A method for polarization detection based on the superposition of orthogonal vortex beams

Abstract

This invention discloses a method for polarization detection based on the superposition of orthogonal vortex beams, comprising the following steps: An incident beam generated by a tunable laser passes sequentially through a linear polarizer and a quarter-wave plate (QWP). The transmission axis α of the linear polarizer and the fast axis β of the QWP are rotated to achieve an arbitrary polarization state in the transmitted beam. An optical lens is used to focus the incident light with the arbitrary polarization state. The beam reflected from the metasurface is then processed by an objective lens to magnify the beam. The focused incident beam illuminates the optical metasurface, and the composite beam generated from the metasurface is collected by a CCD camera after passing through an analyzer. This invention overcomes the problems of large size and poor integration inherent in traditional polarization measurement systems. Metasurfaces offer potential for vertical integration and design flexibility, and provide a new direction for combining polarization information with other related technologies. It also meets the requirements of device miniaturization and system integration.

Description

Technical Field

[0001] This invention belongs to the field of polarization detection technology, specifically relating to a method for polarization detection based on the superposition of orthogonal vortex beams. Background Technology

[0002] The polarization of light contains valuable information about the imaging environment (e.g., material and structural properties, surface roughness, shape and texture of reflective surfaces, orientation of light emitters, or optical activity of various materials), determining how it interacts with anisotropic, chiral, and magnetized matter, and forming the basis for various optical techniques. Therefore, precise polarization detection has important applications in many research fields. For example, in defense and security, polarization information can be used to distinguish between artificial materials and natural surfaces. In atmospheric monitoring, it can be used to track the size and distribution of particles in the atmosphere, thus enabling air quality monitoring.

[0003] In early polarization detection methods, to measure the polarization state of a beam, a number of polarization elements (such as polarizers, waveplates, and polarization modulators) were typically used. These elements were placed in front of a power meter in a beam of light, and the polarization state of the incident light could be determined by measuring the luminous flux of the incident light as it passed through these polarization elements. For example, the intensity of light after passing through waveplates with different rotation angles could be collected (Berry HG, Gabrielse G, Livingston A E. Measurement of the Stokes parameters of light[J]. Applied Optics,1977,16(12):3200-3205.) or the intensity of different polarization components generated by the incident light passing through a special beam splitter could be detected. (Azzam RM A. Division-of-amplitude photopolarimeter (DOAP) for the simultaneous measurement of all four Stokes parameters of light[J]. Optica Acta: International Journal of Optics, 1982, 29(5): 685-689.) Traditional measurement systems have advantages in measurement speed and accuracy, but due to the wide variety of polarization elements used, the data processing system is complex, bulky, and costly, limiting its application range. Considering the problems of large size and high cost of traditional discrete optical elements, this method based on traditional polarization detection is not compatible with the trend of integration and miniaturization in photonics. In addition, in practical applications, it is necessary to overcome the limitations of poor resolution and low damage threshold of optical elements. There are many fundamental or technical challenges in building compact, efficient, and integrable devices.

[0004] Over the past two decades, metamaterials have attracted widespread attention from researchers due to their extraordinary electromagnetic properties. As artificial bulk structures composed of periodically arranged dielectric structures of subwavelength metals, metamaterials generate groundbreaking electromagnetic and photonic phenomena through resonant coupling with the electric field of incident electromagnetic waves. However, metamaterials face numerous challenges in practical applications due to high losses, strong dispersion characteristics associated with resonant responses, and the difficulty in fabricating nanoscale 3D structures. Therefore, many researchers have focused their research on single-layer or few-layer planar structures, known as metasurfaces. As two-dimensional equivalents of bulk metamaterials, metasurfaces retain all the advantages of metamaterials, exhibiting remarkable wavefront manipulation capabilities through the interaction between electromagnetic waves and the structural units and functional arrangement of the metasurface. The use of optical metasurfaces for polarization detection of incident light has been extensively studied. Compared to traditional methods, this approach not only significantly reduces the number of optical components used but also reduces the space occupied by the system. Furthermore, the potential for vertical integration and design flexibility of metasurfaces offers new opportunities for combining polarization information with other related technologies. Therefore, metasurfaces not only meet the industry's growing demand for device miniaturization and system integration, but also provide new ideas for further research into other methods of polarization detection and related applications.

[0005] Therefore, based on the above research results, this paper proposes a method for polarization detection based on the superposition of mutually orthogonal vortex beams. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for polarization detection based on the superposition of orthogonal vortex beams. The polarization state of the incident light is inferred by the composite intensity pattern formed by the interference superposition of two OAM beams with different topological charges.

