 As shown, Figure 4 This is a scanning electron microscope (SEM) photograph of the superatom on the supersurface in this embodiment. Figure 4 The arrangement of the metaatoms is consistent with the design: starting at the junction of the second and first quadrants, eight types of metaatoms are arranged counterclockwise, each occupying a specific region. Each region is then filled with the same type of metaatom, and this cycle repeats twice to form the metasurface. Some locations may have fallen off due to machining errors, but this does not affect the final appearance.

[0072] like Figure 5 As shown, Figure 5 Schematic diagram of incident light and reflected light in this embodiment. As can be seen from Figure 5, when LCP light is incident, the reflected beam forms an OAM with a topological number equal to 2; when RCP light is incident, the reflected beam is still a plane beam. Assuming the area of ​​the metasurface S ≥ (17λ) 2 , λ is the wavelength of incident visible light, taking the period of the super atom as 380nm as an example, see Figure 2 、 6 , 7, Figure 6 is the metasurface phase distribution diagram, Figure 6 The mechanism for achieving polarization and orbital angular momentum conversion in the present invention is revealed. The design of the metasurface can make different phase distributions correspond to different polarization states, thereby achieving the manipulation of the incident light properties. Figure 7 The far-field diagram of the experimental measurement of left-handed and right-handed light with an incident wavelength of 632.8nm. Figure 7It can be seen that when incident at the same angle, the left-handed light presents a vortex beam with an orbital angular momentum of 2 in the far field, and the right-handed wave presents a plane wave beam with an orbital angular momentum of 0, indicating that the metasurface successfully manipulates the polarization state of the incident light and generates the desired orbital angular momentum. Figure 8 These are experimental results for incident light at a wide wavelength. The designed functionality is essentially achieved from 730nm to 760nm. Under LCP light, the far-field exhibits a Gaussian spot, appearing as a plane wave with l = 0. Under RCP light, the far-field exhibits a four-lobed vortex beam with l = 2. Due to fabrication errors and the limited area of ​​the fabricated device, performance deteriorates at other wavelengths, but overall, operation in the visible band is achievable.

[0073] In summary, the embodiment of the present application provides a spin-decoupled visible light orbital angular momentum control metasurface device, which uses superatoms to form a plasma supersurface to design a spin-decoupled visible light orbital angular momentum control metasurface device. The superatom is equivalent to a half-wave plate as a whole, so the thickness of the middle SiO2 dielectric layer t2 = 30nm, the thickness of the upper I-shaped nanostructure t1 = 190nm, and the bottom is a metal substrate with a thickness that satisfies total reflection, such as t3 = 100nm. Through the above principle, on the basis of ensuring that the reflection amplitude is high enough, by scanning the parameters and changing the length l, width w and bilateral spacing d of the I-shaped nanostructure, the transmission phase of the superatom for left-handed and right-handed light can be dynamically changed. By adjusting the width w to 60nm, the bilateral spacing d to vary between 100 and 260nm, and the length l to vary between 60 and 360nm, a series of superatoms with transmission phases varying between 0° and 360° are obtained. Combined with the PB phase principle, by rotating each meta-atom with different transmission phases to obtain a geometric phase difference of 2, arbitrary LCP (left-handed light) and RCP (right-handed light) phase distributions can be achieved. Starting from the vortex phase distribution calculation formula, it is assumed that under normal incidence, the desired transformation from left-handed incident light to right-handed vortex light with an orbital angular momentum of 2, and from right-handed incident light to left-handed plane wave with an orbital angular momentum of 0, is achieved. By calculating the phase distribution required for LCP and RCP, the phase distribution at each position of the metasurface can be substituted into the corresponding formula to calculate the transmission phase. and the rotation angle α, further rotate the obtained calculation angle, and the eight different transmission phase superatoms obtained by changing the length l of the I-shaped structure, the required eight superatoms can be obtained. By arranging the superatoms according to the corresponding phase (taking the geometric center of the metasurface as the origin, dividing the metasurface into 16 regions at equal angles, starting from the junction between the second quadrant and the first quadrant, arranging the eight superatoms in the counterclockwise direction, each region distributes the same superatom, and circulates twice to form a metasurface, and the size of each superatom is different; for left-handed light incidence, in the adjacent regions of the metasurface, the superatom in the latter region produces a geometric phase difference of -22.5° compared with the superatom in the former region, and the superatom in the last region produces a geometric phase difference of -360° compared with the superatom in the first region, combined with the transmission phase difference -360° produced by the superatom distributed in each region due to the size change, so that each region of the metasurface forms a vortex phase gradient in the counterclockwise direction for left-handed light, and further makes the reflected light of the incident left-handed light into right-handed vortex light with orbital angular momentum of 2; for right-handed light incidence, in the adjacent regions of the metasurface, the superatom in the latter region produces a geometric phase difference of 22.5° compared with the superatom in the former region, and the superatom in the last region produces a geometric phase difference of 360° compared with the superatom in the first region, combined with the transmission phase difference -360° produced by the superatom distributed in each region due to the size change, so that each region of the metasurface has no phase gradient, and further makes the reflected light of the incident right-handed light into left-handed plane wave with orbital angular momentum of 0), the predetermined function can be realized, that is, the right-handed vortex light with orbital angular momentum of 2 is obtained after left-handed incidence, and the left-handed plane wave with orbital angular momentum of 0 is obtained after right-handed incidence. The spin decoupling visible light orbital angular momentum control metasurface device designed in this way can work in the visible light band, overcoming the defects of the prior art.

