CN114879288B

CN114879288B - Collimating emitter capable of simultaneously emitting multiple single-photon circularly polarized light and emitting method thereof

Info

Publication number: CN114879288B
Application number: CN202210451621.6A
Authority: CN
Inventors: 李智; 薛泽洋; 贾尚曈; 陈建军
Original assignee: Peking University
Current assignee: Peking University
Priority date: 2022-04-27
Filing date: 2022-04-27
Publication date: 2023-03-24
Anticipated expiration: 2042-04-27
Also published as: CN114879288A

Abstract

The invention discloses a collimation emitter for simultaneously emitting a plurality of single-photon circularly polarized light and an emission method thereof. The invention adopts a plurality of chiral scattering units which are arranged on the Archimedes spiral at the center on the upper surface of a metal substrate, each chiral scattering unit can scatter SPP excited by an incident quantum dot single photon source to a far field in a circular polarization state with the same chirality, and the SPP is the same as the final expected circular polarization state of far field scattered light, the superposition process is scalar superposition when the far field interferes, the polarization state of far field scattered light spots is not changed, and the position of the quantum dot single photon source is insensitive, so that a plurality of quantum dot single photon sources can be simultaneously coupled with the same collimation emitter; the focusing light spots with the light spot size being the diffraction limit can be used for selectively exciting the multiple quantum dot single photon sources, and the multiple quantum dot single photon sources can be independently controlled; the invention can be applied to the fields of quantum optics, quantum communication, quantum computing and the like.

Description

Collimation emitter capable of simultaneously emitting multiple single photon circular polarized light and emission method thereof

Technical Field

The invention relates to the field of nanophotonics, in particular to a collimating emitter for simultaneously emitting a plurality of single-photon circularly polarized light and an emitting method thereof.

Background

Single photon sources are one of the most important fundamental resources in modern quantum information science. Among the most promising types of single photon sources are solid-state single photon sources based on atomic-like emitters, such as quantum dots, which combine the excellent optical properties of atoms with the convenience and scalability of solid-state systems. However, single photons emitted by quantum dot single photon sources are generally emitted in a spatially isotropic manner and the polarization state of the emitted light is random, which greatly limits their applications. It is important to collimate the single photons emitted by a quantum dot single photon source to a desired direction with a determined polarization state. In particular circularly polarized single photons of specific chirality, each photon carrying a spin angular momentum

(the positive sign corresponds to the left-handed circular polarization, and the negative sign corresponds to the right-handed circular polarization), and has important application in the fields of chiral quantum optics and the like.

Various methods have been proposed to regulate the radiation rate and emission direction of a single photon source by nanostructures. For example, by properly coupling the quantum dot single-photon source with the metal nano-antenna structure, the single-photon radiation rate of the quantum dot single-photon source can be greatly improved by using the Purcell enhancement effect, and the single-photon emission direction of the quantum dot single-photon source can be controlled by using the nano-antenna with a specific configuration, such as a yagi antenna, a bulls-eye structure antenna and the like, so that unidirectional emission is realized. However, the mode volume of the metal nano antenna is in the nano level, and in order to realize the effective coupling of the quantum dot single photon source and the metal nano antenna, the quantum dot single photon source must be accurately positioned in a specific hot spot area which is dozens of nanometers near the metal nano antenna.

In recent years, the use of a super-surface structure to manipulate the single photon radiation properties of a quantum dot single photon source, particularly to generate single photon radiation with a specific polarization state, has been proposed. For example, by accurately preparing the nano-diamond containing the color center to the center of the super-surface of a bullseye structure, researchers successfully realize far-field collimated emission of a circularly polarized single photon at room temperature for the first time; by accurately integrating the quantum dot single photon source and the mirror image thereof to two focuses of the super surface of a bifocal super lens structure, researchers realize the beam splitting emission of circularly polarized single photons with different chiralities to different directions for the first time. However, the positioning accuracy of the quantum dot single-photon source relative to the super-surface structure in the above work must be accurately controlled to be several tens of nanometers, that is, about one tenth of a wavelength. The reason is that in the above work, scattered lights from different scattering units in the super-surface structure have different polarization states or different electric field vector directions, when the scattered lights interfere with each other in a far field, the process of superimposing vector fields in different directions is performed, and in order to obtain a certain desired polarization state for the total scattered field, correct phase differences must be provided between different scattering units, so that the quantum dot single-photon source must also be accurately positioned within about one tenth of the wavelength range near the designed position to ensure that the single photons emitted by the quantum dot single-photon source can provide correct propagation phases when propagating to each scattering unit.

The above precise positioning requirement means that the same nano structure can only be effectively coupled with one quantum dot single photon source generally, and even if a plurality of quantum dot single photon sources exist at the same position at the same time, because the size of the light spot of the exciting light is limited by the diffraction limit, the exciting light can only simultaneously excite a plurality of quantum dot single photon sources in the coupling region of the nano scale, and the independent control of the plurality of quantum dot single photon sources cannot be realized. Therefore, a collimating emitter capable of simultaneously controlling a plurality of quantum dot single-photon sources to independently emit a plurality of single-photon circularly polarized lights has not been reported so far.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a collimation emitter for simultaneously emitting a plurality of single-photon circularly polarized light and an emission method thereof, which can realize independent control on a plurality of quantum dot single-photon sources.

It is an object of the invention to propose a collimating emitter that emits a plurality of single-photon circularly polarized light simultaneously.

The collimating emitter of the invention which simultaneously emits a plurality of single photon circularly polarized lights comprises: the device comprises a metal substrate, a chiral scattering unit and a medium spacing layer; wherein, a plurality of chiral scattering units are carved on the upper surface of the metal substrate, the upper surface of the metal substrate is an xy plane, the centers of the chiral scattering units are arranged on an Archimedes spiral, and the Archimedes spiral satisfies r = r ₀ ±q(θ-θ ₀ ) , + represents a positive Archimedes spiral, -represents a negative Archimedes spiral, r is a distance from a point on the Archimedes spiral to an origin, θ is an azimuth angle with respect to the x-axis, q is a pitch, r is a pitch ₀ Is the distance from the origin of the Archimedes spiral to the origin, θ ₀ Is the azimuth angle of the origin of the archimedean spiral with respect to the x-axis; each chiral scattering unit comprises two rectangular nano grooves with the same shape engraved on the surface of the metal substrate, the long sides of the two rectangular nano grooves are mutually vertical and form an included angle of 45 degrees with the radial direction, the adjacent vertexes of the long sides of the inner sides of the two rectangular nano grooves are positioned on the same radius taking an origin as the center, and the distance between the two vertexes in the radial direction is 1/5-1/4 of the wavelength of Surface Plasmon Polariton (SPP), so that the scattered light of the two rectangular nano grooves has a phase difference of pi/2; along the cornerThe distance between centers of adjacent chiral scattering units is less than 1 mu m, and the interval between the chiral scattering units is more than or equal to 50nm; pitch q is equal to the SPP wavelength; arranging a medium spacing layer on a metal substrate engraved with a chiral scattering unit, wherein the medium spacing layer is used for eliminating the fluorescence quenching effect of a quantum dot single photon source on the surface of the metal; multiple quantum dot single photon sources are arranged near the origin on the medium spacing layer, and the eccentricity between each quantum dot single photon source and the origin is not more than 0.2r ₀ And the distance between adjacent quantum dot single photon sources among the multiple quantum dot single photon sources is greater than the diffraction limit;

when the quantum dot single photon source is positioned at the original point, the excitation light spot irradiates the quantum dot single photon source, so that the quantum dot single photon source excites the SPP on the surface of the metal, and the excited SPP uniformly propagates along the surface of the metal substrate in all directions along the radial direction; the SPP is scattered to form scattered light when encountering the chiral scattering unit, and the scattered light finally propagates to a far field; the chiral scattering unit enables SPP incident to each chiral scattering unit to be scattered to the far-field z direction in the same polarization state, namely the direction perpendicular to the surface of the metal substrate, and because two rectangular nano-grooves of the chiral scattering unit are perpendicular to each other and respectively form an included angle of 45 degrees with the radial direction, and the distance between two adjacent vertexes on the long edge of the inner side of the two rectangular nano-grooves along the radial direction is 1/5-1/4 SPP wavelength, the scattered light of the two rectangular nano-grooves has a phase difference of pi/2, so that the polarization state of the scattered light transmitted to the far-field z direction is a circular polarization state; when all scattered light from different chiral scattering units propagating to a far field interferes in the far field, a common circular polarization state can be extracted, and the superposition process is simplified into scalar superposition of complex amplitudes of the scattered light, so that the polarization state of the total scattered light of the far field after superposition is the same circular polarization state as the common polarization state of the scattered light of each chiral scattering unit; and the spatial arrangement of the different chiral scattering units ensures that the circularly polarized light scattered by each chiral scattering unit to the far-field z direction is in phase, so that the circularly polarized light scattered by the different chiral scattering units is coherent and has long phase in the far-field z direction and is k in the direction _x k _y The origin of the plane forms a collimated emitted bright scattered lightThe spot is a far-field scattering light spot; k is a radical of _x And k _y K respectively representing the x-component and y-component of the wave vector and consisting of the x-component and y-component of the wave vector as horizontal and vertical coordinates _x k _y A plane represents the emission direction of the far-field scattered light spot, and the projections of the vacuum wave vector along the emission direction in the x direction and the y direction are respectively k _x And k _y ；