[0007] This invention achieves the aforementioned technical effects through the following technical solution: a method for polarization detection based on the superposition of mutually orthogonal vortex beams, comprising the following steps:

[0008] (1) First, a tunable laser (NKT-Super K EXTREME) is used as a light source to generate incident plane waves with different wavelengths;

[0009] (2) Secondly, a linear polarizer and a quarter-wave plate (QWP) are used, wherein the angles formed by the transmission axis and fast axis of the linear polarizer and the QWP with the horizontal direction are α and β, respectively; when the incident beam generated by the tunable laser passes through a linear polarizer and a quarter-wave plate (QWP) in sequence, the transmitted beam is made to have an arbitrary polarization state by rotating α and β.

[0010] (3) Again, an optical lens is used to focus the incident light with arbitrary polarization state and place it behind the QWP to focus the incident laser beam so that the beam can completely cover the metasurface; similarly, the beam reflected from the metasurface needs to be processed by an objective lens to magnify the beam.

[0011] (4) Next, the focused incident beam illuminates the pre-designed optical metasurface. In actual processing, taking OAM beams with different integer orders of topological charge values ​​(e.g., l1 = +1, l2 = -3) as an example, the metasurface used to generate and control their superposition can be used to characterize the effectiveness of the method;

[0012] (5) Finally, a CCD camera is used to collect the output beam of the composite beam generated from the metasurface after passing through an analyzer.

[0013] Furthermore, the method for measuring the principal axis of the incident light is as follows:

[0014] First, in a MATLAB simulation, linearly polarized (LP) light with principal axis ψ = π / 4 is simulated by setting the transmission axis α = π / 4 of the polarizer and the fast axis β = π / 4 of the QWP. The interference pattern corresponding to the incident linearly polarized light with principal axis ψ = π / 4 contains 600×600 pixels. The red circle represents the position of the first peripheral vortex, whose position is determined by the polar radius r. p and polar angle φ p Defined; Determine φ p With the center point of the interference pattern (phase singularity) as the center, and r as the center, p Draw a circle with radius φ; by intercepting the intensity distribution along this dashed circle, the polar angle φ of the outer vortex can be accurately measured. p The interference pattern obtained through experiments;

[0015] To improve the signal-to-noise ratio, the intensity of the annular region at each azimuth angle is considered. In the interference pattern, the width d of the red dashed ring is set to 20 pixels. The intensity distribution along the red ring is extracted from the experimental interference pattern. The ring width intensity from 0 to 2π at each azimuth angle is integrated to obtain the corresponding ring width intensity distribution. There are four minimum intensity points along the ring, which correspond to the outer vortex in the interference pattern. The azimuth angle corresponding to the minimum intensity point reflects the polar angle of the outer vortex. Based on the azimuth angle corresponding to the minimum intensity point, the principal axis direction of the incident light can be determined.

[0016] Furthermore, the method for measuring the chirality of the incident light is as follows:

[0017] This invention uses the superposition of OAM modes with topological charges of l1 = +1 and l2 = -3 as an example to illustrate that the interference superposition of two OAM beams with non-equivalent integer orders l1 and l2 when the topological charges have opposite signs can also be used to identify the chirality of incident light. In MATLAB, polarized incident light with different ellipticities and principal axis ψ = 0 can be obtained by continuously changing the transmission axis angle α of the polarizer while keeping the fast axis β of QWP fixed along the horizontal direction. Among the interference patterns produced by five polarized lights with the same principal axis: LCP, LEP, LP, REP, and RCP, LEP and REP have the same ellipticity, but the interference patterns have different intensity distributions. Under LP illumination, the outer vortex in the interference pattern is located at a relatively far position; when the polarization state is in the process of LP gradually evolving into RCP, the left-hand component will gradually decrease (r p As the polarization state decreases, the outer vortexes move towards the inner center, causing the intensity cleavages to move outwards. Therefore, the distance between the symmetrical intensity cleavages increases. When the polarization state is RCP, the left-handed component completely disappears, and the amplitude ratio between the two components tends to infinity (r). p Approaching zero), the outer vortex and the inner central vortex have completely merged, and the intensity profile exhibits a "donut" shape with its radius reaching its maximum (corresponding to l = -3); when the polarization state of the incident light is in the process of gradually evolving from LP to LCP, the left-handed component dominates, and the outer vortex in the interference pattern will diverge outward (r p (Increase), which causes the intensity cleavage to move slowly inward, and the distance between the intensity cleavages will decrease; when the incident polarization is LCP, the right-hand component has completely disappeared, and the light intensity profile once again shows the shape of a "donut", and the radius reaches its minimum at this time (corresponding to l=+1). Therefore, the chirality of the incident light can be determined by comparing the distance between the two intensity cleavages.

[0018] Furthermore, the method for measuring the ellipticity is as follows:

[0019] The interference superposition pattern of two OAM beams of non-equivalent integer orders l1 and l2 with opposite topological charges can also be used to determine the ellipticity of the incident light. Under the incident polarized light with different ellipticities, the metasurface has different intensity profiles. The reason for this phenomenon is that the amplitude ratio between the RCP and LCP components depends on the polar radius of the outer vortex in the interference pattern. It is conceivable that the ellipticity of the incident light can be determined by measuring the ratio between the minimum intensity point and the maximum intensity point on the circle with the phase singularity as the center and the polar radius as the radius on the interference pattern.