[0074] The above only describes the embodiments of the present application and is not used to limit the protection scope of the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. Spin-decoupled visible light orbital angular momentum control metasurface device, characterized by: include: A metasurface is formed by 8 metaatom arrangements. The metasurface is divided into 16 regions at equal angles, with its geometric center as the origin. Starting from the junction of the second and first quadrants, the 8 metaatoms are arranged in a counterclockwise direction, with the same metaatom distributed in each region. This cycle is repeated twice to form the metasurface. Each metaatom has a different size. For left-handed light incident on the metasurface, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of -22.5° compared to the metaatoms in the previous region, and the metaatoms in the last region produce a geometric phase difference of -360° compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size change of the metaatoms distributed in each region, each region of the metasurface forms a vortex phase gradient in the counterclockwise direction for left-handed light, thereby converting the reflected light of the incident left-handed light into right-handed vortex light with an orbital angular momentum of 2. For right-handed light incident on the metasurface, in adjacent regions of the metasurface, the metaatoms in the latter region produce a geometric phase difference of 22.5° compared to the metaatoms in the previous region, and the metaatoms in the last region produce a geometric phase difference of 360° compared to the metaatoms in the first region. Combined with the -360° transmission phase difference caused by the size change of the metaatoms distributed in each region, there is no phase gradient in each region of the metasurface, thereby converting the reflected light of the incident right-handed light into a left-handed plane wave with an orbital angular momentum of 0. Each superatom consists of an upper I-shaped nanostructure with a thickness of t1 = 190 nm, a middle SiO2 dielectric layer with a thickness of t2 = 30 nm, and a bottom metal substrate with a thickness of t3 = 100 nm. The I-shaped nanostructure includes a length l, a width w, and a bilateral spacing d, wherein the length l represents the length of the long side of the I-shaped nanostructure, the width w represents the width of each side of the I-shaped nanostructure, and the bilateral spacing d represents the interval between the two long sides of the I-shaped nanostructure. The length l varies between 60 and 360 nm, and the bilateral spacing d varies between 100 and 260 nm, varying with the change of the geometric phase of the superatom in which the I-shaped nanostructure is located. The width w is 60 nm.

2. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 1, characterized in that: The material of the I-shaped nanostructure is one of Au, Ag, Al and Na.

3. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 2, characterized in that: The material of the I-shaped nanostructure is Au, and the corresponding Drude model is: Where ε(ω) is the frequency-dependent dielectric constant, ε ∞ is the limiting dielectric constant, which is 12, ω p is the plasma frequency, which is 1.37×10 16 rad / s, ω is the angular frequency of light, and γ is the electron collision frequency, which is 1.05×10 14 s -1 , i represents an imaginary number.

4. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 1, characterized in that: The distance between the centers of two adjacent metaatoms is defined as the period p, which is calculated using the generalized Snell's law: Among them, θ r is the reflection angle of the reflected light, θ i is the angle of incidence of the incident visible light, is the rotation angle between adjacent metaatoms, k0 is the free space wave vector, and p is the period of the metaatom.

5. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 4, characterized in that: The phase distribution expression of the vortex phase is: where x and y are the distances along the x-axis and y-axis of the metasurface, l is the topological charge number, and Φ is the azimuthal angle.

6. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 5, characterized in that: The phase of the meta-atom in each region of the metasurface under left-handed light incidence is calculated by the following formula and the phase under right-handed incident light in, is the transmission phase of the metaatom, and 2α is the geometric phase difference.

7. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 4, characterized in that: The central wavelength of the incident visible light is 632.8 nm, and the period of the super atom is 380 nm.

8. The spin-decoupled visible light orbital angular momentum control metasurface device according to claim 1, characterized in that: The total area of ​​the metasurface S≥(17λ) 2 , λ is the wavelength of incident visible light.

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