A single-photon emitter with controllable spin angular momentum and momentum direction and method thereof

Abstract

The present invention discloses a single-photon emitter with controllable spin angular momentum and momentum direction, and a method thereof. The present invention comprises a metal substrate, nano-scattering units, a dielectric spacer, and a single-photon source. The present invention independently controls the spin angular momentum of the emitted photons through the internal structure of each nano-scattering unit, in a scheme that is independent of the scattered light phase; and controls the momentum direction of the emitted photons through the position of each nano-scattering unit, in a scheme that is dependent on the scattered light phase. Because these two schemes do not affect each other, the control of the spin angular momentum and momentum direction of the emitted photons can be independently designed, realizing a single-photon emitter in which both the spin angular momentum and momentum direction are independently controllable. The maximum emission angle can reach 53°, and the phase-independent scheme makes the spin angular momentum insensitive to the precise position of the single-photon source relative to the metasurface, eliminating the need for high-precision single-photon source positioning and alignment technology.

Description

Technical Field

[0001] The present invention relates to the field of nanophotonics, and in particular to a single-photon emitter with controllable spin angular momentum and momentum direction and a method for realizing the same. Background Art

[0002] Quantum information science has developed rapidly in recent years and is one of the frontiers of modern physics. Compared with other physical systems, photons interact less strongly with their environment and can maintain good quantum coherence, making them ideal carriers of quantum information. Single-photon sources play an important role in photon-based quantum information technology. In particular, solid-state single-photon sources, such as quantum dots and nitrogen vacancy color centers, are considered to be important basic units for building scalable photon-based quantum information technology. For photons, their energy, momentum, and spin angular momentum are all fundamental degrees of freedom that can be used to carry quantum information and increase information capacity, such as for wavelength multiplexing and momentum multiplexing. Generally, the energy of a photon can be simply controlled by the energy gap of a solid-state single-photon source. However, the photons emitted by solid-state single-photon sources are usually emitted isotropically in space, and the light wave vector corresponds to the momentum direction, that is, the momentum direction is random; at the same time, the polarization state of the photons emitted by solid-state single-photon sources is usually also random, and the polarization state corresponds to the spin angular momentum of the photon, so its spin angular momentum is also random. To achieve control of the momentum direction and spin angular momentum of photons emitted by solid-state single-photon sources, special plans and designs are required.

[0003] The rapid development of metasurface technology provides a very promising platform for controlling light. Metasurfaces are planar optical elements constructed by artificial nanostructures that can control the amplitude, phase and polarization of light on a subwavelength scale. Compared with traditional optical elements, metasurfaces are an ideal platform for miniaturization, integration and multifunctionality of optical devices. They have a wide range of applications, such as beam focusing, polarization state control of light, wavefront shaping, multiplexing and holography. Recently, there have been some studies that have achieved control over the photons emitted by a single-photon source by integrating appropriately designed metasurfaces with single-photon sources. For example, by precisely placing a single-photon source on a metasurface constructed by circular nanoridges with different centers on a silver film, the momentum direction of the emitted photons can be controlled based on phase matching conditions, realizing a single-photon emitter with controllable momentum direction; by preparing a metasurface constructed by dielectric nanoridges with azimuthal widths on a silver film around a nitrogen vacancy color center, the emitted single photons can be controlled to have a positive reduced Planck constant at room temperature. 's spin angular momentum, that is, the emitted photon is in a left-handed circularly polarized state, realizing a circularly polarized single-photon emitter. The total scattering field of the metasurface in free space originates from the coherent superposition of the scattering fields from all nanoscattering units. However, the existing works on the control of the momentum direction and spin angular momentum of the photons are based on the precise control of the phase of the scattered light from the nanoscattering units in the metasurface. This phase-dependent scheme means that the control of the momentum direction and spin angular momentum of the photons is very sensitive to the phase of the scattered light. The positional deviation of the single-photon source and the nanoscattering units in the metasurface will affect the interference results. Therefore, precise mutual positioning between the single-photon source and the nanoscattering units is required. For example, the nitrogen vacancy color center must be placed with a high precision of less than tens of nanometers relative to the metasurface constructed by dielectric nanoridges whose width varies with the azimuth angle, and the emitted photons can obtain the designed normalized Planck constant. The spin angular momentum of the emitted photon is controlled simultaneously, i.e., the polarization state is left-handed circular polarization. On the one hand, the position sensitivity caused by the phase sensitivity mentioned above greatly increases the difficulty of manufacturing. On the other hand, the phase sensitivity makes it a huge challenge to simultaneously and independently control the momentum direction and spin angular momentum of the emitted photon. To date, no single-photon emitter has been able to simultaneously and independently control the momentum direction and spin angular momentum of the emitted photon. Summary of the Invention

[0004] In response to the problems existing in the above-mentioned prior art, the present invention proposes a single-photon emitter with controllable spin angular momentum and momentum direction and an implementation method thereof. At the same time, phase-independent and phase-dependent schemes are used to construct an anisotropic metasurface, and a single-photon source is integrated with the metasurface. The spin angular momentum and momentum direction of the emitted photons are independently controlled by the phase-independent scheme and the phase-dependent scheme, respectively. Since these two schemes do not affect each other, the control of the spin angular momentum and momentum direction of the emitted photons can be independently designed, thereby realizing a single-photon emitter with independently controllable spin angular momentum and momentum direction.

[0005] One object of the present invention is to provide a single-photon emitter with controllable spin angular momentum and momentum direction.