[0020] The simulated interferogram under LP light illumination with ψ=0 is obtained by drawing a circle with the center point of the interference pattern as the center and the polar radius of the outer vortices as the radius. All four outer vortices are distributed on the circle, and their intensity values ​​are represented by I. min_1 I min_2 I min_3 and I min_4 This is represented by I. Simultaneously, the four maximum intensity values ​​on the circle are respectively represented as I. max_1 I max_2 I max_3 and I max_4 By combining the formula for calculating ellipticity The ellipticity of the incident light can be measured based on the intensity distribution of the interference pattern, where... and These are the average values ​​of the four minimum intensity points and four maximum intensity points on the circle, respectively; when the incident polarization is LP, η = 0; when the incident polarization is LCP or RCP, η = 1; the experimental results are also measured using the same method.

[0021] The beneficial effects of this invention are:

[0022] (1) The method of the present invention is easy to operate. It only requires a system consisting of a metasurface and a polarizer to complete the polarization detection of incident light, overcoming the shortcomings of the traditional polarization detection system such as complex logic design.

[0023] (2) Metasurfaces have the ability to manipulate light in the subwavelength range, so the overall size of the detection system is easier to integrate compared to traditional optical systems;

[0024] (3) Metasurfaces also have the ability to be designed vertically and have been developed for use in multiple fields. Therefore, they also provide new ideas for combining polarization information with other related technologies.

[0025] (4) The OAM technology of light has been extensively studied, proving that vortex beams have great potential in improving the capacity of optical devices and communication. This invention combines metasurfaces with vortex beams for polarization detection of incident light, providing new ideas for other applications of vortex beams. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 Polarization detection based on metasurfaces; (a) Schematic diagram of metasurfaces for generating and superimposing OAM beams with different topological charges; (b) Interference patterns corresponding to LCP or RCP light illumination; (c) Interference patterns corresponding to LP light incident; where the left side is the original composite pattern; the right side is the interference pattern generated after passing through the analyzer; the white solid circle indicates the position of the outer vortex;

[0028] Figure 2 A schematic diagram of the structural unit of the metasurface; the gold nanorod has a length of 220 nm, a width of 80 nm, and a thickness of 30 nm, while the thicknesses of the SiO2 spacer layer and the gold ground layer are 85 nm and 150 nm, respectively.

[0029] Figure 3 Flowchart of plasma metasurface processing technology;

[0030] Figure 4 Scanning electron microscope (SEM) image of the metasurface and flowchart of the experimental setup; (a) SEM image of the metasurface; (b) Flowchart of the experimental setup;

[0031] Figure 5 A method for measuring the principal axis of incident light using a composite beam interference pattern is presented. (a) and (b) are the simulation and experimental results corresponding to LP with ψ = π / 4, respectively. (c) Intensity profile along the red ring. (d) Normalized intensity distribution along the red ring in the simulation and experimental results.

[0032] Figure 6 Interference patterns (a) and (b) corresponding to LP beams with different principal axes represent simulation and experimental results, respectively; (c) normalized intensity distribution along the corresponding rings in the simulation and experimental results figures;

[0033] Figure 7 Chirality was measured using composite beam interference patterns. (a) and (b) show the simulation and experimental results, respectively; (c) shows the intensity distribution along the vertical direction under different ellipticities.

[0034] Figure 8Ellipticity measurement was achieved by superimposing interference patterns of composite beams; (a) Simulation diagram of composite beam formed by LP with ψ = 0; (b) Represents the corresponding experimental results; (c) Measured and simulated ellipticity values ​​of incident light; the black line represents the ellipticity of the simulation results, and the red triangle represents the ellipticity of the experimental results. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1

[0037] See Figures 1 to 8 As shown, a method for polarization detection based on the superposition of orthogonal vortex beams is described, and the specific operation process is as follows:

[0038] (1) First, a tunable laser (NKT-Super K EXTREME) is used as a light source to generate incident plane waves with different wavelengths;

[0039] (2) Secondly, a linear polarizer and a quarter-wave plate (QWP) are required, wherein the angles formed by the transmission axis and fast axis of the linear polarizer and the QWP with the horizontal direction are α and β, respectively. When the incident beam generated by the tunable laser passes through a linear polarizer and a quarter-wave plate (QWP) in sequence, the transmitted beam can be made to have an arbitrary polarization state by rotating α and β;