When the ith quantum dot single-photon source has an eccentricity delta relative to the origin, defining the eccentricity direction of the ith quantum dot single-photon source as x _i Axial direction, vertical direction y _i In the axial direction, i is more than or equal to 1 and less than or equal to N, N is the number of a plurality of quantum dot single photon sources, N is a natural number more than or equal to 2, the excitation light spot irradiates to the quantum dot single photon source, the quantum dot single photon source excites the SPP on the surface of the metal, the excited SPP uniformly propagates along the surface of the metal substrate by taking the quantum dot single photon source as the center to all directions, the SPP is scattered to form scattered light when meeting the chiral scattering unit, and the scattered light finally propagates to a far field; the polarization change of far-field scattered light of each chiral scattering unit is small and negligible, the chiral scattering units can enable SPPs (dispersed phase P) incident to the chiral scattering units to be scattered to the z direction of a far field in a circular polarization state with the same chirality, when scattered light from different chiral scattering units is subjected to far-field interference, the polarization state of total scattered light of the far field after superposition is still in the circular polarization state which is the same as the common polarization state of the scattered light of each chiral scattering unit, the polarization state of far-field scattered light spots is still in the circular polarization state, and the far-field scattered light spots are insensitive to the position of a quantum dot single photon source, namely high robustness is shown; on the other hand, the source position deviation of the quantum dot single photon source is influenced by the fact that the complex amplitude of scattered light of each chiral scattering unit changes, and the complex amplitude change is represented by an additional phase shift delta psi caused by the change of the SPP propagation distance of the ith quantum dot single photon source to the chiral scattering unit _i Namely: for in x _i y _i In-plane with respect to x _i The axial direction having an azimuth angle theta _i When the eccentricity delta is not more than 0.2r ₀ The additional phase shift Δ ψ _i ＝-Re(k _SPP )δcosθ _i Wherein k is _SPP Represents the SPP wave vector; extra phase shift delta psi _i Is proportional to eachX of a chiral scattering unit _i Coordinate because of x _i ＝rcosθ _i (ii) a Thus, the far-field interference pattern still appears as a bright scattering spot, i.e. a far-field scattering spot, except that the emission direction of the far-field scattering spot will be along k _xi The direction is deflected by a corresponding deflection angle

Deflection angle->

Satisfies the relation>

Wherein k is ₀ Represents the vacuum wave vector, at which the additional phase shift Δ ψ results from the change in the SPP propagation distance _i Is changed by the emission direction, i.e. the deflection angle>

Introduced phase shift compensation, whether or not the signal is present>

Or the amount of horizontal wave vector shift Δ k equivalent to giving a far-field scattered spot _xi ＝-Re(k _SPP ) Delta/r, namely the eccentricity delta of the quantum dot single photon source relative to the original point only influences the emission direction of the far-field scattering light spot and does not influence the polarization state of the far-field scattering light spot; k is a radical of _xi And k _yi X respectively representing wave vectors of ith quantum dot single-photon source _i Component sum y _i Component, x in wave vector _i Component sum y _i Component is k formed by horizontal and vertical coordinates _xi k _yi The plane represents the emission direction of the far-field scattered light spot, and the vacuum wave vector along the emission direction is in x _i Direction and y _i The projection of the direction is k _xi And k _yi (ii) a All x _i Axis and y _i The axes are all positioned in the xy plane of the upper surface of the metal substrate;

the eccentricity is not more than 0.2r ₀ The multiple quantum dot single photon sources can be coupled with the same collimation emitter at the same time, the mutual distance of the multiple quantum dot single photon sources is kept above a diffraction limit, an excitation light spot is adopted, the excitation light spot is a focusing light spot, the size of the light spot is the diffraction limit, one or more quantum dot single photon sources are selected, the excitation light spot irradiates to one quantum dot single photon source or among the multiple quantum dot single photon sources, and therefore the selected one or more quantum dot single photon sources are excited to form corresponding one or more far-field scattering light spots, and independent control over the multiple quantum dot single photon sources is achieved.

The above-described far-field scattered spot deflection phenomenon of the present invention is very similar to the process of imaging a point source as an object by a conventional lens: when the point source is displaced from the optical axis of the lens, the image point will be displaced in a proportional manner in the opposite direction from the optical axis to ensure that the aplanatism is still satisfied from the object to the image point. Similarly, the equivalent quantum point single-photon source has a distance r from the origin to the origin that is significantly less than the center of the Archimedes 'spiral, i.e., the origin position, relative to the center of the Archimedes' spiral in which the chiral scattering elements are arranged ₀ With a smaller eccentricity δ, the far-field scattered light is still a collimated scattered spot with circular polarization, but with a small deflection of the emission direction. For example, if it is considered that the SPP wave vector and the vacuum wave vector are approximately equal, when the eccentricity δ =0.2r ₀ When the far field collimation scattering light spot is corresponding to the deflection angle

Below this emission angle, the polarization of the far-field scattered light of the subwavelength chiral scattering unit can be designed to be approximately constant, i.e. to remain circularly polarized, so that the eccentricity δ =0.2r can be approximated ₀ The method is used as the upper limit of the eccentric amount which can be tolerated by the quantum dot single-photon source, namely the critical eccentric amount. Correspondingly, centered at the origin and at 0.2r ₀ The circular region of radius acts as a robust coupling region where a quantum dot single photon source can be effectively coupled to a collimated emitter. It is clear that it is possible to use,here the tolerance for the eccentricity of a quantum dot single photon source is the distance r from the origin of the Archimedes spiral to the origin ₀ Is proportional. Selecting a distance r from the origin to the origin of a larger Archimedes spiral ₀ The tolerance degree of the eccentricity of the quantum dot single-photon source is larger, and the distance r from the starting point to the origin of the Archimedes spiral ₀ Satisfies the following conditions: r is not less than 5 mu m ₀ Less than or equal to 30 mu m. For example, when the distance r from the origin of the Archimedes spiral to the origin ₀ At 10 μm, the upper limit of the amount of eccentricity of the single-photon source of acceptable quantum dots is equal to 2 μm, which results in about three times the wavelength of visible light. Azimuth angle theta of starting point of Archimedes spiral ₀ Is 0 to 360 degrees.

The long side l of the rectangular nanometer groove is 200 nm-300 nm, and the short side w is 50 nm-100 nm; the groove depth is related to the long side and the short side, and the chiral scattering unit has high scattering efficiency and high circular polarization degree through calculation of a finite element calculation method. The distance between two adjacent vertexes on the long edge of the inner side of the two rectangular nano grooves along the radial direction is 1/5-1/4 SPP wavelength, and the scattered light of the two rectangular nano grooves has a phase difference of pi/2 through calculation and optimization of a finite element calculation method. The number of turns of the Archimedes spiral is 3-7.

The thickness of the metal substrate is more than or equal to 300nm.

The medium spacing layer is made of medium material transparent to visible light, and the thickness is 5 nm-20 nm.

Another object of the present invention is to provide a method for emitting a plurality of single-photon circularly polarized light emitters simultaneously.

The invention discloses an emission method of a collimation emitter capable of simultaneously emitting a plurality of single-photon circularly polarized light, which comprises the following steps:

1) Preparation of a collimation emitter:

a) Providing a metal substrate, wherein the upper surface of the metal substrate is an xy plane;

b) Carving a plurality of chiral scattering units on the upper surface of the metal substrate, wherein the centers of the plurality of chiral scattering units are arranged on an Archimedes spiral, and the Archimedes spiral satisfies r = r ₀ ±q(θ-θ ₀ ) And + represents ortho-ArchimedesDe-spire, -representing a negative Archimedes spiral, r being the distance of a point on the Archimedes spiral to the origin, θ being the azimuth angle with respect to the x-axis, q being the pitch, r ₀ Is the distance from the origin of the Archimedes spiral to the origin, θ ₀ Is the azimuth angle of the origin of the archimedean spiral with respect to the x-axis;

c) Each chiral scattering unit comprises two rectangular nano grooves which are carved on the surface of the metal substrate and have the same shape, the long sides of the two rectangular nano grooves are mutually vertical and form an included angle of 45 degrees with the radial direction, the adjacent vertexes of the long sides of the inner sides of the two rectangular nano grooves are positioned on the same radius with the origin as the center, and the distance between the two vertexes in the radial direction is 1/5-1/4 of the wavelength of the surface plasmon polariton SPP, so that the scattered light of the two rectangular nano grooves has a phase difference of pi/2; the distance between the centers of the angularly adjacent chiral scattering units is less than 1 mu m, and the interval between the chiral scattering units is more than or equal to 50nm; the pitch q is equal to the SPP wavelength;

d) Arranging a medium spacing layer on a metal substrate engraved with a chiral scattering unit, wherein the medium spacing layer is used for eliminating the fluorescence quenching effect of a quantum dot single photon source on the surface of the metal;

e) Multiple quantum dot single photon sources are arranged near the origin on the medium spacing layer, and the eccentricity between each quantum dot single photon source and the origin is not more than 0.2r ₀ And the distance between adjacent quantum dot single photon sources among the multiple quantum dot single photon sources is greater than the diffraction limit;