[0006] The single-photon emitter with controllable spin angular momentum and momentum direction of the present invention comprises: a metal substrate, a nano-scattering unit, a dielectric spacer, and a single-photon source; wherein, in a Cartesian coordinate system, the upper surface of the metal substrate is an xy plane; a plurality of nano-scattering units are engraved on the upper surface of the metal substrate, each nano-scattering unit comprising two rectangular nano-grooves of identical shape engraved on the upper surface of the metal substrate, the rectangular nano-grooves located in the counterclockwise direction of the radius being the first nano-groove, the rectangular nano-groove located in the clockwise direction of the radius being the second nano-groove, the adjacent vertices on the inner long sides of the first and second nano-grooves being located on the same radius centered at the origin, the vertex belonging to the first nano-groove being the first vertex, and the vertex belonging to the second nano-groove being the second vertex, the angle between the long side of the first nano-groove and the radius being the orientation of the first nano-groove being denoted as α, the angle between the long side of the second nano-groove and the radius being the orientation of the second nano-groove being denoted as β, and the radial distance s being the distance from the center of the second nano-groove to the origin minus the distance from the center of the first nano-groove to the origin. r The orientation and radial distance of the first and second nanogrooves satisfy the following set of spin angular momentum governing equations:

[0007]

[0008]

[0009]

[0010] Among them, k spp is the wave vector of surface plasmon, k0 is the wave vector of vacuum light, is the azimuth angle of the nanoscattering unit, that is, the azimuth angle of the radius where the first vertex is located; the nanoscattering unit is structurally anisotropic, that is, the internal structural parameters of the nanoscattering unit at different azimuth angles are different, and it is an anisotropic metasurface; k is the light wave vector of the set emission direction, which represents the set emission direction. The set emission direction is in the xz plane, and the angle between the set emission direction and the z axis is taken as the emission angle θ, l and w are the length and width of the rectangular nanogroove respectively; where the right side of the equation in equation (3) takes "+" to represent left-handed circular polarization, that is, the spin angular momentum of the photon in the set emission direction is the positive reduced Planck constant Taking “-” indicates right-handed circular polarization, that is, setting the spin angular momentum of the photon in the emission direction to the negative reduced Planck constant

[0011] The position vector r of the nanoscattering unit satisfies the constructive interference condition in the set emission direction:

[0012] k spp r+δ-k·r=2mπ+C0 (4)

[0013] Wherein, δ is the scattering phase of the nanoscattering unit, r is the modulus of the position vector r of the nanoscattering unit, the midpoint between the first vertex and the second vertex of the nanoscattering unit is taken as the position of the nanoscattering unit, that is, r is the distance from the midpoint between the first vertex and the second vertex to the origin, C0 is a constant applicable to all nanoscattering units, and m is an integer;

[0014] Polar coordinate position of the nanoscattering unit Satisfies the momentum direction control equation:

[0015]

[0016] The distance from the starting point of the elliptical spiral to the origin is r0 = (C0 / k spp +λ spp ), λ spp and λ are the surface plasmon wavelength and light wavelength, respectively; m represents the number of turns of the elliptical helix, m=1,…,M, 3≤M≤15, and M is the total number of turns of the elliptical helix;

[0017] The scattering phase δ of the nanoscattering unit satisfies the geometric phase equation:

[0018]

[0019] Where θ is the emission angle, which is the angle between the set emission direction and the z-axis. σ represents the photon spin, which is +1 for left-handed circularly polarized (LCP) light and -1 for right-handed circularly polarized (RCP) light, and the corresponding photon spin angular momentum is the positive reduced Planck constant. and the negative reduced Planck constant According to the momentum direction control equation and the geometric phase equation, the position of the nanoscattering unit is also anisotropic. For obliquely emitted LCP light and RCP light, the solution to the momentum direction control equation is an elliptical spiral, that is, the position of the nanoscattering unit is arranged in a multi-turn elliptical spiral, each turn of the elliptical spiral is confocal and the focus is at the origin, and the spiral directions of the multi-turn elliptical spirals corresponding to the LCP light and RCP light are opposite. For the forward-emitting LCP light and RCP light, the solution to the momentum direction control equation is an Archimedean spiral.

[0020] A dielectric spacer layer is provided on the metal substrate of the nano-scattering unit to eliminate the fluorescence quenching effect of the single-photon source on the metal surface; a single-photon source is placed on the dielectric spacer layer, and the single-photon source is located within the multi-ring nano-scattering unit;

[0021] The excitation spot is irradiated to the single-photon source, causing the single-photon source to emit photons, exciting surface plasmon polariton (SPP) on the metal surface. The excited SPP propagates uniformly in all directions along the radial direction along the surface of the metal substrate; the SPP encounters the nano-scattering unit and is scattered to form scattered light. The position vector r of each nano-scattering unit satisfies the constructive interference condition in the set emission direction, so that the scattered light scattered by all the nano-scattering units coherently constructively forms an emission spot along the set emission direction, so that the direction of the scattered light is along the set emission direction. The set emission direction corresponds to the momentum direction of the photon, so that the momentum direction is independently controlled by the arrangement position of each nano-scattering unit, and the momentum direction is related to the phase of the scattered light from the nano-scattering unit; each nano-scattering unit satisfies the spin angular momentum control equation group, and the first and second phases within a nano-scattering unit are obtained. The projections of the linearly polarized light scattered by the nanogrooves into the far field on a plane perpendicular to the set emission direction are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, has the same target polarization state. The polarization state corresponds to the spin angular momentum of the photon, so that the spin angular momentum of the scattered light is independently controlled by the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit; the scattered light finally propagates to the far field along the set emission direction with circularly polarized spin angular momentum, realizing a single-photon emitter with simultaneous and independent control of momentum direction and spin angular momentum.