[0040] (3) Furthermore, considering the extremely small size of the metasurface, an optical lens is needed to focus the incident light with arbitrary polarization. By placing the laser beam behind the QWP and focusing it, the beam can completely cover the metasurface, thus improving the accuracy of the example. Similarly, the beam reflected from the metasurface needs to be processed by an objective lens to magnify the beam for easier collection and observation;

[0041] (4) Next, the focused incident beam illuminates the pre-designed optical metasurface. In practice, fabricating a metasurface capable of generating and controlling the superposition of OAM beams of unequal integer orders with topological charges of l1 and l2 (e.g., l1 = +1, l2 = -3) can be used to characterize the effectiveness of this method. Scanning electron microscope images of the fabricated metasurface are shown below. Figure 4 As shown in (a);

[0042] (5) Finally, the composite beam generated from the metasurface is collected by a CCD camera after passing through an analyzer. A schematic diagram of the specific experimental procedure is shown below. Figure 4 As shown in (b).

[0043] Example 2

[0044] The principal axis of the incident light is measured by using the interference pattern generated by the superposition of OAM beams with l1 as a positive integer and l2 as an integer:

[0045] Figure 4 This invention demonstrates a specific method for measuring the principal axis of incident light. Firstly, in a MATLAB simulation, linearly polarized (LP) light with a principal axis of ψ = π / 4 is simulated by setting the transmission axis α = π / 4 of the polarizer and the fast axis β = π / 4 of the QWP. The interference pattern corresponding to linearly polarized light with a principal axis of ψ = π / 4 is shown below. Figure 5 As shown in (a), the image contains 600×600 pixels. The red circle represents the position of the first outer vortex, whose position is determined by the polar radius r. p and polar angle φ p Defined. To determine φ p With the center point of the interference pattern (phase singularity) as the center, and r as the center, p Draw a circle with radius as follows: Figure 5 (a) shows the white dashed circle. The polar angle φ of the outer vortex is accurately measured by intercepting the intensity distribution along this dashed circle. p The interference pattern obtained through experiments, such as Figure 5 As shown in (b). Simultaneously, to improve the signal-to-noise ratio, the intensity of the annular region at each azimuth angle is considered. Figure 5 As shown in the red ring region in (b), the width d of the red dashed ring is 20 pixels. The intensity distribution along the red ring is extracted from the experimental interference pattern, as shown in 5(c). Integrating the ring width intensity from 0 to 2π at each azimuth angle yields the corresponding ring width intensity distribution. It is easy to see that there are four minimum intensity points along the ring, which correspond to the outer vortex in the interference pattern. Specifically, the azimuth angle corresponding to the minimum intensity point reflects the polar angle of the outer vortex. Therefore, the principal axis direction of the incident light can be determined based on the azimuth angle corresponding to the minimum intensity point. The red curve in the figure represents the simulation result. Comparing it with the experimental result (black curve), both show a minimum value at 3π / 8, verifying the accuracy of equation (2-7). At the same time, the experimental results and simulation results have good agreement.

[0046] To further verify the effectiveness of this method, in the simulation, LP light with different principal axes (0, π / 4, π / 2, -π / 4) was simulated by setting α = β to different values. The corresponding interference patterns are as follows: Figure 6As shown in the figure, it is easy to see from the figure that when the incident light has different principal axes, the polar radius r of the outer vortex in the interference pattern changes. p Nothing changed, but the polar angle φ p However, it rotates around the beam axis. According to Figure 5 The described measurement principal axis method, along with the normalized intensity distribution of the white dashed circle, is extracted and shown in... Figure 6 (c) The red curve shows the azimuth angles θ corresponding to the four minimum intensity points. min This represents the polar angles of the four outer vortices, i.e., φ. p =θ min The experimental results show a high degree of consistency with the simulation results. Without loss of generality, the experimental results are processed in the same manner, and the obtained normalized intensity distribution along the ring is shown... Figure 6 (c) The black curved section, where the blue dashed line represents the azimuth angle (n=1) corresponding to the first outer vortex, i.e., when θ min When π / 4, the principal polarization axis ψ = 0. The experimental results and simulation results are in high agreement and verify the accuracy of formula (2-7).

[0047] Example 3

[0048] The chirality of incident light can be measured by using the interference pattern generated by the superposition of OAM beams with positive integer l1 and negative integer l2.