2) When the quantum dot single-photon source is located at the origin:

the excitation light spot irradiates to a quantum dot single photon source, so that the quantum dot single photon source excites the SPP on the metal surface, and the excited SPP uniformly propagates along the surface of the metal substrate along the radial direction to all directions; the SPP is scattered to form scattered light when encountering the chiral scattering unit, and the scattered light finally propagates to a far field; the chiral scattering units enable SPP incident to each chiral scattering unit to be scattered to the far-field z direction, namely the direction perpendicular to the surface of the metal substrate, in the same polarization state, and two rectangular nano grooves of the chiral scattering units are perpendicular to each other and respectively form an included angle of 45 degrees with the radial direction, and the two rectangular nano grooves areThe distance between two adjacent vertexes on the long edge of the inner side of the nano groove along the radial direction is 1/5-1/4 SPP wavelength, so that the scattered light of the two rectangular nano grooves has a phase difference of pi/2, and the polarization state of the scattered light which is transmitted to the far field in the z direction is a circular polarization state; when all scattered light from different chiral scattering units propagating to a far field interferes in the far field, a common circular polarization state can be extracted, and the superposition process is simplified into scalar superposition of complex amplitudes of the scattered light, so that the polarization state of the total scattered light of the far field after superposition is the same circular polarization state as the common polarization state of the scattered light of each chiral scattering unit; and the spatial arrangement of the different chiral scattering units ensures that the circularly polarized light scattered to the far field z direction by each chiral scattering unit is in phase, so that the circularly polarized light scattered by the different chiral scattering units is coherent and has a longer phase in the far field z direction, and the direction is k _x k _y The origin of the plane forms a bright scattering light spot which is emitted in a collimation mode, namely the far-field scattering light spot; k is a radical of _x And k _y K respectively representing the x-component and y-component of the wave vector and consisting of the x-component and y-component of the wave vector as horizontal and vertical coordinates _x k _y A plane represents the emission direction of the far-field scattering light spot, and the projections of the vacuum wave vectors along the emission direction in the x direction and the y direction are respectively k _x And k _y ；；

3) When the ith quantum dot single photon source has an eccentricity δ relative to the origin:

defining the eccentric direction of the ith quantum dot single-photon source as x _i Axial direction, vertical direction y _i In the axial direction, i is more than or equal to 1 and less than or equal to N, N is the number of a plurality of quantum dot single photon sources, N is a natural number more than or equal to 2, the excitation light spot irradiates to the quantum dot single photon source, the quantum dot single photon source excites the SPP on the surface of the metal, the excited SPP uniformly propagates along the surface of the metal substrate by taking the quantum dot single photon source as the center to all directions, the SPP is scattered to form scattered light when meeting the chiral scattering unit, and the scattered light finally propagates to a far field; the polarization change of far-field scattered light of each chiral scattering unit is negligible, and the chiral scattering unit can enable SPP incident to the chiral scattering unit to be scattered to the far-field z direction in a circular polarization state with the same chiralityWhen scattered light from different chiral scattering units is subjected to far-field interference, the polarization state of total scattered light of a far field after superposition is still a circular polarization state which is the same as the common polarization state of the scattered light of each chiral scattering unit, and the polarization state of a far-field scattering light spot is still a circular polarization state and is insensitive to the position of a quantum dot single photon source, namely high robustness is shown; on the other hand, the influence of the position deviation of the single-photon source of the quantum dot is reflected in that the complex amplitude of scattered light of each chiral scattering unit changes, and the complex amplitude change is represented by an additional phase shift delta psi caused by the change of the SPP propagation distance of the ith single-photon source of the quantum dot to the chiral scattering unit _i Namely: for in x _i y _i In-plane relative to x _i Axial direction having an azimuth angle θ _i When the eccentricity delta is not more than 0.2r ₀ The additional phase shift Δ ψ _i ＝-Re(k _SPP )δcosθ _i Wherein k is _SPP Represents the SPP wave vector; extra phase shift delta psi _i Proportional to x of each chiral scattering unit _i Coordinate because of x _i ＝rcosθ _i (ii) a Thus, the far-field interference pattern still appears as a bright scattering spot, i.e. a far-field scattering spot, except that the emission direction of the far-field scattering spot will be along k _xi The direction is deflected by a corresponding deflection angle

Deflection angle->

Satisfies the relation>

Introduced phase shift compensation>

Or the amount of horizontal wave vector shift Δ k equivalent to giving a far-field scattered spot _xi ＝-Re(k _SPP ) Delta/r, namely the eccentricity delta of the quantum dot single photon source relative to the original point only influences the emission direction of the far-field scattering light spot and does not influence the polarization state of the far-field scattering light spot; k is a radical of _xi And k _yi X respectively representing wave vectors of ith quantum dot single-photon source _i Component sum y _i Component, x in wave vector _i Component sum y _i Component k formed by horizontal and vertical coordinates _xi k _yi The plane represents the emission direction of the far-field scattered light spot, and the vacuum wave vector along the emission direction is in x _i Direction and y _i The projection of the direction is k _xi And k _yi ；

4) The eccentricity is not more than 0.2r ₀ The multiple quantum dot single photon sources can be coupled with the same collimation emitter at the same time, the mutual distance of the multiple quantum dot single photon sources is kept above a diffraction limit, an excitation light spot is adopted, the excitation light spot is a focusing light spot, the size of the light spot is the diffraction limit, one or more quantum dot single photon sources are selected, the excitation light spot irradiates to one quantum dot single photon source or among the multiple quantum dot single photon sources, and therefore the selected one or more quantum dot single photon sources are excited to form corresponding one or more far-field scattering light spots, and independent control over the multiple quantum dot single photon sources is achieved.

Further, the handedness of circular polarization determines the handedness of the Archimedes spiral, the left-hand circular polarization corresponds to the positive Archimedes spiral, and the right-hand circular polarization corresponds to the negative Archimedes spiral; two adjacent vertexes of the two rectangular nano-grooves of each chiral scattering unit are located on the same radius with the origin as the center, of the two vertexes, the vertex belonging to the rectangular nano-groove located in the counterclockwise direction of the radius is a first vertex, the vertex belonging to the rectangular nano-groove located in the clockwise direction of the radius is a second vertex, and the handedness of the far-field scattered circularly polarized light can be simply switched by exchanging the front and back positions in the radial direction of the two rectangular nano-grooves, namely, the first vertex is in front, and the Archimedes spiral is positive corresponding to right-handed circularly polarized (LCP) light, and the second vertex is in front, and the Archimedes spiral is negative corresponding to right-handed circularly polarized (RCP) light.

The invention has the advantages that:

the invention adopts the technical scheme that a plurality of chiral scattering units are engraved on the upper surface of a metal substrate, and the centers of the plurality of chiral scattering units are arranged on an Archimedes spiral; each chiral scattering unit can scatter SPP excited by a quantum dot single photon source incident to the chiral scattering unit to a far field in a circular polarization state with the same chirality, and the common circular polarization state is the same as the final expected circular polarization state of the far field scattered light; when all scattered light with the same circular polarization state interferes in a far field, the superposition process is simplified into a scalar superposition process instead of complex vector superposition; at the moment, the quantum dot single-photon source has a not too large offset relative to the original point, the scalar superposition ensures that the polarization state of the far-field scattering light spot is hardly changed, and the extra phase difference between different chiral scattering units caused by the position offset of the quantum dot single-photon source only causes the deflection of the emission direction of the far-field scattering light spot, namely the polarization state of the far-field scattering light is insensitive to the position of the quantum dot single-photon source, and the result shows that the positioning accuracy of the quantum dot single-photon source relative to the collimation emitter can be widened to 2 mu m, which is dozens of times of the typical positioning accuracy in other researches, namely high robustness is shown; the high robustness and the dependency relationship between the scattering light spot direction of far-field collimation emission and the position of the quantum dot single-photon source enable the circularly polarized far-field collimation emission of a plurality of quantum dot single-photon sources to be realized on the same collimation emitter, the plurality of quantum dot single-photon sources can be simultaneously coupled with the same collimation emitter, the mutual distance of the plurality of quantum dot single-photon sources is kept above the diffraction limit, the plurality of quantum dot single-photon sources can be selectively excited by using the focusing light spot with the light spot size being the diffraction limit, and the independent control of the plurality of quantum dot single-photon sources is realized; the invention provides a new idea for single photon radiation control of a quantum radiator, and can be applied to the fields of quantum optics, quantum communication, quantum computing and the like.