[0022] When the launch angle θ≤15°, the control equations of spin angular momentum are approximately simplified to obtain an approximate analytical solution:

[0023]

[0024]

[0025] The offset of the single photon source from the z-axis is less than 600 nm.

[0026] The metal substrate is made of silver and has a thickness of 300 nm or greater. The distance between angularly adjacent nanoscattering units is less than 1 μm, and the spacing between nanoscattering units is 50 nm or greater. The rectangular nanogrooves have a length of 200 nm to 300 nm and a width of 50 nm to 100 nm. The groove depth is related to the length and width. Finite element calculations are used to ensure that the nanoscattering units have high scattering efficiency and high circular polarization degree.

[0027] The dielectric spacer layer is made of a dielectric material transparent in the visible light band, such as aluminum oxide, glass or quartz, and has a thickness of 5nm to 20nm.

[0028] The excitation light spot is a focused light spot, and the size of the light spot is the diffraction limit.

[0029] Another object of the present invention is to provide a method for realizing a single-photon emitter with controllable spin angular momentum and momentum direction.

[0030] The method for realizing a single-photon emitter with controllable spin angular momentum and momentum direction of the present invention comprises the following steps:

[0031] 1) A dielectric spacer layer is provided on the metal substrate of the nano-scattering unit to eliminate the fluorescence quenching effect of the single-photon source on the metal surface;

[0032] 2) In a Cartesian coordinate system, the upper surface of the metal substrate is the xy plane. A single-photon source is placed on the dielectric spacer layer and located within the multi-ring nanoscattering unit. The excitation spot is irradiated onto the single-photon source, causing it to emit photons, exciting surface plasmons (SPPs) on the metal surface. The excited SPPs propagate uniformly in all directions along the surface of the metal substrate in a radial direction.

[0033] 3) SPP encounters nano-scattering units and is scattered to form scattered light;

[0034] The position vector r of the nanoscattering unit satisfies the constructive interference condition in the set emission direction:

[0035] k spp r+δ-k·r=2mπ+C0

[0036] Among them, k spp is the wave vector of the surface plasmon, δ is the scattering phase of the nanoscattering unit, r is the modulus of the position vector r of the nanoscattering unit, the midpoint between the first vertex and the second vertex of the nanoscattering unit is taken as the position of the nanoscattering unit, that is, r is the distance from the midpoint between the first vertex and the second vertex to the origin, C0 is a constant applicable to all nanoscattering units, and m is an integer; k is the light wave vector of the set emission direction, which represents the set emission direction. The set emission direction is in the xz plane, and the angle between the set emission direction and the z axis is taken as the emission angle θ;

[0037] Polar coordinate position of the nanoscattering unit Satisfies the momentum direction control equation:

[0038]

[0039] Where r0=(C0 / k spp +λ spp ), is the azimuth angle of the nanoscattering unit; m represents the number of turns of the elliptical spiral, m = 1, ..., M, 3 ≤ M ≤ 15, and M is the total number of turns of the elliptical spiral;

[0040] The scattering phase δ of the nanoscattering unit satisfies the geometric phase equation:

[0041]

[0042] Where θ is the emission angle, σ represents the photon spin, which is +1 for left-handed circularly polarized (LCP) light and -1 for right-handed circularly polarized (RCP) light, and the corresponding photon spin angular momentum is the positive reduced Planck constant. and the negative reduced Planck constant According to the momentum direction control equation and the geometric phase equation, the position of the nanoscattering unit is anisotropic. For obliquely emitted LCP light and RCP light, the solution to the momentum direction control equation is an elliptical spiral, that is, the position of the nanoscattering unit is arranged in a multi-turn elliptical spiral, each turn of the elliptical spiral is confocal and the focus is at the origin. The spiral directions of the multi-turn elliptical spirals corresponding to the LCP light and RCP light are opposite. For the forward-emitted LCP light and RCP light, the solution to the momentum direction control equation is an Archimedean spiral.

[0043] The position vector r of each nano-scattering unit satisfies the constructive interference condition in the set emission direction, so that the scattered light after being scattered by all the nano-scattering units coherently constructively forms an emission spot along the set emission direction, so that the direction of the scattered light is along the set emission direction. The set emission direction corresponds to the momentum direction of the photon, so that the momentum direction is independently controlled by the arrangement position of each nano-scattering unit, and the momentum direction is related to the phase of the scattered light from the nano-scattering unit;

[0044] 4) In each nano-scattering unit, the orientation and radial distance of the first and second nano-grooves satisfy the following spin angular momentum control equations:

[0045]

[0046]

[0047]

[0048] Where k0 is the vacuum light wave vector, l and w are the length and width of the rectangular nanogroove, respectively, α is the angle between the long side and the radius of the first nanogroove as the orientation of the first nanogroove, β is the angle between the long side and the radius of the second nanogroove as the orientation of the second nanogroove, and s ris the radial distance; k spp is the wave vector of the surface plasmon; the nanoscattering unit is anisotropic in structure and is an anisotropic metasurface; in the third equation of the spin angular momentum control equation group, the right side of the equation is “+” to represent left-handed circular polarization, that is, the spin angular momentum of the photon in the emission direction is set to the positive reduced Planck constant Taking “-” indicates right-handed circular polarization, that is, setting the spin angular momentum of the photon in the emission direction to the negative reduced Planck constant

[0049] Each nanoscattering unit satisfies a set of spin angular momentum control equations. The projections of linearly polarized light scattered from the first and second nanogrooves within a nanoscattering unit into the far field on a plane perpendicular to a set emission direction are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, the same target polarization state. The polarization state corresponds to the spin angular momentum of the photon. Therefore, the spin angular momentum of the scattered light is independently controlled by the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit.

