A vortex beam orbital angular momentum detector and detection method based on surface plasmon resonances

Abstract

This invention discloses a vortex beam orbital angular momentum detector and detection method based on surface plasmons, belonging to the field of micro-nano photonics and photoelectric detection. The detector is based on a planar photodetector of the "metal-semiconductor-metal" type. A vortex beam-splitting structure, including an incident coupling grating, is provided on a metal electrode on one side of the detector or between two metal electrodes. This structure can split the incident light field carrying different orbital angular momentum into plasmons in different directions. By utilizing the differences in the direction and propagation loss of the plasmons, single-detector resolution of the vortex beam's orbital angular momentum is achieved. The detector of this invention is compatible with existing semiconductor processes, has a simple structure, can detect vortex beams carrying high-order orbital angular momentum, and can also detect the rotation direction of the vortex beam.

Description

Technical Field

[0001] This invention relates to the field of micro-nano photonics and photoelectric detection, specifically to a vortex beam orbital angular momentum detector and detection method based on surface plasmons. Background Technology

[0002] A vortex beam, a structured optical field, possesses a helical phase wavefront and a ring-shaped intensity distribution, and can carry a specific orbital angular momentum (OAM). Vortex beams with different integer orbital angular momentum are orthogonal to each other, theoretically enabling ultra-high-capacity optical communication with infinite channels. Therefore, vortex beams carrying OAM have attracted widespread attention from researchers. Laguerre-Gaussian beams of different orders carry different OAMs and are the most readily obtainable type of vortex beam, widely used in optical field OAM research. The vortex beams mentioned below refer to Laguerre-Gaussian beams.

[0003] In research, interference or diffraction methods are commonly used to verify the orbital angular momentum (OAM) of a vortex beam. The former involves interfering the vortex beam under test with a zero-order Laguerre Gaussian beam, and the number of crosshairs in the interference pattern is observed using a CCD to determine the OAM. The latter involves observing the diffraction pattern after the vortex beam passes through a hole of a specific shape, and the OAM of the vortex beam is determined from the diffraction pattern. Clearly, both methods are only suitable for laboratory environments.

[0004] By using spatial light modulators or carefully designed metasurface structures, vortex light fields excited by different OAMs can be split into different directions and then focused at different positions on the image plane through lenses, achieving multi-channel vortex beam detection. With advancements in metasurface design methods, phase modulation and focusing imaging functions can be integrated into a single metasurface, enabling multiplexing of multiple information dimensions such as OAM, photon spin, and wavelength, providing a solution for high-capacity free-space optical communication. Research on surface plasmons in vortex beams offers a new approach to optical field OAM detection. Using line gratings or ring gratings, surface plasmons excited by different OAM vortex beams can be split into different directions or focused to different positions, allowing for OAM resolution through near-field optical detection. Furthermore, when a vortex beam strikes a ring groove, it can excite surface plasmons carrying a certain angular momentum, the sum of the spin angular momentum, orbital angular momentum of the incident light field, and the angular momentum introduced by the structural geometry. A ring-shaped aperture of a specific size or a carefully designed holographic stripe allows only surface plasmons with specific angular momentum to pass through, thereby enabling the selection of specific OAM vortex beams.

[0005] The above are all optical resolution methods for OAM, and most of them require imaging systems for detection. Currently, direct electrical detection of OAM light fields is still extremely rare, with most methods based on the orbital angular momentum photovoltaic (OPGE) effect in low-symmetry materials (such as WTe2, TaIrTe4, etc.). OPGE can be phenomenologically understood as the light field transferring orbital angular momentum to electrons, thereby exciting photocurrents. The photocurrent excited by the OPGE effect is obtained by measuring the difference between the photocurrents excited by left- and right-handed spin light, and its magnitude is proportional to the OAM carried by the incident light field. Currently, detection of ±4 OAM in the mid-infrared band has been achieved based on the OPGE effect. However, this method requires measuring the photocurrents excited by different spin light fields and depends on the crystal symmetry of the material, making it unsuitable for common silicon-based or III-V semiconductor photodetectors.