Drawings

Fig. 1 is a schematic diagram of an embodiment of the collimated emitter for simultaneously emitting multiple single-photon circularly polarized light according to the present invention, wherein (a) is a general schematic diagram of the distribution of the chiral scattering units on the upper surface of the metal substrate, and (b) is a partially enlarged schematic diagram of the distribution of the chiral scattering units on the upper surface of the metal substrate;

FIG. 2 is a graph of the results of numerical simulations of an embodiment of a collimated emitter for simultaneous emission of multiple single-photon circularly polarized light according to the present invention, wherein (a) is the chirality C of the scattered light in the z-direction of the far field with a single chiral scattering unit located on the x-axis and y-axis _z The variation curve chart of the eccentricity delta along with the quantum dot single photon source has the eccentricity direction in the x direction and the upper and lower curves corresponding to the results of the chiral scattering unit in the x axis and the y axis, and (b) is the integral chirality C of the far field scattering light spot of the single-photon circularly polarized light collimating emitter when the quantum dot single photon source deviates from the original point _int A graph showing the variation of the eccentricity δ, (c) a far-field distribution graph of a far-field scattering spot of the single-photon circularly polarized light collimation emitter when the quantum dot single-photon source is right at the origin, namely the eccentricity δ =0, and the excitation polarization is along the z direction, wherein the height in the vertical direction in the graph represents the intensity of a scattering field, and the brightness depth represents the chirality of the scattering field, and (d) a far-field distribution graph of a far-field scattering spot of the single-photon circularly polarized light collimation emitter when the quantum dot single-photon source has critical eccentricity, namely the eccentricity δ =2 μm, and the excitation polarization is along the z direction;

fig. 3 is a graph of experimental results obtained from two samples of a collimated emitter for simultaneously emitting a plurality of single-photon circularly polarized light according to the present invention, wherein (a) is a graph of a Charge Coupled Device (CCD) of an approximately non-eccentric sample on a sample surface, and left and right graphs respectively show the results of large-area excitation and selective excitation of single quantum dots, (b) are graphs of a CCD of an approximately non-eccentric sample in a far field, and left and right graphs respectively show intensity profiles of LCP and RCP components, (c) are graphs of experimental results of an approximately non-eccentric sample displayed in a manner similar to the simulation results, i.e., a graph of height in a vertical direction shows light intensity and lightness in a dark direction shows chirality, (d) is a graph of a CCD of a sample in a critical state approximately at an eccentric amount on a sample surface, and left and right graphs respectively show the results of large-area excitation and selective excitation of single quantum dots, (e) is a graph of a CCD of an approximately critical eccentric sample in a far field, and left and right graphs respectively show intensity profiles of RCP components, and (f) is a graph of the results of an approximately eccentric sample displayed in a manner similar to the experimental results of an approximately eccentric sample;

FIG. 4 is a statistical chart of experimental results obtained for multiple samples of a collimated emitter for simultaneous emission of multiple single-photon circularly polarized light according to the present invention, wherein (a) the integrated chirality C of the far-field scattered light spot obtained for multiple different samples _int A graph of variation with the eccentricity delta of a quantum dot single photon source, and (b) is a second-order correlation function g of emitted light ⁽²⁾ (t), the upper graph and the lower graph are respectively a result graph of measuring scattering light spots in a far field by the same quantum dot and measuring direct emitted light on the surface of a sample;

fig. 5 is a graph showing results obtained by an embodiment of the collimating emitter for simultaneously emitting multiple single-photon circularly polarized light according to the present invention, wherein (a) is a schematic diagram showing that multiple quantum dot single-photon sources located in a robust coupling region and spaced apart from each other by a distance greater than a diffraction limit are simultaneously coupled with the collimating emitter, (b) is a CCD graph showing a sample surface of a sample containing multiple quantum dot single-photon sources under large-area excitation, (c) is a graph showing experimental results when upper and lower two quantum dot single-photon sources are respectively and independently excited, and (d) is a graph showing experimental results when upper and lower two quantum dot single-photon sources are simultaneously excited.

Detailed Description

The invention will be further elucidated by means of specific embodiments in the following with reference to the drawing.

As shown in fig. 1, the collimating emitter of this embodiment that emits a plurality of single-photon circularly polarized light simultaneously includes: the device comprises a metal substrate, a chiral scattering unit, a medium spacing layer and a plurality of quantum dot single photon sources;wherein, a plurality of chiral scattering units are engraved on the upper surface of the metal substrate, the upper surface is an xy plane, the centers of the plurality of chiral scattering units are arranged on an Archimedes spiral, and the Archimedes spiral satisfies r = r ₀ ±q(θ-θ ₀ ) , + represents positive Archimedes 'spiral, -represents negative Archimedes' spiral, r is the distance from the point on the Archimedes 'spiral to the origin, theta is the azimuth angle, q is the pitch, r is the distance between the point on the Archimedes' spiral and the origin, q is the pitch ₀ Is the distance from the origin of the Archimedes spiral to the origin, θ ₀ An azimuth angle that is the origin of the archimedean spiral; each chiral scattering unit comprises two rectangular nano grooves with the same shape engraved on the surface of the metal substrate, the long sides of the two rectangular nano grooves are mutually vertical and form an included angle of 45 degrees with the radial direction, the adjacent vertexes of the long sides of the inner sides of the two rectangular nano grooves are positioned on the same radius with the origin as the center, and the distance between the two vertexes in the radial direction is 1/5-1/4 of the wavelength of Surface Plasmon Polariton (SPP), so that the scattered light of the two rectangular nano grooves has a phase difference of pi/2; the distance between the centers of the angularly adjacent chiral scattering units is less than 1 mu m, and the interval between the chiral scattering units is more than or equal to 50nm; pitch q is equal to the SPP wavelength; arranging a medium spacing layer on a metal substrate engraved with a chiral scattering unit, wherein the medium spacing layer is used for eliminating the fluorescence quenching effect of a quantum dot single photon source on the surface of the metal; multiple quantum dot single photon sources are arranged near the origin on the medium spacing layer, and the eccentricity between each quantum dot single photon source and the origin is not more than 0.2r ₀ 。

Numerical simulation

Numerical simulations were performed with finite element software COMSOL Multiphysics. In the simulation, a quantum dot single photon source was represented by an electric dipole located 10nm above the metal. The wavelength of the excitation light spot is 623 nanometers, which is consistent with the luminescence center wavelength of the quantum dot single photon source used in the later experiment. The effective refractive index of the metal substrate is set to 0.0363+3.13i. The total field with the chiral scattering unit is subtracted from the total field without the chiral scattering unit to obtain a pure scattering field component, and the process can eliminate the optical field component directly radiated to the free space by the quantum dot single photon source. And finally, calculating the far field distribution of the scattering field by utilizing a Fresnel-kirchhoff diffraction integral formula.

According to the previous analysis, to obtain far-field collimated emission of single-photon circularly polarized light, each chiral scattering unit should scatter the incident SPP as far-field circularly polarized light, and the scattered light has the same target chirality. This requirement is satisfied by composing the chiral scattering unit from a combination of two rectangular nano-grooves etched into a metal substrate, as shown in fig. 1 (b), where the dotted line in fig. 1 (b) indicates the radial direction. Each rectangular nano-groove has a length of 300nm, a width of 100nm and a depth of 90nm, can scatter the incident SPP to a far field, and the scattered light is approximately linearly polarized light with an electric field direction perpendicular to the long axis direction of the rectangular nano-groove. The major axes of the two rectangular nano grooves forming a chiral scattering unit form an included angle of 45 degrees with the radial direction and are perpendicular to each other, so that the scattered light of SPP incident along the radial direction after being scattered by the two nano grooves has approximately the same intensity and mutually perpendicular linear polarization. And then setting two adjacent vertexes of the long edges of the inner sides of the two rectangular nanometer grooves to be positioned on the same radius with the origin as the center, wherein the vertex belonging to the rectangular nanometer groove positioned in the anticlockwise direction of the radius is a first vertex A, the vertex belonging to the rectangular nanometer groove positioned in the clockwise direction of the radius is a second vertex B, the radial distance between the two vertexes A and B is 130nm and is approximately equal to 1/4 SPP wavelength, and the distance is the result of optimization through numerical simulation, so that scattered light of the two rectangular nanometer grooves has a phase difference of pi/2. The scattered light of the two rectangular nano-grooves will be superimposed into circularly polarized light when interfering in the far field z direction. Here, the two rectangular nano-groove structures constitute a chiral scattering unit, which scatters the incident SPP into far-field circularly polarized light. Due to the handedness of the far-field scattered circularly polarized light, switching can be simply performed by exchanging the radial front and back positions of the two rectangular nano-grooves, that is, the first vertex a is preceded by left-handed circularly polarized (LCP) light, and the second vertex B is preceded by right-handed circularly polarized (RCP) light.

The scattered field of a single chiral scattering unit is first simulated. Assuming that the electric dipole is located at the origin of the xy plane, i.e. the eccentricity δ =0, the electric dipole is 10 μm from the chiral scattering unit. The degree of circular polarization of far-field scattered light is characterized by the degree of chirality C, which defines C = (I) _L –I _R )/(I _L +I _R ) In which I _L And I _R Representing the intensity of the LCP light and the RCP light, respectively, in a particular direction. Thus, C =1,0, -1 respectively represent ideal LCP light, linearly polarized light, and ideal RCP light. The calculation result shows that the chirality of far-field scattered light in the z direction is C _z =0.99, indicating that the far-field scattered light is LCP light of very high purity in the z-direction. Further calculating the change of the chirality of the scattered light when the quantum dot single-photon source deviates from the origin, defining the eccentricity direction as the x direction and the eccentricity as delta, and respectively displaying the chirality C of the scattered light in the z direction of the far field when the chiral scattering unit is located on the x axis and the y axis by the calculation in the upper line and the lower line in fig. 2 (a) _z As a function of δ. For a chiral scattering cell on the x-axis, the chirality C of the scattered light in the z-direction of the far field _z Hardly changes with delta because for a chiral scattering unit on the x-axis the SPP remains incident along the x-axis at all times, corresponding to the angle of incidence of the SPP with respect to the radial direction remaining unchanged. And for a chiral scattering unit on the y-axis, the chirality C of the scattered light in the z-direction of the far field _z Decreases with increasing delta due to the angle of incidence alpha of the SPP with respect to the radial direction for chiral scattering units on the y-axis _y Increases with increasing delta, resulting in a slight decrease in the handedness of the scattered light. However, when the eccentricity delta is less than or equal to 2 μm, the handedness C of the scattered light _z Always kept above 0.66. Whereas for the general case, i.e. the case where the chiral scattering element is neither on the x-axis nor on the y-axis, the variation of the angle of incidence of the SPP with respect to δ is between 0 and α _y Between the two special cases of the chiral scattering unit in the x-axis and in the y-axis, the corresponding handedness C of the scattered light _z The variation with δ should be between the upper and lower lines in fig. 2 (a). Therefore, as long as the eccentricity δ ≦ 2 μm, the scattered light of each chiral scattering element is LCP light with higher purity.