[0050] 5) The scattered light eventually propagates to the far field along the set emission direction with circularly polarized spin angular momentum, realizing a single-photon emitter that can simultaneously and independently control the momentum direction and spin angular momentum.

[0051] Advantages of the present invention:

[0052] The present invention independently controls the spin angular momentum of emitted photons through the internal structure of each nanoscattering unit, and the scheme is independent of the phase of the scattered light; and controls the momentum direction of emitted photons through the position of each nanoscattering unit, and the scheme is related to the phase of the scattered light. Since these two schemes do not affect each other, the control of the spin angular momentum and momentum direction of the emitted photons can be designed independently, realizing a single-photon emitter in which the spin angular momentum and momentum direction are simultaneously and independently controllable; the maximum emission angle can reach 53°, and the phase-independent scheme makes the spin angular momentum insensitive to the precise position of the single-photon source relative to the metasurface, eliminating the need for high-precision single-photon source positioning and alignment technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 Schematic diagram of an embodiment of a single-photon emitter with controllable spin angular momentum and momentum direction according to the present invention, wherein (a) is a top view of the entire device, and (b) is a schematic diagram of the internal structure of a nano-scattering unit;

[0054] Figure 2Figures 1 and 2 are numerical simulation results of far-field emission obtained according to an embodiment of a single-photon emitter with controllable spin angular momentum and momentum direction according to the present invention, wherein (a) is a far-field scattered light intensity distribution diagram of the single-photon emitter when the single-photon source is located exactly on the z-axis and polarized along the z-direction; (b) is a distribution diagram of the far-field scattered light, where the vertical height represents the intensity of the scattered field, and the brightness represents the chirality of the scattered field; (c) is the integrated chirality of the far-field emission spot at different emission angles θ, where the solid line and the dashed line correspond to the results given by the rigorous numerical solution and the approximate analytical solution, respectively; and (d) is the integrated chirality of the far-field emission spot when the single-photon source deviates from the z-axis at different distances, where the solid line and the dashed line correspond to the results of deviation along the x-direction and the y-direction, respectively. DETAILED DESCRIPTION

[0055] The present invention will be further described below through specific embodiments in conjunction with the accompanying drawings.

[0056] like Figure 1 As shown, the single-photon emitter with controllable spin angular momentum and momentum direction of this embodiment includes: a metal substrate, a nano-scattering unit, a dielectric spacer and a single-photon source; wherein, in a Cartesian coordinate system, the upper surface of the metal substrate is an xy plane; a plurality of nano-scattering units are engraved on the upper surface of the metal substrate, each nano-scattering unit includes two rectangular nano-grooves of the same shape engraved on the upper surface of the metal substrate, the length and width of the rectangular nano-grooves are represented by l and w respectively, the rectangular nano-grooves located in the counterclockwise direction of the radius are the first nano-grooves, and the rectangular nano-grooves located in the clockwise direction of the radius are the second nano-grooves. The rectangular nano-groove with a first and a second nano-groove on the inner long side and adjacent vertices are located on the same radius centered at the origin. The vertex belonging to the first nano-groove is the first vertex, and the vertex belonging to the second nano-groove is the second vertex. The angle between the long side and the radius of the first nano-groove is expressed as the orientation of the first nano-groove and is denoted as α. The angle between the long side and the radius of the second nano-groove is expressed as the orientation of the second nano-groove and is denoted as β. The distance from the center of the second nano-groove to the origin minus the distance from the center of the first nano-groove to the origin is the radial distance s. r The orientation and radial distance of the first and second nanogrooves satisfy the following set of spin angular momentum governing equations:

[0057]

[0058]

[0059]

[0060] Among them, k spp is the wave vector of surface plasmon, k0 is the wave vector of vacuum light, is the azimuth angle of the nanoscattering unit, that is, the azimuth angle of the radius where the first vertex is located; the nanoscattering unit is structurally anisotropic, that is, the internal structural parameters of the nanoscattering unit at different azimuth angles are different, and it is an anisotropic metasurface; k is the light wave vector of the set emission direction, which represents the set emission direction. The set emission direction is in the xz plane, and the angle between the set emission direction and the z axis is the emission angle θ. l and w are the length and width of the rectangular nanogroove, respectively; where "+" on the right side of equation (3) represents left-handed circular polarization, that is, the spin angular momentum of the photon in the set emission direction is the positive reduced Planck constant Taking “-” indicates right-handed circular polarization, that is, setting the spin angular momentum of the photon in the emission direction to the negative reduced Planck constant The center of the first nano-groove is closer to the origin, indicating left-handed circular polarization, and the center of the second nano-groove is closer to the origin, indicating right-handed circular polarization.

[0061] When the launch angle θ≤15°, the control equations of spin angular momentum are approximately simplified to obtain an approximate analytical solution:

[0062]

[0063]

[0064] Each nanoscattering unit satisfies a set of spin angular momentum control equations. The projections of linearly polarized light scattered from the first and second nanogrooves within a nanoscattering unit into the far field on a plane perpendicular to a set emission direction are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, the same target polarization state. The polarization state corresponds to the spin angular momentum of the photon. Therefore, the spin angular momentum of the scattered light is independently controlled by the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit.