[0006] In summary, there is currently a lack of direct detection methods for vortex optical field OAM that are compatible with existing semiconductor process systems, and most existing methods cannot distinguish vortex beams carrying higher-order OAM. Summary of the Invention

[0007] The purpose of this invention is to propose a vortex beam orbital angular momentum detector and its detection method that are compatible with existing semiconductor processes and capable of detecting high-order OAM.

[0008] To achieve direct detection of vortex beams carrying high-order OAM, this invention proposes a vortex beam OAM photodetector based on a metal wire grid. This device is compact, has no specific material limitations, is compatible with existing semiconductor processes, and can resolve high-order OAM, showing great promise for applications in on-chip optical communication based on optical field OAM.

[0009] A vortex beam possesses a helical phase wavefront, meaning its wave vector has a component perpendicular to the propagation direction, which increases with increasing OAM (Optical Amplitude). When a vortex beam is incident perpendicularly on a grating, according to the momentum conservation condition, the wave vector of the excited surface plasmons is the sum of the in-plane wave vector of the vortex beam and the reciprocal lattice vector provided by the grating. In other words, the direction of the excited surface plasmons is determined by the magnitude of the OAM carried by the vortex beam. By designing the structure of the plasmon propagation region, plasmons in different propagation directions experience different losses before being converted into photoelectric signals, ultimately resulting in electrical signals of varying magnitudes, thus achieving photoelectric detection of the vortex beam's OAM.

[0010] This invention is based on a planar photodetector of the "metal-semiconductor-metal" (MSM) type. A vortex beam splitting structure is provided on a metal electrode on one side of the detector or between two metal electrodes. The structure includes a plasmon excitation region, a plasmon transmission region and a plasmon detection region.

[0011] The plasmon excitation region, plasmon transmission region, and plasmon detection region are separated from each other. During testing, the vortex beam irradiates the plasmon excitation region. The generated plasmons propagate through the plasmon transmission region and are coupled to the free space light field in the plasmon detection region, where they are converted into photoelectric signals and detected.

[0012] Preferably, the plasmon excitation region, transmission region and detection region use metals with low plasmon loss, such as silver, gold, sodium and potassium.

[0013] The plasmon excitation region includes a periodic structure that provides momentum matching and excites plasmons. This structure can be an input coupling grating in the form of a one-dimensional wire grating or a two-dimensional aperture array. The period of the input coupling grating should satisfy the phase-matching condition; preferably, the period of the input coupling grating is equal to the wavelength of the plasmons on the metal film surface.

[0014] The one-dimensional wire grid or two-dimensional aperture array structure can be fabricated by photolithography during the fabrication of the metal structure; or obtained by etching and cutting the metal with a focused ion beam; or prepared by depositing additional material on the surface of the metal film through photolithography, deposition, and lift-off; or obtained by fabricating groove or ridge structures on the substrate through standard photolithography processes and then depositing metal.

[0015] The plasmon transmission region can be planar, without any fabricated structure (as in Embodiment 1), and the plasmons propagate on the upper or lower surface of the metal plane. In this case, the plasmons excited by the vortex beam carrying a large OAM have a large angle with the grating axis, resulting in a long propagation distance, high loss, and weak photoelectric signal.

[0016] Preferably, the plasmon transmission region can be fabricated on a metal plane using a plasmon dielectric lens (see Example 2), or by fabricating a differentiated loss structure, or by specially designing the shape of the transmission region to increase the loss difference between plasmons excited by different OAM vortex beams during propagation, thereby improving the device's detection performance.

[0017] The plasmon detection region can be planar, without any fabricated structure, and the plasmons are converted into an optical field through edge scattering before detection.

[0018] Preferably, an output coupling grating can be fabricated in the plasmon detection region to improve the efficiency of plasmon conversion into an optical field.

[0019] Plasmon polarization is converted into an optical field, which is then converted into an electrical signal through the photovoltaic or photothermal-electric effect. When the device uses the photovoltaic effect, the semiconductor optoelectronic material used should be able to form a Schottky junction with the metal used. When using a silver film, the semiconductor optoelectronic material can be silicon, gallium arsenide, indium gallium arsenide, etc. When the device uses the photothermal-electric effect, the semiconductor optoelectronic material used should have a strong photothermal-electric effect, such as tellurium, tin selenide, etc.