[0049] The superposition of OAM modes with topological charges of positive integers l1 and negative integers l2 can also be used to identify the chirality of incident light. In MATLAB, polarized incident light with different ellipticities and principal axis ψ = 0 can be obtained by continuously changing the transmission axis angle α of the polarizer while keeping the fast axis β of QWP fixed along the horizontal direction. Among the interference patterns produced by five polarized lights with the same principal axis: LCP, LEP, LP, REP, and RCP, LEP and REP have the same ellipticity, but the interference patterns have different intensity distributions. Under LP illumination, the outer vortex in the interference pattern is located at a relatively far position; when the polarization state is in the process of gradually evolving from LP to RCP, the left-hand component gradually decreases (r p As the polarization state decreases, the outer vortexes move towards the inner center, causing the intensity cleavages to move outwards. Therefore, the distance between the symmetrical intensity cleavages increases. When the polarization state is RCP, the left-handed component completely disappears, and the amplitude ratio between the two components tends to infinity (r). pApproaching zero), the outer vortex and the inner central vortex have completely merged, and the intensity profile exhibits a "donut" shape with its radius reaching its maximum (corresponding to l = -3); when the polarization state of the incident light is in the process of gradually evolving from LP to LCP, the left-handed component dominates, and the outer vortex in the interference pattern will diverge outward (r p (Increase), which causes the intensity cleavage to move slowly inward, and the distance between the intensity cleavages will decrease; when the incident polarization is LCP, the right-hand component has completely disappeared, and the light intensity profile once again shows the shape of a "donut", and the radius reaches its minimum at this time (corresponding to l=+1). Therefore, the chirality of the incident light can be determined by comparing the distance between the two intensity cleavages.

[0050] Based on the variation of the ellipticity of the intensity cleavage in the interference pattern with the incident light, the distance between two intensity cleavages in the perpendicular direction of the interference pattern can be measured. (Compared with simulation results) Figure 7 (a) The corresponding experimental results are as follows Figure 7 As shown in (b), for ease of data analysis, a rectangular region with a width of 20 pixels (marked with red lines) was selected in the experimental results. The chirality of the incident polarized light is determined by the distance between the two peak intensities. The normalized intensity distributions from numerical calculations and experimental observations are as follows: Figure 7 As shown in (c), the thin red curve and the thick colored curve correspond to the simulation and experimental results, respectively. When the incident light polarization state changes from LCP to RCP, the distance L between the two lobes gradually increases. Specifically, when the distance L is greater than L... LP When the incident light is right-handed, it is right-handed. Otherwise, it is left-handed. Therefore, the chirality of the incident light can be easily distinguished by comparing the distance between the two peak intensities.

[0051] Example 4

[0052] Ellipticity is measured using the interference pattern generated by the superposition of OAM beams with positive integer l1 and negative integer l2.

[0053] Interference patterns generated by the superposition of orthogonal OAM beams with positive integer topological charges l1 and negative integers l2 can also be used to determine the ellipticity of the incident light. Based on the above analysis, the metasurface exhibits different intensity profiles under incident polarized light with different ellipticities. This phenomenon is caused by the amplitude ratio between the RCP and LCP components depending on the polar radius of the peripheral vortex in the interference pattern. Therefore, it is conceivable to determine the ellipticity of the incident light by measuring the ratio between the minimum and maximum intensity points on a circle centered at the phase singularity and with the polar radius as its radius on the interference pattern. Figure 8 This method for measuring ellipticity is demonstrated in detail. Among other things, Figure 8(a) and (b) are the simulated and experimental interferograms under LP light illumination with ψ = 0, respectively. By drawing a circle with the center point of the interference pattern as the center and the polar radius of the outer vortices as the radius, all four outer vortices are distributed on the circle, and their intensity values ​​are represented by I... min_1 I min_2 I min_3 and I min_4 This is represented by I. Simultaneously, the four maximum intensity values ​​on the circle are respectively represented as I. max_1 I max_2 I max_3 and I max_4 By combining the formula for calculating ellipticity The ellipticity of the incident light can be measured based on the intensity distribution of the interference pattern, where... and These are the average values ​​of the four minimum intensity points and four maximum intensity points on the circle, respectively. Specifically, when the incident polarization is LP, η = 0. When the incident polarization is LCP or RCP, η = 1. The experimental results also use the same measurement method, such as... Figure 8 As shown in (b). Figure 8 (c) Simulation and experimental results of the ellipticity of incident light under different polarization states are presented. The black line represents the ellipticity of the simulation results. Here, five representative experimental results were selected. The inset in the figure shows the experimental results of the light intensity distribution at the above angles, and the corresponding ellipticity values ​​are calculated, as shown by the red triangle. The relative error between the experiment and the simulation is between 5.21% and 10.65%. Furthermore, the specific differences between the experimental, simulation, and theoretical values ​​are shown in Table 1-1.

[0054] Theoretical, simulation, and experimental results under different ellipticities

[0055]

[0056]

[0057] Example 5

[0058] Design theory of three-layer metasurfaces:

[0059] In 2011, the Capasso group from Harvard University first proposed a class of nanoscale artificially synthesized planar array structures, thus initiating a surge of research on metasurfaces (Yu N., MA Genevet P Fau-Kats, F. Kats Ma Fau-Aieta, et al. Light propagation with phase discontinuities: generalized laws of reflection and refraction[J]. Science, 2011, 334(6054):333-337.). Due to the inherent ohmic loss of metallic structures, early plasma metasurface devices composed of single-layer nanostructures had low conversion efficiency. Therefore, in order to solve the problem of low efficiency, researchers developed a three-layer plasma metasurface to improve conversion efficiency (Zheng G., H. Muhlenbernd, M. Kenney, et al. Metasurface holograms reaching 80% efficiency[J]. Nature Nanotechnology, 2015, 10(4):308-312.).