To be no longerThe LCP light scattered by homochiral scattering elements into the far field is collimated into the z direction, the different chiral scattering elements are arranged in a forward archimedean spiral, and the pitch q is set to the SPP wavelength, as shown in figure 1. The reason for this arrangement is that the principal axis coordinate system of each chiral scattering unit is rotated with the azimuth angle theta of the chiral scattering unit, and this rotation will contribute an exp (-i theta) geometric phase to the LCP light, and the forward archimedean spiral will introduce an exp (i theta) SPP propagation phase to the chiral scattering units with different azimuth angles to compensate the geometric phase, so as to ensure that all the LCP light scattered to far field by the chiral scattering units has the same phase in the z direction, and coherent constructive interference can be performed in the z direction of the far field, and a bright scattering LCP spot is formed in this direction. Distance r from the origin of the Archimedes spiral to the origin ₀ Set to 10 μm, the distance r from the starting point to the origin of the Archimedes spiral can be ensured ₀ The large SPP propagation length can be tolerated, and the larger eccentricity of the quantum dot single photon source can be tolerated, and is still obviously smaller than 36 mu m, so that the smaller SPP propagation loss is ensured. The Archimedes spiral has 5 circles, and 100 chiral scattering units are uniformly distributed in each circle. The results of performing the simulation calculation are shown in fig. 2 (b) to (d).

FIG. 2 (c) shows a typical simulation result with the corresponding quantum dot single photon source located right at the origin and the excitation polarization along the z-direction. The height in the vertical direction in the figure represents the intensity of the scattered field, while the lightness indicates the handedness of the scattered field. It can be seen that the far field scattering pattern appears around k _x k _y A sharp, single peak configuration at the plane origin, with a peak having a full width at half maximum of only 2.3 deg., indicates that the scattered light is well collimated into the far-field z-direction. At the same time, this peak is almost completely black, indicating high purity LCP light. To further quantify the chirality of the far-field scattered spot, an integral chirality C is used _int Is defined as C _int ＝(I _Lint –I _Rint )/(I _Lint +I _Rint ) In which I _Lint And I _Rint The integrated light intensity of LCP and RCP components of the far-field scattering light spot respectively takes the maximum light intensity position of the far-field scattering light spot as the center and the full width at half maximum of the peak as the centerRadius, and thus contains a significant fraction of the far field scattered spot intensity. The calculation result shows that when the equivalent quantum point single photon source is not eccentric, namely delta =0, the integral chirality of the far-field scattering light spot is as high as C _int =0.99, high purity LCP light. The numerical simulation result also shows that the total radiation power of the electric dipole with the z polarization reaches 4.15P ₀ In which P is ₀ Representing the total radiated power of the electric dipole in vacuum, indicating a Purcell enhancement factor equal to 4.15. This enhancement factor is almost independent of the presence of peripheral chiral scattering units, and depends mainly on the coupling effect between the electric dipole and the metal substrate. Specifically, the SPP power excited by electric dipole, the SPP power propagated to the chiral scattering unit and the power scattered to far field by the chiral scattering unit reach 2.96P ₀ 、2.24P ₀ And 1.85P ₀ . While the scattered light scattered to the far field by the chiral scattering unit is collected with a power of 0.99P by an objective lens with a Numerical Aperture (NA) of 0.8, that is, an objective lens used in the following experiment ₀ 。

Next, the situation of eccentricity of the quantum dot single photon source is simulated. According to the previous analysis, when the eccentricity delta is much smaller than the distance r from the origin to the origin of the Archimedes spiral ₀ When the far field scattering light spot is off center from the far field k _x k _y The coordinate origin of the plane, and the offset is approximately proportional to the eccentricity delta of the quantum dot single photon source. Numerical simulations confirm this and find the center of the far-field scattered spot in the far-field k _x k _y The offset ratio of the plane is about 0.092k ₀ Mu m. For calculating integral chirality C under the condition of quantum dot single photon source eccentricity _int The center of the integration area in the calculation is set to 0.092k ₀ The deviation rate of/mum moves along with the center of the far-field scattering light spot, so that the integral chirality C of the far-field scattering light spot under the condition that the quantum dot single-photon source deviates from the origin point is calculated _int The calculation results are shown in fig. 2 (b). It can be observed that under the condition that the eccentricity delta of the quantum dot single-photon source is not too large, the integral chirality of the far-field scattering light spot is only slightly reduced until the eccentricity delta reaches 2 mu m, and the far-field scattering light spot is scatteredThe integrated handedness of the spot is still as high as 0.92. This result is much higher than the worst z-direction chirality C of the far-field scattered light of a single chiral scattering unit, i.e., when the chiral scattering unit is located on the y-axis, the eccentricity delta is equal to 2 μm for the corresponding z-direction chirality C of the far-field scattered light _z =0.66, because of the z-direction chirality C of the far-field scattered light of other chiral scattering units not located on the y-axis _z Much higher. For the case that the eccentricity delta of the quantum dot single-photon source is larger than 2 μm, the integral chirality of the far-field scattering light spot is rapidly reduced, mainly because under a larger emission angle, the polarization of the scattered light of each chiral scattering unit can deviate from the polarization state of the LCP as a target, and meanwhile, the approximate linear relation between the additional SPP propagation phase shift caused by the eccentricity of the quantum dot single-photon source and the eccentricity delta is not established under the larger eccentricity delta. Based on the above results, the critical eccentricity amount of the quantum dot single-photon source that can be tolerated is set to δ =2 μm, which is more than three times the radiation wavelength (623 nm) of the quantum dot single-photon source. Under the critical eccentricity, the intensity and chirality distribution of the far-field scattered light obtained by simulation are shown in fig. 2 (d), although the collimation and the integral chirality of the far-field scattered light spot are reduced compared with the prior situation that the eccentricity of the quantum dot single-photon source is zero, the full width at half maximum of the far-field scattered light spot is only 3.9 degrees, and the integral chirality reaches C _int =0.92, i.e. the far-field scattered spot can still be considered as a very well far-field collimated LCP light, except that the emission direction of the maximum intensity is deflected from the original 0 ° to 10.6 °. Therefore, the region satisfying delta less than or equal to 2 μm near the origin is regarded as a robust coupling region where the quantum dot single-photon source can effectively act on the structure consisting of the chiral scattering units.

In the above analysis, only the quantum dot single photon source polarized in the z direction is considered, because the energy of the quantum dot single photon source coupled into the SPP mode is proportional to the coupling strength of the electric dipole and the SPP field, and the z component of the SPP field is much stronger than the horizontal component, so the coupling strength of the electric dipole and the SPP in the z direction is much greater than the coupling strength of the electric dipole and the SPP in the horizontal direction. The numerical simulation result shows that the SPP work of electric dipole excitation in one horizontal directionThe ratio was 0.16P ₀ The fact that there is only about one twentieth of the power of the SPP excited by the electric dipole in the z direction means that the signal detected in practical experiments is mainly due to the z-polarization contribution of the quantum dot single photon source, whereas the contribution from the horizontal polarization is negligible.

Experiment of

In order to experimentally verify the scheme, a silver film with the thickness of 400nm is evaporated on a glass substrate, and the thickness of the rest part of the silver film minus the depth of the rectangular nanometer groove is far larger than the skin depth of visible light in silver, so the silver film is approximately considered to be optically equivalent to infinite thickness. Then, a designed chiral scattering unit structure is etched on the silver surface by a focused ion beam etching method, and a layer of Al with the thickness of 10nm is evaporated ₂ O ₃ The quantum dot single photon source is used as a medium spacing layer to reduce the fluorescence quenching effect of the quantum dot single photon source on the silver surface. The quantum dot single photon source used in the experiment is CdSe colloid quantum dots, has better single photon emission performance, and is characterized in that CdSe colloid quantum dot solution is spin-coated on the surface of a sample, so that the CdSe colloid quantum dots are randomly distributed on the surface of the sample. Adjusting the concentration of the CdSe colloidal quantum dot solution to adjust the density of the CdSe colloidal quantum dots on the surface of a sample, and selecting proper concentration to enable the average distance between the CdSe colloidal quantum dots to be about 2 mu m, wherein the average distance is larger than the diameter of a light spot of tightly-focused excitation light by 1 mu m on one hand, so that only one quantum dot single-photon source can be excited by the tightly-focused excitation light every time; on the other hand, the method can ensure that at least one quantum dot single photon source exists within the range of 2 mu m from the center of the prepared collimation emitter structure, and is used for realizing single photon circular polarized light emitted by far-field collimation.