[0020] Preferably, by designing a plasmonic excitation structure or electrode structure, the positive or negative orbital angular momentum carried by the vortex beam can be detected. One approach is to place the vortex beam splitting structure on a metal film between two electrodes. The plasmonic excitation region is located at the center of the metal film, and the plasmonic detection region is located at both edges of the metal film. The portion between the plasmonic excitation region and the plasmonic detection region is the plasmonic transmission region. The input coupling grating of the plasmonic excitation region is a composite grating structure, comprising upper and lower gratings with different periods. The periods Λ1 and Λ2 of the two gratings should satisfy the following conditions:

[0021] (1)

[0022] (2)

[0023] In equations (1) and (2), λ spp For plasmon wavelengths, k OAM Let be the in-plane angular momentum of the vortex light field.

[0024] Preferably, it should be made , ,in, k OAM1 This refers to the in-plane angular momentum of a vortex beam with a topological charge m=1. It ensures that for vortex beams with different rotational directions, the excited SPPs propagate in opposite directions, thus enabling the resolution of vortex beams carrying topological charges in different directions.

[0025] Furthermore, considering that the intensity of the upper and lower SPP beams generated by the single grating structure is different for vortex optical fields with different rotation directions, a single-period grating as described above can also be used to divide the plasmon transmission region and the detection region into upper and lower parts to realize the detection of the rotation direction of the vortex optical field.

[0026] In some specific embodiments of the present invention, the detector is a silicon-based photodetector, comprising an SOI substrate and two silver electrodes processed thereon, wherein the top silicon layer of the SOI substrate is located between the two silver electrodes and forms a Schottky contact with the two silver electrodes; the vortex beam splitting structure is located on one silver electrode or on a silver film between the two silver electrodes, wherein, in the plasmon excitation region, the silver film is not etched through when processing the input coupling grating; in the plasmon detection region, if an output coupling grating is processed, the silver film is etched through to the top silicon layer.

[0027] Furthermore, a transparent protective layer (such as an aluminum oxide protective layer) is applied to the surface of the silver electrode to protect it from oxidation. Additionally, to reduce dark noise, an insulating dielectric layer can be provided between the silver electrode and the top silicon layer, except for the top portion.

[0028] Based on the above-mentioned vortex beam orbital angular momentum detector, the present invention provides a method for detecting the orbital angular momentum of a vortex beam. The vortex beam to be measured is perpendicularly incident on the center position of the input coupling grating, and the polarization direction of the vortex beam is perpendicular to the period of the input coupling grating. Then, the orbital angular momentum carried by the incident vortex beam is determined based on the photoelectric signal measured by the detector.

[0029] For the detection of the orbital angular momentum (OAM) of a vortex beam, the vortex beam splitting structure of the detector is located on a metal electrode. After the vortex beam to be measured hits the center of the input coupling grating perpendicularly, the measured responsivity decreases as the order of the incident vortex beam increases. Thus, the magnitude of the orbital angular momentum of the incident vortex beam can be determined based on the magnitude of the responsivity.

[0030] For the detection of the OAM direction of the vortex beam, the vortex beam splitting structure of the detector is located on the metal film between two metal electrodes, and the plasmon excitation region is located in the center of the metal film, while the plasmon detection region is located at the two sides of the metal film. The upper and lower halves of the input coupling grating of the plasmon excitation region have different grating periods, which makes the excited SPP propagate in opposite directions for vortex beams with different rotation directions. The direction of the orbital angular momentum of the incident vortex beam is determined by measuring the positive and negative signs of the photocurrent excited near the two electrodes.

[0031] The vortex beam orbital angular momentum detector of this invention is based on a grating structure. It splits the light field carrying different orbital angular momentum into plasmons in different directions. By utilizing the differences in the direction and propagation loss of the plasmons, a single detector can resolve the orbital angular momentum of the vortex beam. Its main advantages are:

[0032] 1. The working mechanism is not dependent on specific materials and is compatible with existing semiconductor processes. The detection of the orbital angular momentum of a vortex beam can be achieved by fabricating a simple grating structure on the device electrodes.