[0060] Furthermore, in metasurface design, there are generally two ways to manipulate the phase of light. One is to introduce phase discontinuities by changing the geometry of the structural units, a method known as "transport phase." The other technique, called Pancharatnam-Berry "(PB) phase" or "geometric phase," generates phase abrupt changes by using anisotropic structural units with the same geometry but different orientation angles. When a circularly polarized (CP) beam is incident on a metasurface based on the geometric phase principle, the outgoing beam splits into two parts: a converted beam and an unconverted beam. The converted beam has the opposite polarization state to the incident beam and carries a helical phase structure described by exp(±2iφ), where the sign is related to the chirality of circular polarization. The unconverted beam has the same polarization state as the incident beam. Therefore, metasurfaces provide a way to manipulate phase by designing the orientation angle of gold nanorods.

[0061] This invention designs the phase profile of a metasurface based on the geometric phase principle. Since the polarization state of the incident light depends on the intensity profile of the outgoing diffracted beam, the conversion efficiency of the metasurface is particularly important. The metasurface unit structure used here is a reflective three-layer structure, such as... Figure 2As shown, the structure consists of a gold reflective layer, a silicon dioxide spacer layer, and a top layer of gold nanorods. The addition of the dielectric layer can offset the ohmic losses inherent in the metal structure, thereby achieving the highest conversion efficiency and increasing measurement accuracy. Through optimization of the structural parameters in the CST microwave studio, a conversion efficiency of 80% can be achieved with gold nanorods of 220 nm in length, 80 nm in width, and 30 nm in thickness, and with SiO2 spacer layer and gold ground layer thicknesses of 85 nm and 150 nm, respectively.

[0062] Example 6

[0063] Theories for generating and superimposing vortex beams using metasurfaces:

[0064] In order to convert the incident plane wave into an OAM beam with a helical phase structure, the phase distribution of the metasurface needs to satisfy...

[0065]

[0066] In equation (2-1), E1 and E2 represent the amplitude coefficients of the OAM beam generated by the metasurface, respectively; l1 and l2 represent the topological charges of the two OAM beams generated by the metasurface, respectively. Δφ x This is the additional phase difference between adjacent gold nanorods along the x-direction, its main function being to cause the two OAM beams generated in the x-direction to deflect off-axis. In this example, the additional phase difference is π / 5, resulting in an off-axis angle of 12.2° between the two OAM beams. The off-axis design is intended to better match with other optical devices, broadening the scope for practical applications. It is also noted that the phase profile Ω(x) of the metasurface does not depend on the incident light wavelength, indicating that the metasurface has broadband characteristics and can operate over a wide wavelength range.

[0067] The plasma metasurface designed based on the above phase profile then achieves the superposition of two OAM beams in the x-direction, such as... Figure 1 As shown in (a), when a left-hand circularly polarized (LCP) Gaussian beam illuminates a metasurface, by controlling the phase profile of the metasurface, it can simultaneously generate a pair of off-axis OAM beams with topological charges l1 and l2, which are symmetrical about the normal axis. This example differs from the case described by geometric phase. Because the metasurface is a reflective type, the specular reflection of the beam changes the chirality of the polarization; therefore, the polarization of the resulting off-axis converted OAM beam remains LCP. When the polarization of the incident beam is converted from LCP to right-hand circularly polarized (RCP), the sign of the phase abrupt change caused by the metasurface depends on the chirality of the circular polarization. The propagation direction of the reflected OAM beam will be reversed, and the signs of l1 and l2 will also be inverted (e.g., ...). Figure 1(a) As shown in red). The specific principle of this part can also be found in the literature (Yue F., D. Wen, C. Zhang, et al. Multichannel polarization-controllable superpositions of orbital angular momentum states[J]. Advanced Materials, 2017, 29(15): 1603-838.).