Two excitation modes were used in the experiment. One is to use a large laser spot of about 70 μm at 30 degree oblique incidence on the sample while exciting many quantum dot single photon sources in a relatively large sample area. The central wavelength of the light emitted by the quantum dot single photon source is 623nm, the spectral width of the fluorescence is only 10nm, and the light is approximately considered as quasi-monochromatic light. The fluorescence signal emitted by the quantum dot single photon source is collected by a high-power objective lens with the magnification of 100 times and the numerical aperture NA =0.8 and imagedTo a charge-coupled device (CCD) for detection. The left image of fig. 3 (a) is a typical CCD image of a sample under large-area excitation, and the structure of a hand scattering unit and a plurality of quantum dot single photon sources distributed randomly are obviously observed, and by using this excitation mode and the corresponding CCD image, the eccentricity delta of the quantum dot single photon source relative to the origin can be quantitatively measured. Another excitation mode is to focus 405nm pulse laser to a tightly focused spot with a diameter of about 1 μm through a 100-fold high-power objective lens, and then selectively excite only one single quantum dot single photon source. The right image in fig. 3 (a) is a typical CCD image under excitation of a single quantum dot photon source alone, where only one single quantum dot photon source near the origin obtains excitation, and the white dotted circle in the image marks the position corresponding to the critical value δ =2 μm for the tolerable eccentricity of the single quantum dot photon source. Meanwhile, other quantum dot single photon sources on the surface of the metal substrate can hardly be effectively excited, because the quantum dot single photon sources have very weak absorption on light at the fluorescence emission wavelength of the quantum dot single photon sources. The structural area formed by the chiral scattering units is relatively large (about 200 mu m) ² ) Whereas the emission of a single quantum dot single photon source is relatively weak, no significant scattered light signal is observed in the right diagram of fig. 3 (a), but the scattered light signal is observable in the far field because the scattered light is collimated to be emitted in the far field to a small divergence angle. By inserting a focusing lens into the detection light path and making the distance from the focusing lens to the CCD detector just equal to the focal length of the focusing lens, the detection position of the CCD detector can be switched from the surface of the sample to the back focal plane of the 100 times high power objective lens, namely the space spectrum plane of the sample, and the far field distribution of the scattered light is detected by the CCD. And a quarter-wave plate and a polaroid are added in front of the focusing lens in the detection light path, and the LCP component and the RCP component of the far-field scattered light can be respectively detected by changing the angle of the polaroid.

FIG. 3 (b) shows the typical far-field LCP and RCP component intensity distributions of a quantum-dot single-photon source with an eccentricity of 0.36 μm, which can be approximately considered as a typical single-photon circularly polarized light collimating emitterWithout eccentricity of the sample. It can be seen that the LCP light in the left image has a bright far-field scattering spot near the center of the far field, which is the far-field scattering light signal from the single-photon circularly polarized light collimating emitter, and the full width at half maximum of the far-field scattering spot is only 3.6 °, which shows good collimation characteristics. And the spot was almost completely extinguished in the RCP component of the right plot, indicating that the far-field scattered spot was highly pure LCP light. In fig. 3 (C), the experimental results are shown in a similar manner as in the simulation results, i.e. the intensity is represented by the height in the vertical direction and the degree of chirality C is represented by the lightness and lightness, and a very significant phenomenon of LCP light collimated emission can be seen, which is consistent with previous analytical and numerical simulation predictions. Keeping the same with the processing mode in the simulation, calculating the integral chirality of the far-field scattering facula in the experiment, integrating the signal in the radius by taking the full width at half maximum of the facula as the radius, removing the contribution of the uniform background signal in the integral area, thereby respectively obtaining the integral intensity of LCP light and RCP light, then dividing the difference of the integral intensities of the LCP light and the RCP light by the sum of the integral intensities of the LCP light and the RCP light to obtain the integral chirality in the experiment, and the result is C _int =0.95. The full width at half maximum and the integral chirality of the far-field scattering light spot measured by the experiment are very close to the result of numerical simulation, and the small deviation from the numerical simulation can be mainly attributed to the limited sample preparation precision. In general, the full width at half maximum and integral chirality (3.6 degrees and 0.95) of far-field scattered light spots measured by using the single-photon circularly polarized light collimation emitter provided by the invention are superior to the results reported in the prior literature by adopting a bull's eye structure super surface (4.7 degrees, more than or equal to 0.8) and a bifocal super lens structure (LCP is 4.83 degrees and 0.88 degrees).

Fig. 3 (d) to (f) show the experimental results corresponding to a typical single-photon circularly polarized light collimation emitter approximately in the critical state of tolerable quantum-dot single-photon-source eccentricity, which is δ =1.70 μm, and the quantum-dot single-photon-source is located close to the critical eccentricity δ =2 μm indicated by the white dotted circle in the figure. Consistent with numerical predictions, far field scattered spotsIs offset from the far field center, the quantum dot single photon source, as marked by the white dashed circle located in fig. 3 (e), has exactly the 10.6 ° direction where the critical eccentricity δ =2 μm numerically simulates the emission direction of the far field scattered spot given by the results. Although the collimation of the off-center far-field scattered spot is degraded relative to the un-off far-field scattered spot, the measured integrated chirality still reaches C _int =0.89, good LCP characteristics are shown. Similar experiments are carried out on other samples, the dependence relationship between the integral chirality of the far-field scattering light spot and the eccentricity of the quantum dot single-photon source on the origin is obtained through statistics, the result is shown as a data point in fig. 4 (a), a numerical simulation result is also given through a solid line in fig. 4 (a), and the experimental result is basically consistent with the numerical simulation result. For the sample with the quantum dot single photon source eccentricity less than 2 mu m, the measured lowest integral chirality still reaches C _int The result is that the positioning accuracy of the quantum dot single-photon source relative to the origin in the experiment can be relaxed to about 2 μm, which is more than dozens of times of the positioning accuracy requirement of dozens of nanometers in the previous research. The invention can use a single-step micro-nano preparation process which is much simpler and can realize far-field collimated emission of chiral single photons at room temperature experimentally without matching with a high-precision positioning technology due to the high robustness. It is worth pointing out that the deviation of at least one quantum dot single photon source from the origin of the structure center on each sample structure surface prepared in the experiment does not exceed the critical eccentricity delta =2 μm. In other words, the existence of at least one quantum dot single-photon source in all prepared single-photon circularly polarized light collimation emitters can realize single-photon circularly polarized light far-field collimation emission with integral chirality not lower than 0.76, which means that the sample preparation efficiency is high.

The invention also examines the single photon character of the experiment, specifically, the second order correlation function g of the emitted light is measured by adopting the experimental scheme proposed by Hanbury Brown and Twiss ⁽²⁾ (t) of (d). The lower graph in FIG. 4 (b) shows typical results of an experiment in which the radiation of a quantum dot single photon source is measured at the surface of a sample, and the results of curve fitting indicate g ⁽²⁾ (0)＝0.12, the value is far smaller than the critical value 0.5 corresponding to the single photon state, which indicates that the directly emitted light of the quantum dot single photon source is in a good single photon state. The upper graph in FIG. 4 (b) is a second order correlation function of the far field scattered light spot detected in the experiment at the back focal plane of a 100-fold high power objective lens, and the fitting result of the curve shows that g ⁽²⁾ (0) =0.39, still less than the threshold value of single photon state of 0.5, indicating that the far-field scattering spot of the quantum dot single photon source still satisfies the single photon state, but the single photon property is somewhat lower than the direct emission of the quantum dot single photon source because of the contribution of the background signal, and it is easily seen from fig. 4 (b) that the background signal of the upper graph is stronger than that of the lower graph.

The coupling area of the collimation emitter provided by the invention, which can effectively act with a quantum dot single photon source, is large (30 lambda) ² ) The multiple quantum dot single photon sources can be coupled with the same collimation emitter at the same time, and the mutual distance is kept above the diffraction limit, as shown in fig. 5 (a), therefore, a focusing light spot with the light spot size being the diffraction limit is adopted as an excitation light spot to selectively excite the multiple quantum dot single photon sources, and the excitation of the multiple quantum dot single photon sources is independently controlled by controlling the positions of the excitation light spots. In addition, the direction of a far-field scattering light spot of collimation emission of the collimation emitter provided by the invention in a far field is related to the position of the quantum dot single-photon source, and the scattering light spot direction of the ith quantum dot single-photon source is relative to the far field k _xi k _yi The offset direction of the plane origin is opposite to the offset direction of the quantum dot single-photon source relative to the xy plane origin, and the deflection angle is

Satisfy->

Therefore, the single photon emission of the quantum dot single photon source at different positions can be scattered to different directions by the same collimating emitter, namely, a plurality of single photon circular polarized light can be emitted simultaneously, so that a novel nano photonics device can be realized, such as a plurality of single photon circular polarized light emission devicesOperate independently and simultaneously. To demonstrate this function, the present embodiment selects samples with more than one quantum dot single photon source QD within a robust coupling region not exceeding the critical eccentricity δ =2 μm from the collimated emitter center, the robust coupling region being represented by the dashed circles in fig. 5 (a). As shown in fig. 5 (b), there is a quantum dot single photon source in each of the upper and lower portions of the robust coupling region, represented by the dashed circles, at a distance of 0.27 μm and 0.82 μm from the center of the collimated emitter, respectively. The two quantum dot single photon sources can be excited independently and used for generating respective single photon circularly polarized light far-field collimation emission. The upper row diagram and the lower row diagram in fig. 5 (c) respectively show a CCD diagram, an LCP distribution diagram in a far field, and an RCP distribution diagram in a far field of a sample surface when the upper quantum dot single photon source and the lower quantum dot single photon source are respectively and independently excited, so that it can be obviously observed that the two quantum dot single photon sources independently realize the far field collimation emission of circularly polarized light, and the result shows that the upper quantum dot single photon source and the lower quantum dot single photon source can independently realize the single photon circularly polarized light far field collimation emission by using the same collimation emitter. Moreover, the two quantum dot single photon sources can be excited simultaneously by adjusting the excitation light spot to be in the middle of the two quantum dot single photon sources or adjusting the excitation light spot to be large so that the excitation light spot just covers the two quantum dot single photon sources simultaneously. Fig. 5 (d) shows the CCD detection results for the corresponding sample surface and far field, which can be seen by the simultaneous appearance of two different LCP emission spots in the far field. The intensities of the two LCP emission light spots are obviously different, mainly because the excitation light spots are not accurately adjusted to the middle of the two quantum dot single photon sources, the excitation intensities of the two quantum dot single photon sources are different. The invention shows the independent emission and simultaneous emission of a plurality of single-photon circularly polarized light from a plurality of quantum dot single-photon sources on the same structure for the first time, which provides great elasticity and convenience for practical application, such as spatial multiplexing or frequency multiplexing of a single-photon source. This complex function was not possible in previous research schemes, one reason being that the coupling area of the quantum dot single photon source to the structure was relatively small in previous work: (<λ ² 30) exciting light due to lightThe size of the spot is limited by the diffraction limit, and the multiple quantum dot single photon sources in the coupling region can not be excited respectively and independently; another reason is that the structures proposed in previous work usually provide only one emission direction in the far field, such as yagi antennas, which do not allow the separation of the far field scattered spots of different quantum dot single photon sources in the far field. />

Finally, it is noted that the disclosed embodiments are intended to aid in further understanding of the invention, but those skilled in the art will appreciate that: various substitutions and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the embodiments disclosed, but the scope of the invention is defined by the appended claims.