[0033] 2. It can detect vortex beams carrying high-order OAM, and can detect vortex beams with a topological charge number of m=9, which exceeds the current known reports (m=±4). Attached Figure Description

[0034] Figure 1This is a schematic diagram of the vortex beam OAM detector in Embodiment 1 of the present invention. In the diagram, a is the overall xy-plane view of the detector, b is a schematic diagram of the plasmon excitation, transmission, and detection region, c is an xy-plane view near the detection region, and d is an xz-plane view near the detection region. In the figure, 1-silver electrode, 2-substrate, 3-input coupling grating, and 4-output coupling grating.

[0035] Figure 2 As described in Embodiment 1 of the present invention, the working mechanism of the device is based on the phase matching condition, and the in-plane wave vector ( k OAM The sum of the lattice reciprocal vector (G) and the plasmonic wave vector (G) equals the plasmonic wave vector (G). k spp For specific k OAM It only supports plasmons in specific directions, and so on. k OAM As the x-axis increases, the angle between the direction of plasmon propagation and the x-axis also increases.

[0036] Figure 3 The images shown are electron microscope images (a) and photoelectric test results (b) of the device in Embodiment 1 of the present invention. The photocurrent decreases with the increase of the vortex beam order, but the rate of decrease gradually slows down. The device can realize photoelectric detection of vortex beams with order m≤4.

[0037] Figure 4 This is a schematic diagram of the vortex optical OAM detector in Embodiment 2 of the present invention. In the diagram, a is the overall xy-plane view of the detector, b is a schematic diagram of the plasmon excitation, transmission, and detection region, c is an xy-plane view near the detection region, and d is an xz-plane view near the detection region. In the figure, 1-silver electrode, 2-substrate, 3-input coupling grating, 4-output coupling grating, and 5-plasmon lens.

[0038] Figure 5 The electron microscope image (a) and photoelectric test results (b) of the device in Embodiment 2 of the present invention show that the photocurrent decreases approximately linearly with the increase of the vortex beam order, realizing photoelectric detection of vortex beams in the order range of m=1~9.

[0039] Figure 6 For other structural designs that can increase the loss of plasmons excited by higher-order vortex beams, the input coupling grating of the device shown in a is ring-shaped; the device shown in b has a trapezoidal array of gratings arranged at the edge of the plasmon transmission region; the device shown in c has a wedge-shaped loss structure in the plasmon transmission region; and the device shown in d has multiple circular holes in the plasmon transmission region, and the hole positions are optimized by simulated annealing algorithm to obtain a random hole array structure.

[0040] Figure 7This is a schematic diagram of the vortex optical OAM detector in Embodiment 3 of the present invention, wherein a is the overall xy-plane view of the detector, b is a schematic diagram of the structure of the plasmon excitation, transmission and detection region (within the black dashed box in figure a), c is the xy-plane view near the detection region, and d is the xz-plane view near the detection region.

[0041] Figure 8 This describes the working mechanism of the input coupling grating in Embodiment 3 of the present invention, where 'a' represents the phase distribution diagram and in-plane wave vector of the vortex beam carrying a topological charge of +2. k OAM Directional schematic diagram; b and c are momentum matching schematic diagrams of the upper and lower half of the grating for the vortex beam carrying positive OAM shown in figure a, respectively. Only plasmons propagating to the left can be excited in both cases.

[0042] Figure 9 The images show the propagation directions of plasmons acquired by CCD under different vortex light field excitations in Embodiment 3 of the present invention. The vortex beam carrying positive OAM propagates to the left (left image), and the vortex beam carrying negative OAM propagates to the right (right image). Detailed Implementation

[0043] This invention designs a vortex beam orbital angular momentum detector based on surface plasmon resonances. The technical solution of this invention will be further described in detail below with reference to the accompanying drawings and specific implementation examples.