[0065] In order to control the momentum direction of the emitted photons, that is, to control the emission direction of the emitted photons, the positions of the nano-scattering units are arranged based on the constructive interference condition. The position vector r of the nano-scattering unit satisfies the constructive interference condition in the set emission direction:

[0066] k spp r+δ-k·r=2mπ+C0 (4)

[0067] Wherein, δ is the scattering phase of the nanoscattering unit, r is the modulus of the position vector r of the nanoscattering unit, the midpoint between the first vertex and the second vertex of the nanoscattering unit is taken as the position of the nanoscattering unit, that is, r is the distance from the midpoint between the first vertex and the second vertex to the origin, C0 is a constant applicable to all nanoscattering units, and m is an integer;

[0068] Polar coordinate position of the nanoscattering unit The momentum direction control equation should be satisfied:

[0069]

[0070] The distance from the starting point of the elliptical spiral to the origin is r0 = (C0 / k spp +λ spp ); m=1,2,3,4,5;

[0071] The scattering phase δ of the nanoscattering unit satisfies the geometric phase equation:

[0072]

[0073] Where θ is the emission angle, which is the angle between the set emission direction and the z-axis. σ represents the photon spin, which is +1 for left-handed circularly polarized (LCP) light and -1 for right-handed circularly polarized (RCP) light, and the corresponding photon spin angular momentum is the positive reduced Planck constant. and the negative reduced Planck constant According to the momentum direction control equation and the geometric phase equation, the position of the nanoscattering unit is anisotropic. For obliquely emitted LCP light and RCP light, the solution to the momentum direction control equation is an elliptical spiral, that is, the position of the nanoscattering unit is arranged in a multi-turn elliptical spiral with opposite spiral directions. Each turn of the elliptical spiral is confocal and the focus is at the origin. The spiral directions of the multi-turn elliptical spirals corresponding to the LCP light and RCP light are opposite. For the positively emitted LCP light and RCP light, the solution to the position equation is an Archimedean spiral.

[0074] A dielectric spacer is set on the metal substrate of the nano-scattering unit to eliminate the fluorescence quenching effect of the single-photon source on the metal surface; a single-photon source is placed on the dielectric spacer, and the offset of the single-photon source from the coordinate z-axis is less than 600nm.

[0075] The excitation spot is irradiated to the single-photon source, causing the single-photon source to emit photons, exciting surface plasmon polariton (SPP) on the metal surface. The excited SPP propagates uniformly in all directions along the radial direction along the surface of the metal substrate; the SPP encounters the nano-scattering unit and is scattered to form scattered light. The position vector r of each nano-scattering unit satisfies the constructive interference condition in the set emission direction, so that the scattered light scattered by all the nano-scattering units coherently constructively forms an emission spot along the set emission direction, so that the direction of the scattered light is along the set emission direction. The set emission direction corresponds to the momentum direction of the photon, so that the momentum direction is independently controlled by the arrangement position of each nano-scattering unit, and the momentum direction is related to the phase of the scattered light from the nano-scattering unit; each nano-scattering unit satisfies the spin angular momentum control equation group, and the first and second phases within a nano-scattering unit are obtained. The projections of the linearly polarized light scattered by the nanogrooves into the far field on a plane perpendicular to the set emission direction are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, has the same target polarization state. The polarization state corresponds to the spin angular momentum of the photon, so that the spin angular momentum of the scattered light is independently controlled by the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit; the scattered light finally propagates to the far field along the set emission direction with circularly polarized spin angular momentum, realizing a single-photon emitter with simultaneous and independent control of momentum direction and spin angular momentum.

[0076] The internal structure of the nanoscattering unit is designed to scatter light with the same target spin angular momentum in the set emission direction. This process makes the spin angular momentum of the total scattered field in the far field almost independent of the phase of the scattered light from the nanoscattering unit, and is insensitive to the precise positional relationship between the single-photon source and the nanoscattering unit. This is a phase-independent mechanism. The control of the momentum direction is achieved by reasonably controlling the position of the nanoscattering unit in the metasurface, thereby obtaining the required phase of the scattered light to ensure that the scattered light from all nanoscattering units in the set emission direction constructively interferes with each other. This is a phase-dependent mechanism. The specific structure of the nanoscattering unit is different for different spin angular momenta and momentum directions.

[0077] In order to verify the present invention, numerical simulation was performed using the finite element software COMSOL Multiphysics. In the simulation, an electric dipole located 20 nm above the metal substrate is used to represent the single-photon source, and the x and y coordinates of the single-photon source are equal to 0. The wavelength of light emitted by the single-photon source under the excitation of the excitation spot is λ = 623 nanometers. The metal substrate is made of silver, and the effective refractive index is 0.0363 + 3.13i. Each nano-groove etched into the metal substrate has a length of 250 nm, a width of 100 nm and a depth of 70 nm. The elliptical spiral has 5 turns, with m = 1, 2, 3, 4, 5, and there are 70 nano-scattering units in each turn. The value of the starting point r0 of the elliptical spiral is set to 7.5 μm. Take the emission angle θ = 15° and the polarization of the emitted photons as LCP as a typical example. These conditions correspond to the design of the emitted photon momentum of (sin15°, 0, cos15°), the spin angular momentum is Different azimuths The anisotropic orientation angle and radial spacing of the nanogrooves in the nanoscattering units are determined by the governing equations for spin angular momentum, while the positions of the nanoscattering units within the metasurface are determined by the coherent constructive conditions and the geometric phase equation. Therefore, the internal structure and positions of the nanoscattering units can be independently designed.