Claims

1. A collimating emitter for simultaneously emitting a plurality of single-photon circularly polarized light, said collimating emitter for simultaneously emitting a plurality of single-photon circularly polarized light comprising: the device comprises a metal substrate, a chiral scattering unit and a medium spacing layer; wherein, a plurality of chiral scattering units are carved on the upper surface of the metal substrate, the upper surface of the metal substrate is an xy plane, the centers of the chiral scattering units are arranged on an Archimedes spiral, and the Archimedes spiral satisfies r = r ₀ ±q(θ-θ ₀ ) (+ represents the positive Archimedes spiral,

-representing a negative Archimedes spiral, r being the distance of a point on the Archimedes spiral to the origin, θ being the azimuth angle with respect to the x-axis, q being the pitch, r ₀ Is the distance from the origin of the Archimedes spiral to the origin, θ ₀ Is the azimuth angle of the origin of the archimedean spiral with respect to the x-axis; each chiral scattering unit comprises two rectangular nano grooves with the same shape engraved on the surface of the metal substrate, the long sides of the two rectangular nano grooves are mutually vertical and form an included angle of 45 degrees with the radial direction, the adjacent vertexes of the long sides of the inner sides of the two rectangular nano grooves are positioned on the same radius with the origin as the center, and the distance between the two vertexes in the radial direction is 1/5-1/4 of the Surface Plasmon Polariton (SPP) wavelength, so that the scattered light of the two rectangular nano grooves has piA phase difference of/2; the long side l of the rectangular nanometer groove is 200 nm-300 nm, and the short side w is 50 nm-100 nm; the distance between the centers of the angularly adjacent chiral scattering units is less than 1 mu m, and the interval between the chiral scattering units is more than or equal to 50nm; pitch q is equal to the SPP wavelength; arranging a medium spacing layer on a metal substrate engraved with a chiral scattering unit, wherein the medium spacing layer is used for eliminating the fluorescence quenching effect of a quantum dot single photon source on the surface of the metal; multiple quantum dot single photon sources are arranged near the origin on the medium spacing layer, and the eccentricity between each quantum dot single photon source and the origin is not more than 0.2r ₀ And the distance between adjacent quantum dot single photon sources among the multiple quantum dot single photon sources is greater than the diffraction limit;

when the quantum dot single photon source is positioned at the original point, the excitation light spot irradiates the quantum dot single photon source, so that the quantum dot single photon source excites the SPP on the surface of the metal, and the excited SPP uniformly propagates along the surface of the metal substrate in all directions along the radial direction; the SPP is scattered to form scattered light when encountering the chiral scattering unit, and the scattered light finally propagates to a far field; the chiral scattering unit enables SPP incident to each chiral scattering unit to be scattered to the far-field z direction in the same polarization state, namely the direction perpendicular to the surface of the metal substrate, and because two rectangular nano-grooves of the chiral scattering unit are perpendicular to each other and respectively form an included angle of 45 degrees with the radial direction, and the distance between two adjacent vertexes on the long edge of the inner side of the two rectangular nano-grooves along the radial direction is 1/5-1/4 SPP wavelength, the scattered light of the two rectangular nano-grooves has a phase difference of pi/2, so that the polarization state of the scattered light transmitted to the far-field z direction is a circular polarization state; when all scattered light from different chiral scattering units propagating to a far field interferes in the far field, a common circular polarization state can be extracted, and the superposition process is simplified into scalar superposition of complex amplitudes of the scattered light, so that the polarization state of the total scattered light of the far field after superposition is the same circular polarization state as the common polarization state of the scattered light of each chiral scattering unit; and the spatial arrangement of the different chiral scattering units ensures that the circularly polarized light scattered to the far field z direction by each chiral scattering unit is in the same phase, so that the circularly polarized light scattered by the different chiral scattering units is coherent and has a longer coherence in the far field z direction, andin the direction k _x k _y The origin of the plane forms a bright scattering light spot which is emitted in a collimation mode, namely the far-field scattering light spot; k is a radical of _x And k _y K respectively representing the x-component and y-component of the wave vector and consisting of the x-component and y-component of the wave vector as horizontal and vertical coordinates _x k _y A plane represents the emission direction of the far-field scattered light spot, and the projections of the vacuum wave vector along the emission direction in the x direction and the y direction are respectively k _x And k _y ；

When the ith quantum dot single-photon source has an eccentricity delta relative to the origin, defining the eccentricity direction of the ith quantum dot single-photon source as x _i Axial direction, vertical direction y _i In the axial direction, i is more than or equal to 1 and less than or equal to N, N is the number of a plurality of quantum dot single photon sources, N is a natural number more than or equal to 2, the excitation light spot irradiates to the quantum dot single photon source, the quantum dot single photon source excites the SPP on the surface of the metal, the excited SPP uniformly propagates along the surface of the metal substrate by taking the quantum dot single photon source as the center to all directions, the SPP is scattered to form scattered light when meeting the chiral scattering unit, and the scattered light finally propagates to a far field; the polarization change of far-field scattered light of each chiral scattering unit is small and negligible, the chiral scattering units can enable SPPs (dispersed phase P) incident to the chiral scattering units to be scattered to the z direction of a far field in a circular polarization state with the same chirality, when scattered light from different chiral scattering units is subjected to far-field interference, the polarization state of total scattered light of the far field after superposition is still in the circular polarization state which is the same as the common polarization state of the scattered light of each chiral scattering unit, the polarization state of far-field scattered light spots is still in the circular polarization state, and the far-field scattered light spots are insensitive to the position of a quantum dot single photon source, namely high robustness is shown; on the other hand, the influence of the position deviation of the single-photon source of the quantum dot is reflected in that the complex amplitude of scattered light of each chiral scattering unit changes, and the complex amplitude change is represented by an additional phase shift delta psi caused by the change of the SPP propagation distance of the ith single-photon source of the quantum dot to the chiral scattering unit _i Namely: for in x _i y _i In-plane with respect to x _i The axial direction having an azimuth angle theta _i When the eccentricity delta is not more than 0.2r ₀ The additional phase shift Δ ψ _i ＝-Re(k _SPP )δcosθ _i Wherein k is _SPP Represents the SPP wave vector; extra phase shift delta psi _i Proportional to x of each chiral scattering unit _i Coordinate because of x _i ＝rcosθ _i (ii) a Thus, the far-field interference pattern still appears as a bright scattering spot, i.e. a far-field scattering spot, except that the emission direction of the far-field scattering spot will be along k _xi The direction is deflected by a corresponding deflection angle

Deflection angle

Satisfy the relationship

Wherein k is ₀ Represents the vacuum wave vector, at which the additional phase shift Δ ψ results from the change in the SPP propagation distance _i By change of direction of emission, i.e. deflection angle

The compensation of the phase shift introduced is carried out,

or equivalent to a horizontal wave vector shift Δ k giving a far field scattered spot _xi ＝-Re(k _SPP ) Delta/r, namely the eccentricity delta of the quantum dot single photon source relative to the original point only influences the emission direction of the far-field scattering light spot and does not influence the polarization state of the far-field scattering light spot; k is a radical of _xi And k _yi X respectively representing wave vectors of ith quantum dot single-photon source _i Component sum y _i Component, x in wave vector _i Component sum y _i Component k formed by horizontal and vertical coordinates _xi k _yi The plane represents the emission direction of the far-field scattered light spot, and the vacuum wave vector along the emission direction is in x _i Direction and y _i The projection of the direction is k _xi And k _yi ；

The eccentricity is not more than 0.2r ₀ OfThe multiple quantum dot single photon sources can be coupled with the same collimation emitter at the same time, the mutual distance of the multiple quantum dot single photon sources is kept above a diffraction limit, an excitation light spot is adopted, the excitation light spot is a focusing light spot, the size of the light spot is the diffraction limit, one or more quantum dot single photon sources are selected, the excitation light spot irradiates one quantum dot single photon source or irradiates among the multiple quantum dot single photon sources, and therefore the selected one or more quantum dot single photon sources are excited to form corresponding one or more far-field scattering light spots, and independent control over the multiple quantum dot single photon sources is achieved.