[0044] Example 1

[0045] The vortex beam OAM detector provided in this embodiment is as follows: Figure 1 As shown, the device consists of silver electrodes fabricated on a silicon-on-insulator (SOI) substrate. Figure 1 (a) In the SOI substrate, most of the top silicon layer is etched to the insulating layer, leaving only a small amount of silicon between the electrodes to generate a photoelectric response. Figure 1(c, d) In this embodiment, the retained top silicon region is 74 μm × 48 μm, fabricated using negative photoresist electron beam lithography and reactive ion etching. The two electrodes are 10 μm apart, 44 μm wide, and 180 nm thick, fabricated using electron beam lithography, magnetron sputtering deposition of a silver film, and a lift-off process. One electrode has an input coupling grating and an output coupling grating. The output coupling grating is fabricated at the electrode edge and consists of four metal trenches, each 44 μm long and 80 nm wide, with a period of 610 nm to match the plasmon wavelength of the silver surface. The input coupling grating is 40 μm from the edge of the electrode containing the output coupling grating and consists of metal trenches 12 μm long and 80 nm wide, with a period of 610 nm. Both the input and output coupling gratings are fabricated using focused ion beam etching. The output coupling grating penetrates the silver film to the semiconductor layer (silicon), while the input coupling grating must ensure that the silver film is not etched through; in this embodiment, the depth is approximately 120 nm. To protect the silver electrode from oxidation, a 3 nm thick aluminum oxide protective layer was deposited using atomic layer deposition technology after the device was fabricated.

[0046] Device working mechanism such as Figure 2 As shown. Based on the momentum matching condition: k OAM + G=k SPP In the formula, k OAM Let be the in-plane wave vector component of the vortex beam. G For the input coupled grating reciprocal lattice vector, k SPP For plasmonic wave vectors, G Given by the grating period Λ, G =2π / Λ. A vortex beam incident perpendicularly at the input coupling grating will generate two propagating plasmons in the +x direction, propagating at a certain angle to the x-axis. As the orbital angular momentum of the vortex beam increases, k OAM As the angle between the plasmon and the x-axis increases, the plasmon needs to travel a longer distance to reach the electrode edge, resulting in greater plasmon losses. Considering that only the light field reaching the electrode edge can be detected, as the OAM increases, less light intensity is converted into photoelectric signals, reducing the device responsivity and enabling OAM resolution of vortex beams.

[0047] During testing, the electrodes on both sides of the device are first connected to a current or voltage measuring device (source meter or picoammeter) via a probe station, or connected to a self-made transimpedance amplifier circuit board via wire bonding to read the photocurrent generated by the device under no bias conditions. During testing, a slightly focused 633 nm wavelength vortex beam is perpendicularly incident on the center of the input coupling grating, with the polarization direction of the vortex beam perpendicular to the period of the input coupling grating. The measured photocurrent is compared with the intensity of the incident vortex beam to obtain the device's responsivity to this vortex beam, thus determining the OAM carried by the incident light. The device test results shown in Example 1 are as follows. Figure 3 As shown (in this embodiment, the intensity of the incident vortex light is the same during the measurement process, which is 103 μW, so the photocurrent can be used to represent the responsivity), the resolution of vortex beams of order m=1-4 is achieved. For vortex light fields of higher order, the change in photoelectric signal caused by the difference in orbital angular momentum is less than the device error, making it difficult to achieve effective resolution.

[0048] Compared to vortex photodetectors based on the OPGE effect, this device can be fabricated on a silicon substrate and does not require measuring the results under both left-handed and right-handed polarization states, making it easier to fabricate and operate.

[0049]

Example 2

[0050] The photodetector structure used in this embodiment is as follows: Figure 4 As shown, similar to Embodiment 1, the device consists of silver electrodes fabricated on an SOI substrate, with aluminum oxide deposited on the surface of the silver electrodes to prevent oxidation. Compared to Embodiment 1, this device employs an additional insulating layer to reduce dark noise. Figure 4 As shown in Figures c and d, a silicon dioxide insulating layer approximately 20 nm thick was first deposited on the SOI substrate using atomic layer deposition (or thermal oxidation, electron beam evaporation, etc.). Then, the insulating layer in a 12 μm × 48 μm region at the center between the two electrodes was etched away using photolithography and reactive ion beam etching. Subsequently, silver electrodes and input / output coupling gratings were fabricated using the same method as in Example 1. The center of the input coupling grating is 30 μm from the top edge of the electrode, and other structural parameters are consistent with Example 1.