[0078] In the far field, the scattered light intensity distribution of a single photon emitter is as follows: Figure 2 As shown in (a), a well-collimated emission spot with a divergence angle of only 2.7° is obtained. The simulated emission angle is 14.7°, which is very close to the design value of emission angle θ = 15°. The slight difference is attributed to the additional loss and phase shift when the SPP propagates on the nanoscattering unit. Figure 2 (b) The far-field scattered light distribution is further shown in a three-dimensional graph. The vertical height in the graph represents the intensity of the scattered field, while the brightness depth represents the chirality of the scattered field. Here, the circular polarization degree of the far-field scattered light is characterized by the chirality C, which is defined as C = (I L –I R ) / (I L +I R ), where I L and I R Represents the intensity of LCP light and RCP light in a specific direction. Thus, C = 1 and -1 represent ideal LCP light and ideal RCP light, respectively. It can be seen that the far-field scattering pattern shows a sharp single-peak structure, indicating that the scattered light is well collimated to the far field. At the same time, this peak is almost completely black, indicating that it is high-purity LCP light. In order to further quantitatively describe the chirality of the far-field emission spot, the integrated chirality C is used. int , defined as C int =(I Lint –I Rint ) / (ILint +I Rint ), where I Lint and I Rint are the integrated light intensities of the LCP and RCP components of the far-field emission spot, respectively. When integrating, the maximum light intensity position of the far-field emission spot is taken as the center and the full width at half maximum of the peak is taken as the radius, thus including most of the far-field emission spot intensity. The calculation results show that the integrated chirality C int It reaches 0.91, which indicates that the far-field emission spot is high-purity LCP light in the set emission direction, that is, the spin angular momentum of the emitted photon in the set emission direction is These numerical simulation results well validate the proposed design. In addition, the simulations also show that the SPPs excited by the single-photon source and the scattered light from the nanoscattering unit to the far field account for 71% and 49% of the total power of the single-photon source, respectively.

[0079] We further simulated the changes in the integrated chirality of the far-field emission spot at different emission angles θ, and the results are as follows: Figure 2 As shown by the dotted line in (c). It can be observed that when the emission angle θ≤15°, the simulated integrated chirality of the far-field emission spot remains relatively high (≥0.91). For larger emission angles θ, the simulated integrated chirality of the far-field emission spot decreases rapidly. The reason is that the above simulation directly uses an approximate analytical solution, that is, the circular polarization geometric parameter equations are approximately simplified to obtain the internal structural parameters of the nano-scattering unit given in the approximate analytical solution. The above approximation will cause a large deviation when the emission angle θ is large. At this time, the performance of the metasurface can be improved by using an accurate numerical solution, and the orientation angles α, β and radial distance s of the nano-grooves can be obtained through the accurate numerical solution. r The simulation results obtained under the internal structural parameters of the nano-scattering unit given by the exact numerical solution are as follows Figure 2The solid line in (c) shows that at large emission angles θ, the metasurface performance given by the exact numerical solution is significantly superior to the approximate analytical solution. When the emission angle θ is ≤ 53°, and the collection angle of 53° corresponds to a numerical aperture (NA) of 0.8 for the objective lens used in the experiment, the integrated chirality of the far-field emission spot remains consistently high (≥ 0.92). In previous work, the maximum emission angle θ only reached 20°, far less than the maximum emission angle θ = 53° achieved here. This means that by varying the emission angle θ in the control equations for spin angular momentum, momentum direction, and geometric phase, the corresponding metasurface structural parameters can be obtained, allowing the angle of the emission direction of the designed single-photon generator relative to the z-axis to be controlled over a wide range. Furthermore, for a set emission angle θ, the orientation of the single-photon generator's emission direction in the xy plane can be rotated by the same angle around the z-axis by simply rotating the entire metasurface structure around the z-axis. This also causes the projection of the emitted photon's momentum direction in the xy plane to rotate by the same angle around the z-axis.

[0080] Based on the above design principle, the present invention independently controls the spin angular momentum of the emitted photons by designing the internal structure of the nano-scattering unit. For example, the "+" on the right side of the third equation in the spin angular momentum control equation group is changed to "-", and the spin angular momentum of the emitted photons can be reduced from the normalized Planck constant to Switch to negative reduced Planck constant Structurally, this is achieved by exchanging the radial positions of the two nanogrooves in each nanoscattering unit. At the same time, according to the momentum direction control equation and the geometric phase equation, the spiral direction of the elliptical helix needs to be reversed.

[0081] A particular advantage of the proposed single-photon emitter is that the spin angular momentum of the emitted photons is insensitive to the precise position of the single-photon source relative to the metasurface, due to the phase-independent design scheme used to control the spin angular momentum of the emitted photons. Figure 2 The solid and dashed lines in (d) show the integrated chirality of the far-field emission spot when the single-photon source deviates from the z-axis by different distances in the x-direction and y-direction, respectively. When the offset does not exceed 600nm (≈λ), the integrated chirality of the far-field emission spot decreases only slightly (maintaining ≥0.87). This phenomenon could not be observed in previous work based on phase correlation schemes. This positional robustness of the single-photon source allows for sample preparation and experiments using relatively simple sample preparation processes, eliminating the need for high-precision single-photon source positioning and alignment techniques.

[0082] In the above analysis, only single-photon sources with z-polarization were considered. The reason is that the energy coupled into the SPP mode by a single-photon source is proportional to the coupling strength between the electric dipole and the SPP field. The z component of the SPP field is much stronger than the x and y components. Therefore, the coupling strength between the z-direction electric dipole and the SPP is much greater than the coupling strength between the x- and y-direction electric dipoles and the SPP. Numerical simulations show that the SPP power excited by an x- or y-direction electric dipole is only about one twentieth of the SPP power excited by a z-direction electric dipole. Given the random polarization of the single-photon source itself, this fact means that the signal detected in the actual experiment mainly comes from the z-polarization contribution of the single-photon source, while the contributions from the x- and y-directions are negligible.

[0083] Finally, it should be noted that the purpose of disclosing the embodiments is to facilitate a further understanding of the present invention. However, those skilled in the art will appreciate 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 contents disclosed in the embodiments; the scope of protection claimed by the present invention shall be determined by the scope defined in the claims.