[0068] This superposition process is also easily described mathematically. Any polarization state of light can be viewed as a linear superposition of the LCP and RCP components, which can be expressed as:

[0069] |Φ in >=E L ·e -iυ |L>+E R ·e +iυ |R> (2-2)

[0070] Among them, E L and E R Let represent the amplitudes of the LCP and RCP components, respectively, and υ represent the phase of the components. The amplitude ratio and phase difference of the two components together determine the polarization state of the light. For example, linearly polarized (LP) light can be considered as a linear superposition of LCP and RCP components with the same amplitude and a fixed phase difference. Therefore, when a plane wave with arbitrary polarization is incident on a metasurface, a superposition of two OAM eigenstates with different circular polarizations is achieved along the two reflection directions along the x-axis. The superimposed polarization electric field can be expressed as:

[0071]

[0072]

[0073] Among them, LG l This represents a class of OAM beams with different topological charges l. As can be seen from equations (2-3) and (2-4), the resulting composite beam depends entirely on the amplitude ratio and phase difference of the two components of the incident light. Without loss of generality, we will use the example of LCP and RCP components having the same amplitude (LP) to illustrate the characteristics of the composite beam. After the composite beam passes through a linear analyzer, the intensity profile of the output beam is as follows: Figure 1 As shown in (c), the shear of the two modal phases at different radii generates peripheral vortices (shown by white dots). Within the region inside the white dashed circle, the LCP component with l1 dominates, i.e. Conversely, in the region outside the white dashed circle, the RCP component with l2 is dominant, i.e. The boundary between these two regions It is a radius of r p The white dashed circle, r p This can be understood as the radial distance between the center of the beam and the outer vortex, called the polar radius, which can be expressed as...

[0074]

[0075] It is not difficult to observe that the peripheral vortices appear precisely on the boundary region where the two modes have the strength dominance, and there are |l1-l2| of them. They are uniformly distributed around the central vortex at certain angles, which are called polar angles and can be expressed as follows:

[0076]

[0077] Where n is an odd number, representing the position of each peripheral vortex; considering that the physical meaning of the principal polarization axis is the angle between the major axis of the ellipse and the x-axis, which is half the phase difference between two circularly polarized states, i.e., ψ = Δυ / 2. By combining l1 = +1 and l2 = -3, equation (2-6) can be written as

[0078]

[0079] Therefore, the polar angle φ of any outer vortex in the interference pattern can be measured. p To determine the principal axis direction of the incident polarized light. According to equations (2-5) and (2-6), the position of the peripheral vortex depends on the amplitude ratio and phase difference between the LCP and RCP components. That is, by measuring the polar angle and polar radius of the peripheral vortex, the amplitude ratio and phase difference between the two principal components of the incident light can be uniquely determined. Therefore, by determining the position of the peripheral vortex in the interference pattern, the polarization state of the incident light can be measured in reverse. This part can be referenced in the literature (Baumann SM, D.M.Kalb, L.H.Mac.Millan, et al. Propagation dynamics of optical vortices due to Gouy phase[J]. OpticsExpress, 2009, 17(12): 9818-9827.).

[0080] Example 7

[0081] Processing and manufacturing of metasurfaces:

[0082] The fabrication process of plasma metasurfaces is as follows: Figure 3As shown, the ITO-coated glass substrate was first cleaned with ultrasound in acetone and isopropanol (IPA), respectively. Then, a thin layer of positive resistive PMMA (polymethyl methacrylate) 950A2, which is sensitive to electron beams, was coated onto the substrate using a spinning machine at a rotation speed of 1000 rpm, resulting in a PMMA layer thickness of 120 nm. The PMMA-coated substrate was then baked on a hot plate at 180°C for 5 minutes. Next, nanopatterns were designed and drawn using MATLAB software and saved as a txt file, which was then loaded into an electron beam writer (Raith PIONEER). The nanopatterns were defined on the PMMA resist using the electron beam writer. After exposure, the sample was developed in a developer (methyl isobutyl ketone (MIBK):IPA = 1:3) for 45 seconds. Finally, a 30 nm thick gold layer was deposited on the sample using an electron beam evaporator. The vacuum level during the deposition process was 4 × 10⁻⁶. -6 To achieve adhesion, a thin titanium layer (3 nm) is first deposited on the SiO2 layer, followed by a gold layer. Finally, the defined gold nanorod pattern is fabricated during acetone extraction.

[0083] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0084] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for polarization detection based on the superposition of orthogonal vortex beams, characterized in that, Includes the following steps: (1) First, a tunable laser is used as a light source to generate incident plane waves with different wavelengths; (2) Secondly, a linear polarizer and a quarter-wave plate are used, the quarter-wave plate being a QWP, wherein the angles formed by the transmission axis and fast axis of the linear polarizer and the QWP with the horizontal direction are α and β, respectively; when the incident beam generated by the tunable laser passes through a linear polarizer and a quarter-wave plate in sequence, the transmitted beam can have an arbitrary polarization state by rotating α and β. (3) Again, an optical lens is used to focus the incident light with arbitrary polarization state. The incident laser beam is focused by placing it behind the QWP so that the beam can completely cover the metasurface. Similarly, the beam reflected from the metasurface needs to be processed by an objective lens to magnify the beam. (4) Then, the focused incident beam is irradiated onto the pre-designed optical metasurface. Here, the metasurfaces that can generate and control the superposition of OAM beams with topological charges of l=1 and l=-3 are actually fabricated, thus characterizing the effectiveness of the method. (5) Finally, the composite beam generated from the metasurface is collected by a CCD camera after passing through an analyzer.