[0051] This embodiment demonstrates a method for improving the OAM resolution of a device using plasmonic lenses. For example... Figure 4As shown in Figure b, a dielectric lens was fabricated in the plasmon transport region using negative photoresist exposure. The material was cured maN-2405 photoresist with a height of approximately 450 nm. The lens employed a biconcave design with radii R1 and R2 of 12 μm and 11.33 μm, respectively, and a focal length of -8 μm. The geometric center of the lens was located on the central axis between the grating and the electrode, 10 μm from the center of the input coupling grating in the x-direction. Other high-refractive-index dielectric materials such as aluminum oxide and titanium dioxide can also be used for the plasmon lens. This lens can increase the angle between the propagation direction of the plasmons excited by the vortex beam and the x-axis, further reducing the responsivity of the vortex beam carrying higher-order OAM and improving the device's resolution.

[0052] The testing method for the device is the same as in Example 1, and the test results are as follows: Figure 5 As shown, after optimization with a dielectric lens, the photocurrent decreases approximately linearly with the increase of the orbital angular momentum of the vortex beam, and the device can resolve vortex beams carrying 1-9 topological charges.

[0053] Example 2 illustrates a method for optimizing device performance by designing a plasmonic transport region structure and controlling plasmonic transport loss. Alternatively, selective loss structures can be directly etched onto the silver film in the plasmonic transport region, or the shape of the transport region can be designed (see [link to example 2]). Figure 6 This increases the difference in transmission loss of high-order vortex beams, thereby improving the resolution of the device.

[0054]

Example 3

[0055] Example 3 demonstrates a method for detecting the rotation direction (positive or negative topological charge) of an incident vortex light field by designing an input coupling grating structure.

[0056] Device structure such as Figure 7 As shown, the two electrodes are spaced 100 μm apart, with an 80 μm × 44 μm silver film region in the middle of the electrodes for excitation and transport of plasmons. The silver film region is 10 μm away from both electrodes. A composite grating structure is etched in the center of the silver film; the upper half of the grating is 6 μm long with a period of 680 nm, and the lower half is 6 μm long with a period of 560 nm. Output coupling gratings with a period of 610 nm are etched at both ends of the silver film. The substrate fabrication process, electrode and silver film fabrication process, and grating fabrication process are the same as in Example 1.

[0057] The working principle of the device is as follows Figure 8 As shown. The upper half of the vortex beam carrying positive OAM contains only in-plane wave vectors pointing to the left, while the lower half contains only in-plane wave vectors pointing to the right. Figure 8(a) During testing, the center of the vortex beam is aligned with the center of the composite grating. Considering that the upper grating has a larger period and provides a smaller reciprocal lattice vector, after superimposing with the leftward in-plane wave vector, it only supports plasmons propagating to the left. Similarly, the lower grating has a smaller period and provides a larger reciprocal lattice vector. After superimposing with the rightward in-plane wave vector, it also only supports plasmons propagating to the left. Figure 8 (b, c). That is, for vortex light carrying positive OAM, only plasmons propagating to the left can be generated, exciting a photocurrent near the left electrode. Conversely, for vortex light carrying negative OAM, only plasmons propagating to the right can be generated, exciting a photocurrent in the opposite direction. In actual tests, due to factors such as scattering at the grating boundary, the plasmons cannot propagate completely unidirectionally, but a significant asymmetric distribution of plasmon intensity on both sides can be observed. Figure 9 ).

[0058] The device testing method is the same as in Example 1, but care must be taken to align the center of the incident vortex beam with the center of the grating as precisely as possible. The device can resolve vortex beams carrying topological charges in different directions.

[0059] Finally, it should be stated that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the scope of the claims.

Claims

1. A vortex beam orbital angular momentum detector, based on a "metal-semiconductor-metal" type planar photodetector, characterized in that, A vortex light field splitting structure is arranged on a metal electrode on the side of the detector or between two metal electrodes; the vortex light field splitting structure is made of metal and includes a plasmonic excitation region, a plasmonic transmission region and a plasmonic detection region; the plasmonic excitation region includes an input coupling grating capable of providing plasmons matched with the momentum of the vortex light and exciting plasmons; when the vortex light beam is irradiated on the plasmonic excitation region, the generated plasmons propagate through the plasmonic transmission region and are coupled to a free space light field in the plasmonic detection region, and are converted into a photoelectric signal and detected.