2. The collimated emitter for simultaneously emitting a plurality of single-photon circularly polarized light according to claim 1, wherein the number of turns of the archimedean spiral is between 3 and 7.

3. The collimated emitter of claim 1, wherein the Archimedes spiral has a distance r from its origin to its origin ₀ Satisfies the following conditions: r is not less than 5 mu m ₀ ≤30μm。

4. The collimated emitter for simultaneously emitting multiple single-photon circularly polarized light according to claim 1, wherein the thickness of the metal substrate is greater than or equal to 300nm.

5. The collimated emitter according to claim 1, wherein the dielectric spacer layer is made of a visible transparent dielectric material and has a thickness of 5nm to 20nm.

6. The collimated emitter of claim 1, wherein two adjacent vertexes of the two rectangular nano-grooves of each chiral scattering unit on the long side of the inner side are located on the same radius with the origin as the center, of the two vertexes, the vertex belonging to the rectangular nano-groove located in the counterclockwise direction of the radius is a first vertex, the vertex belonging to the rectangular nano-groove located in the clockwise direction of the radius is a second vertex, the first vertex is forward and the archimedean spiral is positive corresponding to left-handed circularly polarized light, the second vertex is forward and the archimedean spiral is negative corresponding to right-handed circularly polarized light.

7. A method of implementing a collimated emitter for simultaneously emitting multiple single-photon circularly polarized light according to claim 1, comprising the steps of:

1) Preparation of a collimation emitter:

b) Carving a plurality of chiral scattering units on the upper surface of the metal substrate, wherein the centers of the plurality of chiral scattering units are arranged on an Archimedes spiral, and the Archimedes spiral satisfies r = r ₀ ±q(θ-θ ₀ ) (+ represents positive Archimedes 'spiral, -represents negative Archimedes' spiral, r is the distance from the point on the Archimedes 'spiral to the origin, theta is the azimuth angle relative to the x axis, q is the pitch, r is the distance from the origin to the point on the Archimedes' spiral, b represents the azimuth angle relative to the x axis, q represents the pitch, r represents the distance from the origin to the x axis ₀ Is the distance from the origin of the Archimedes spiral to the origin, θ ₀ Is the azimuth angle of the origin of the archimedean spiral with respect to the x-axis;

c) Each chiral scattering unit comprises two rectangular nano grooves which are carved on the surface of the metal substrate and have the same shape, the long sides of the two rectangular nano grooves are mutually vertical and form an included angle of 45 degrees with the radial direction, the adjacent vertexes of the long sides of the inner sides of the two rectangular nano grooves are positioned on the same radius with the origin as the center, and the distance between the two vertexes in the radial direction is 1/5-1/4 of the wavelength of the surface plasmon polariton SPP, so that the scattered light of the two rectangular nano grooves has a phase difference of pi/2; the distance between the centers of the angularly adjacent chiral scattering units is less than 1 mu m, and the interval between the chiral scattering units is more than or equal to 50nm; pitch q is equal to the SPP wavelength;

e) Placing multiple quanta near the origin on the dielectric spacer layerThe eccentricity between each quantum dot single photon source and the origin is not more than 0.2r ₀ And the distance between adjacent quantum dot single photon sources among the multiple quantum dot single photon sources is greater than the diffraction limit;

2) When the quantum dot single-photon source is located at the origin:

the excitation light spot irradiates to a quantum dot single photon source, so that the quantum dot single photon source excites the SPP on the metal surface, and the excited SPP uniformly propagates along the surface of the metal substrate along the radial direction to all directions; the SPP is scattered to form scattered light when encountering the chiral scattering unit, and the scattered light finally propagates to a far field; the chiral scattering unit enables SPP incident to each chiral scattering unit to be scattered to the far-field z direction in the same polarization state, namely the direction perpendicular to the surface of the metal substrate, and because two rectangular nano-grooves of the chiral scattering unit are perpendicular to each other and respectively form an included angle of 45 degrees with the radial direction, and the distance between two adjacent vertexes on the long edge of the inner side of the two rectangular nano-grooves along the radial direction is 1/5-1/4 SPP wavelength, the scattered light of the two rectangular nano-grooves has a phase difference of pi/2, so that the polarization state of the scattered light transmitted to the far-field z direction is a circular polarization state; when all scattered light from different chiral scattering units propagating to a far field interferes in the far field, a common circular polarization state can be extracted, and the superposition process is simplified into scalar superposition of complex amplitudes of the scattered light, so that the polarization state of the total scattered light of the far field after superposition is the same circular polarization state as the common polarization state of the scattered light of each chiral scattering unit; and the spatial arrangement of the different chiral scattering units ensures that the circularly polarized light scattered to the far field z direction by each chiral scattering unit is in phase, so that the circularly polarized light scattered by the different chiral scattering units is coherent and has a longer phase in the far field z direction, and the direction is k _x k _y The origin of the plane forms a bright scattering light spot which is emitted in a collimation mode, namely the far-field scattering light spot; k is a radical of _x And k _y K respectively representing the x-component and y-component of the wave vector and consisting of the x-component and y-component of the wave vector as horizontal and vertical coordinates _x k _y The plane represents the emission direction of the far-field scattered light spot, and the vacuum wave vector along the emission direction is in the x direction and the y directionProjection is respectively k _x And k _y ；；

defining the eccentric direction of the ith quantum dot single-photon source as x _i Axial direction, vertical direction y _i In the axial direction, i is more than or equal to 1 and less than or equal to N, N is the number of a plurality of quantum dot single photon sources, N is a natural number more than or equal to 2, the excitation light spot irradiates to the quantum dot single photon source, the quantum dot single photon source excites the SPP on the surface of the metal, the excited SPP uniformly propagates along the surface of the metal substrate by taking the quantum dot single photon source as the center to all directions, the SPP is scattered to form scattered light when encountering the chiral scattering unit, and the scattered light finally propagates to a far field; the polarization change of far-field scattered light of each chiral scattering unit is small and negligible, the chiral scattering units can enable SPPs (dispersed phase P) incident to the chiral scattering units to be scattered to the z direction of a far field in a circular polarization state with the same chirality, when scattered light from different chiral scattering units is subjected to far-field interference, the polarization state of total scattered light of the far field after superposition is still in the circular polarization state which is the same as the common polarization state of the scattered light of each chiral scattering unit, the polarization state of far-field scattered light spots is still in the circular polarization state, and the far-field scattered light spots are insensitive to the position of a quantum dot single photon source, namely high robustness is shown; on the other hand, the source position deviation of the quantum dot single photon source is influenced by the fact that the complex amplitude of scattered light of each chiral scattering unit changes, and the complex amplitude change is represented by an additional phase shift delta psi caused by the change of the SPP propagation distance of the ith quantum dot single photon source to the chiral scattering unit _i Namely: for in x _i y _i In-plane with respect to x _i The axial direction having an azimuth angle theta _i When the eccentricity delta is not more than 0.2r ₀ The additional phase shift Δ ψ _i ＝-Re(k _SPP )δcosθ _i Wherein k is _SPP Representing the SPP wave vector; additional phase shift Δ ψ _i Proportional to x of each chiral scattering unit _i Coordinate because of x _i ＝rcosθ _i (ii) a Thus, the far-field interference pattern still appears as a bright scattering spot, i.e. a far-field scattering spot, except that the emission direction of the far-field scattering spot will be along k _xi DirectionDeflecting by a corresponding deflection angle

Deflection angle

Satisfy the relationship

The compensation of the phase shift introduced is carried out,

or the amount of horizontal wave vector shift Δ k equivalent to giving a far-field scattered spot _xi ＝-Re(k _SPP ) Delta/r, namely the eccentricity delta of the quantum dot single photon source relative to the original point only influences the emission direction of the far-field scattering light spot and does not influence the polarization state of the far-field scattering light spot; k is a radical of _xi And k _yi X representing the wave vectors of the ith quantum dot single photon source _i Component sum y _i Component, x in wave vector _i Component sum y _i Component k formed by horizontal and vertical coordinates _xi k _yi The plane represents the emission direction of the far-field scattered light spot, and the vacuum wave vector along the emission direction is in x _i Direction and y _i The projection of the direction is k _xi And k _yi ；

4) The eccentricity is not more than 0.2r ₀ Multiple quantum dot single photon sources can be coupled with the same collimation emitter at the same time, the mutual distance of the multiple quantum dot single photon sources is kept above the diffraction limit, and an excitation light spot is adopted and is focused lightThe size of the spot is diffraction limit, one or more quantum dot single photon sources are selected, the spot is excited to irradiate one quantum dot single photon source or irradiate among the multiple quantum dot single photon sources, and therefore the selected one or more quantum dot single photon sources are excited to form corresponding one or more far-field scattering spots, and independent control over the multiple quantum dot single photon sources is achieved.

8. The method of claim 7, wherein the two rectangular nanochannels of each chiral scattering unit are located on the long side of the inner side and two adjacent vertexes are located on the same radius with the origin as the center, of the two vertexes, a vertex belonging to the rectangular nanochannels located in the counterclockwise direction of the radius is a first vertex, and a vertex belonging to the rectangular nanochannels located in the clockwise direction of the radius is a second vertex; the chirality of the far-field scattered circularly polarized light is switched by exchanging the radial front and back positions of the two rectangular nanometer grooves, namely that the first vertex is in front and the Archimedes spiral is positive corresponding to left-handed circularly polarized light, and the second vertex is in front and the Archimedes spiral is negative corresponding to right-handed circularly polarized light.