2. The method for polarization detection based on the superposition of orthogonal vortex beams according to claim 1, characterized in that, The method for measuring the principal axis of the incident light is as follows: First, in a MATLAB simulation, linearly polarized (LP) light with principal axis ψ=π / 4 is simulated by setting the transmission axis α=π / 4 of the polarizer and the fast axis β=π / 4 of the QWP. The interference pattern corresponding to the incident linearly polarized light with principal axis ψ=π / 4 contains 600x600 pixels, and the position of the first peripheral vortex is determined by the polarimeter r. p and polar angle φ p Defined as follows: with the center point of the interference pattern, i.e., the phase singularity, as the center, and r as the radius. p By drawing a circle with radius φ, and intercepting the intensity distribution along this circle, the polar angle φ of the outer vortex can be accurately measured. p ; Interference patterns obtained through experiments; Meanwhile, to improve the signal-to-noise ratio, the intensity of the annular region at each azimuth angle is considered, and the intensity distribution along a pre-defined ring width is extracted from the interference pattern. The ring width intensity from 0 to 2π at each azimuth angle is integrated to obtain the corresponding ring width intensity distribution. There are four minimum intensity points along the ring intensity distribution, which correspond to the outer vortex in the interference pattern. The azimuth angle corresponding to the minimum intensity point reflects the polar angle of the outer vortex. Based on the azimuth angle corresponding to the minimum intensity point, the principal axis direction of the incident light can be determined.

3. The method for polarization detection based on the superposition of orthogonal vortex beams according to claim 2, characterized in that, The method for measuring the chirality of incident light is as follows: Based on the topological loads l=+1 and OAM mode superposition can also be used to identify the chirality of incident light. In MATLAB, by continuously changing the transmission axis angle α of the polarizer while keeping the fast axis β of QWP fixed along the horizontal direction, polarized incident light with different ellipticities and principal axis ψ=0 can be obtained. Among the interference patterns produced by five polarized lights with the same principal axis: LCP, LEP, LP, REP, and RCP, LEP and REP have the same ellipticity, and these interference patterns have different intensity distributions. Under LP illumination, the outer vortex in the interference pattern is located at a farther position. When the polarization state is in the process of LP gradually evolving into RCP, the left-hand component is continuously decreasing, i.e., r p As the polarization decreases, the outer vortexes move towards the inner center, causing the intensity cleavages to move outwards. This increases the distance between the symmetrical intensity cleavages. When the polarization state is RCP, the left-handed component completely disappears, and the amplitude ratio between the two components tends to infinity, i.e., r... p Approaching zero, the outer vortex and the inner central vortex have completely merged, and the intensity profile exhibits a "donut" shape, with the radius reaching its maximum, corresponding to l=-3. When the polarization state of the incident light is in the process of gradually evolving from LP to LCP, the left-handed component is dominant, and the outer vortex in the interference pattern will diverge outward, i.e., r p The increase causes the intensity cleavage to move slowly inward, thus reducing the distance between the two intensity cleavages. When the incident polarization is LCP, the right-hand component has completely disappeared, and the light intensity profile presents a shape similar to a "donut," with the radius reaching its minimum, corresponding to l=+1. Therefore, the chirality of the incident light can be determined by comparing the distance between two intensity cleavages.

4. The method for polarization detection based on the superposition of orthogonal vortex beams according to claim 3, characterized in that, The method for measuring ellipticity is as follows: The interference pattern generated by the superposition of orthogonal OAM beams with topological charges of l = +1 and l = -3 can also be used to determine the ellipticity of the incident light. Under the incident polarized light with different ellipticities, the metasurface has different intensity profiles. The reason for this phenomenon is that the amplitude ratio between the RCP and LCP components depends on the polar radius of the peripheral vortex in the interference pattern. It is conceivable that the ellipticity of the incident light can be determined by measuring the ratio between the minimum intensity point and the maximum intensity point on the circle with the phase singularity as the center and the polar radius as the radius on the interference pattern. The simulated interferogram under LP light illumination with ψ=0 is constructed by drawing a circle with the center point of the interference pattern as the center and the polar radius of the outer vortices as the radius. All four outer vortices are distributed on the circle, and their intensity values ​​are represented by I. min_1 I min_2 I min_3 and I min_4 It is represented by; at the same time, the four maximum intensity values ​​on the circle are respectively represented as I. max_1 I max_2 I max_3 and I max_4 By combining the formula for calculating ellipticity The ellipticity of the incident light can be measured based on the intensity distribution of the interference pattern, where... and The values ​​are the average values ​​of the four minimum intensity points and four maximum intensity points on the circle, respectively; when the incident polarization is LP, η=0; when the incident polarization is LCP or RCP, η=1; the experimental results are also measured using the same method.

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