Claims

1. A single-photon emitter with controllable spin angular momentum and momentum direction, characterized in that: The single-photon emitter with controllable spin angular momentum and momentum direction includes: a metal substrate, a nano-scattering unit, a dielectric spacer and a single-photon source; wherein, in a Cartesian coordinate system, the upper surface of the metal substrate is an xy plane; a plurality of nano-scattering units are engraved on the upper surface of the metal substrate, each nano-scattering unit includes two rectangular nano-grooves of identical shape engraved on the upper surface of the metal substrate, the rectangular nano-groove located in the counterclockwise direction of the radius is a first nano-groove, and the rectangular nano-groove located in the clockwise direction of the radius is a second nano-groove, the first and second nano-grooves are on the inner long sides and adjacent vertices are located on the same radius centered at the origin, the vertex belonging to the first nano-groove is the first vertex, and the vertex belonging to the second nano-groove is the second vertex, the angle between the long side of the first nano-groove and the radius is expressed as α as the orientation of the first nano-groove, the angle between the long side of the second nano-groove and the radius is expressed as β as the orientation of the second nano-groove, and the distance from the center of the second nano-groove to the origin minus the distance from the center of the first nano-groove to the origin is the radial distance s. r The orientation and radial distance of the first and second nanogrooves satisfy the following set of spin angular momentum governing equations: Among them, k spp is the wave vector of surface plasmon, k0 is the wave vector of vacuum light, is the azimuth angle of the nanoscattering unit, that is, the azimuth angle of the radius where the first vertex is located; the nanoscattering unit is structurally anisotropic, that is, the internal structural parameters of the nanoscattering unit at different azimuth angles are different, and it is an anisotropic metasurface; k is the light wave vector of the set emission direction, which represents the set emission direction. The set emission direction is in the xz plane, and the angle between the set emission direction and the z axis is taken as the emission angle θ. l and w are the length and width of the rectangular nanogroove, respectively; where "+" on the right side of equation (3) indicates left-handed circular polarization, that is, the spin angular momentum of the photon in the set emission direction is the positive reduced Planck constant "-" represents right-handed circular polarization, that is, the spin angular momentum of the photon in the emission direction is set to the negative reduced Planck constant The position vector r of the nanoscattering unit satisfies the constructive interference condition in the set emission direction: k spp r+δ-k·r=2mπ+C0 (4) Wherein, δ is the scattering phase of the nanoscattering unit, r is the modulus of the position vector r of the nanoscattering unit, the midpoint between the first vertex and the second vertex of the nanoscattering unit is taken as the position of the nanoscattering unit, that is, r is the distance from the midpoint between the first vertex and the second vertex to the origin, C0 is a constant applicable to all nanoscattering units, and m is an integer; Polar coordinate position of the nanoscattering unit Satisfies the momentum direction control equation: The distance from the starting point of the elliptical spiral to the origin is r0 = (C0 / k spp +λ spp ), λ spp and λ are the surface plasmon wavelength and light wavelength, respectively; m represents the number of turns of the elliptical helix, m=1,…,M, 3≤M≤15, and M is the total number of turns of the elliptical helix; The scattering phase δ of the nanoscattering unit satisfies the geometric phase equation: Among them, θ is the emission angle, which is the angle between the set emission direction and the z-axis, σ represents the photon spin, which takes +1 and -1 for left-handed circularly polarized LCP light and right-handed circularly polarized RCP light, respectively, and the corresponding photon spin angular momentum is the positive reduced Planck constant +h and the negative reduced Planck constant -h, respectively; according to the momentum direction control equation and the geometric phase equation, the position of the nanoscattering unit is also anisotropic; for the obliquely emitted LCP light and RCP light, the solution of the momentum direction control equation is an elliptical spiral, that is, the position of the nanoscattering unit is arranged in a multi-turn elliptical spiral, each turn of the elliptical spiral is confocal and the focus is at the origin, and the spiral directions of the multi-turn elliptical spirals corresponding to the LCP light and RCP light are opposite; for the positively emitted LCP light and RCP light, the solution of the momentum direction control equation is an Archimedean spiral; A dielectric spacer layer is provided on the metal substrate of the nano-scattering unit to eliminate the fluorescence quenching effect of the single-photon source on the metal surface; a single-photon source is placed on the dielectric spacer layer, and the single-photon source is located within the multi-ring nano-scattering unit; The excitation light spot is irradiated to the single-photon source, causing the single-photon source to emit photons, exciting the surface plasmons SPP on the metal surface, and the excited SPP propagates uniformly in all directions along the radial direction along the surface of the metal substrate; the SPP encounters the nano-scattering unit and is scattered to form scattered light, and the position vector r of each nano-scattering unit satisfies the constructive interference condition in the set emission direction, so that the scattered light after being scattered by all the nano-scattering units coherently constructively forms an emission light spot along the set emission direction, so that the direction of the scattered light is along the set emission direction, and the set emission direction corresponds to the momentum direction of the photon, so that the momentum direction is independently controlled by the arrangement position of each nano-scattering unit, and the momentum direction is related to the phase of the scattered light from the nano-scattering unit; each nano-scattering unit satisfies the spin angular momentum control equation group, from a nano-scattering unit The linearly polarized light scattered to the far field by the first and second nanogrooves in the Mie scattering unit has projections on a plane perpendicular to the set emission direction that are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, has the same target polarization state, and the polarization state corresponds to the spin angular momentum of the photon, so that the spin angular momentum of the scattered light is independently controlled through the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit; the scattered light finally propagates to the far field along the set emission direction with circularly polarized spin angular momentum, realizing a single-photon emitter with simultaneous and independent control of momentum direction and spin angular momentum.

2. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: When the launch angle θ≤15°, the control equations of spin angular momentum are approximately simplified to obtain an approximate analytical solution:

3. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: The offset of the single photon source from the z-axis is less than 600 nm.

4. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: The thickness of the metal substrate is ≥300 nm.

5. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: The distance between angularly adjacent nano-scattering units is less than 1 μm, and the interval between nano-scattering units is ≥50 nm.

6. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: The length of the rectangular nano-groove is 200nm to 300nm, and the width is 50nm to 100nm.

7. The single-photon emitter with controllable spin angular momentum and momentum direction according to claim 1, characterized in that: The dielectric spacer layer is made of a dielectric material transparent in the visible light band and has a thickness of 5nm to 20nm.

8. A method for realizing a single-photon emitter with controllable spin angular momentum and momentum direction as claimed in claim 1, characterized in that: The implementation method comprises the following steps: 1) A dielectric spacer layer is provided on the metal substrate of the nano-scattering unit to eliminate the fluorescence quenching effect of the single-photon source on the metal surface; 2) In a Cartesian coordinate system, the upper surface of the metal substrate is the xy plane. A single-photon source is placed on the dielectric spacer layer and located within the multi-ring nanoscattering unit. The excitation spot is irradiated onto the single-photon source, causing it to emit photons, exciting surface plasmons (SPPs) on the metal surface. The excited SPPs propagate uniformly in all directions along the surface of the metal substrate in a radial direction. 3) SPP encounters nano-scattering units and is scattered to form scattered light; The position vector r of the nanoscattering unit satisfies the constructive interference condition in the set emission direction: k spp r+δ-k·r=2mπ+C0 Among them, k spp is the wave vector of the surface plasmon, δ is the scattering phase of the nanoscattering unit, r is the modulus of the position vector r of the nanoscattering unit, the midpoint between the first vertex and the second vertex of the nanoscattering unit is taken as the position of the nanoscattering unit, that is, r is the distance from the midpoint between the first vertex and the second vertex to the origin, C0 is a constant applicable to all nanoscattering units, and m is an integer; k is the light wave vector of the set emission direction, which represents the set emission direction. The set emission direction is in the xz plane, and the angle between the set emission direction and the z axis is taken as the emission angle θ; Polar coordinate position of the nanoscattering unit Satisfies the momentum direction control equation: Where r0=(C0 / k spp +λ spp ), is the azimuth angle of the nanoscattering unit; m represents the number of turns of the elliptical spiral, m = 1, ..., M, 3 ≤ M ≤ 15, and M is the total number of turns of the elliptical spiral; The scattering phase δ of the nanoscattering unit satisfies the geometric phase equation: Where θ is the emission angle, σ represents the photon spin, which is +1 and -1 for left-handed circularly polarized LCP light and right-handed circularly polarized RCP light, respectively. The corresponding photon spin angular momentum is the positive reduced Planck constant and the negative reduced Planck constant According to the momentum direction control equation and the geometric phase equation, the position of the nanoscattering unit is anisotropic. For obliquely emitted LCP light and RCP light, the solution to the momentum direction control equation is an elliptical spiral, that is, the position of the nanoscattering unit is arranged in a multi-turn elliptical spiral, each turn of the elliptical spiral is confocal and the focus is at the origin. The spiral directions of the multi-turn elliptical spirals corresponding to the LCP light and RCP light are opposite. For the forward-emitted LCP light and RCP light, the solution to the momentum direction control equation is an Archimedean spiral. The position vector r of each nano-scattering unit satisfies the constructive interference condition in the set emission direction, so that the scattered light after being scattered by all the nano-scattering units coherently constructively forms an emission spot along the set emission direction, so that the direction of the scattered light is along the set emission direction. The set emission direction corresponds to the momentum direction of the photon, so that the momentum direction is independently controlled by the arrangement position of each nano-scattering unit, and the momentum direction is related to the phase of the scattered light from the nano-scattering unit; 4) In each nano-scattering unit, the orientation and radial distance of the first and second nano-grooves satisfy the following spin angular momentum control equations: Where k0 is the vacuum light wave vector, l and w are the length and width of the rectangular nanogroove, respectively, α is the angle between the long side and the radius of the first nanogroove as the orientation of the first nanogroove, β is the angle between the long side and the radius of the second nanogroove as the orientation of the second nanogroove, and s r is the radial distance; k spp is the wave vector of the surface plasmon; the nanoscattering unit is anisotropic in structure and is an anisotropic metasurface; in the third equation of the spin angular momentum control equation group, the right side of the equation is "+" to represent left-handed circular polarization, that is, the spin angular momentum of the photon in the emission direction is set to the positive reduced Planck constant "-" represents right-handed circular polarization, that is, the spin angular momentum of the photon in the emission direction is set to the negative reduced Planck constant Each nanoscattering unit satisfies a set of spin angular momentum control equations. The projections of linearly polarized light scattered from the first and second nanogrooves within a nanoscattering unit into the far field on a plane perpendicular to a set emission direction are orthogonal to each other and have equal amplitudes. At the same time, the scattered light from the first and second nanogrooves has a phase difference of ±π / 2 in the set emission direction, so that the scattered light from the nanoscattering unit has the same circular polarization in the set emission direction, that is, the same target polarization state. The polarization state corresponds to the spin angular momentum of the photon. Therefore, the spin angular momentum of the scattered light is independently controlled by the internal structure of the nanoscattering unit, and the spin angular momentum is independent of the phase of the scattered light from the nanoscattering unit. 5) The scattered light eventually propagates to the far field along the set emission direction with circularly polarized spin angular momentum, realizing a single-photon emitter that can simultaneously and independently control the momentum direction and spin angular momentum.

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