2. The optical vortex beam orbital angular momentum detector of claim 1, wherein, The input coupling grating is a one-dimensional grating or a two-dimensional hole array.

3. The optical vortex beam orbital angular momentum detector of claim 1, wherein, The plasmonic transmission region is a plane, or a plasmonic medium lens is arranged in the plasmonic transmission region, or a differential loss structure is processed, or the shape of the transmission region is designed to increase the loss difference between plasmons excited by different orbital angular momentum vortex light beams during propagation.

4. The optical vortex beam orbital angular momentum detector of claim 1, wherein, The plasmonic detection region is a plane, or an output coupling grating is arranged in the plasmonic detection region.

5. The optical vortex beam orbital angular momentum detector of claim 1, wherein, The vortex light field splitting structure is located on a metal film between two metal electrodes, the plasmonic excitation region is located at the center of the metal film, the plasmonic detection region is located at the two side edges of the metal film, and the part between the plasmonic excitation region and the plasmonic detection region is the plasmonic transmission region; the input coupling grating of the plasmonic excitation region is a composite grating structure including two gratings with different periods, and the periods Λ1 and Λ2 of the two gratings satisfy the following conditions: （1） （2） in formula (1) and formula (2), The detector is a silicon-based photodetector including an SOI substrate and two silver electrodes processed thereon, the top layer silicon of the SOI substrate is located between the two silver electrodes and forms a Schottky contact with the two silver electrodes; the vortex light field splitting structure is located on one silver electrode or a silver film between the two silver electrodes, wherein, in the plasmonic excitation region, the silver film is not punctured when the input coupling grating is processed; in the plasmonic detection region, if the output coupling grating is processed, the silver film is punctured to the top layer silicon. spp is the plasmonic wavelength, k OAM is the in-plane angular momentum of the vortex optical field.

6. The optical vortex beam OAM detector according to any one of claims 1 to 5, wherein the optical vortex beam OAM detector is configured to detect the OAM state of the optical vortex beam by measuring the intensity of the optical vortex beam at the at least one location. The surface of the silver electrode is covered with a transparent protective layer, and / or the silver electrode is provided with an insulating medium layer between the top layer silicon except for the top end part.

7. The optical vortex beam orbital angular momentum detector of claim 6, wherein, After the plasmons are converted into a light field in the plasmonic detection region, an electric signal is converted through photovoltaic effect or photothermal electric effect; when the photovoltaic effect is used, the semiconductor and the metal used in the plasmonic detection region form a Schottky junction; when the photothermal electric effect is used, the semiconductor is a photovoltaic material with photothermal electric effect.

8. The optical vortex beam orbital angular momentum detector of claim 1, wherein, 9. A vortex light beam orbital angular momentum detection method, which is realized by using the vortex light beam orbital angular momentum detector according to any one of claims 1-8, the vortex light beam to be detected is vertically shot at the center position of the input coupling grating, the polarization direction of the vortex light beam is perpendicular to the period of the input coupling grating, and then the orbital angular momentum carried by the incident vortex light beam is judged according to the photoelectric signal measured by the detector. ​ 10. The method of claim 9, wherein the vortex beam is a Laguerre-Gaussian beam. For the detection of the orbital angular momentum size of the vortex beam, the vortex beam field splitting structure of the detector is located on a metal electrode, the measured responsivity decreases with the increase of the incident vortex beam order after the vortex beam to be measured vertically hits the center of the input coupling grating, and thus the orbital angular momentum size of the incident vortex beam is determined according to the responsivity size.

11. The method of claim 9, wherein the vortex beam is a Laguerre-Gaussian beam. For the detection of the orbital angular momentum direction of the vortex beam, the vortex beam orbital angular momentum detector of claim 5 is used, and the grating periods of the upper half and the lower half of the input coupling grating are different, so that the SPPs excited for vortex beams with different rotation directions propagate in opposite directions, and the orbital angular momentum direction of the incident vortex beam is determined by measuring the positive and negative of the photoelectric current excited near the two electrodes.

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