Terahertz metadevice capable of producing observable lateral displacement of optical spin Hall effect and its application in two-dimensional control of optical spin Hall effect

Abstract

The present invention discloses a terahertz meta-device capable of generating observable lateral displacement of optical spin Hall effect and its application method in two-dimensional control of optical spin Hall effect, belonging to the field of optical devices, including a plurality of periodically arranged super units, the super units are prepared by all-silicon medium, and the super units include a rectangular base and a petal-shaped column vertically formed on the top of the rectangular base, the petal-shaped column includes two elliptical columns intersecting vertically. The present invention adopts the above-mentioned terahertz meta-device capable of generating observable lateral displacement of optical spin Hall effect and its application method in two-dimensional control of optical spin Hall effect, which can simultaneously control the core parameters such as phase and polarization of light waves in different channels, and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of optical devices, and in particular to a terahertz meta-device capable of generating observable lateral displacement of optical spin Hall effect and an application method thereof in two-dimensional control of optical spin Hall effect. Background Art

[0002] The Spin Hall Effect of Light (SHEL) can be seen as the evolution of the Spin Hall Effect in electronic systems and is a fundamental phenomenon in optics. The principle is that after a light beam passes through an optical interface or an inhomogeneous medium, transverse (perpendicular to the propagation direction) spin-dependent splitting of photons occurs. Similar to the electron spin Hall effect, SHEL can be used as a carrier of energy and information transmission, showing great potential in energy storage, measurement, and manipulation. At the same time, it has been observed through weak measurements that SHEL offsets are highly sensitive to a variety of medium surface parameters, so SHEL has a strong influence in the characterization of small variables.

[0003] Since Kato et al. first observed SHEL in GaAs semiconductors in 2004, the spin Hall effect has become a hot topic in the field of condensed matter physics. In fact, the optical spin Hall effect originates from SOI (silicon on insulator) and is mainly attributed to two geometric phases of light, namely the Ryto-Vladimirskii-Berry (RVB) phase related to the propagation direction of the wave vector and the Pancharatnam-Berry (PB) phase related to the polarization manipulation of light. In traditional materials, due to the weak spin-orbit interaction, the SHEL spin-related transverse splitting caused by the RVB phase in real space is very small, almost on a sub-wavelength scale, and often requires weak measurement to calculate parameters to be observed, which poses a huge obstacle to its application in all directions. Therefore, in recent years, a large number of studies related to enhancing the SHEL spin-related splitting have emerged.

[0004] Contrary to traditional materials, the SHEL spin splitting achieved by the metasurface is mainly caused by the PB phase related to the polarization state of the light field. When a paraxial light beam passes through the surface of an inhomogeneous medium, a space-varying PB phase is obtained that can cause the light field to spin split in momentum space. The spin splitting caused by the PB phase manifests itself in real space as the angular splitting of photons with opposite spins in two-dimensional space. This splitting increases with the increase of the transmission distance and can even be directly observed by the naked eye.

[0005] However, most of the traditional research on SHEL control is limited to one-dimensional line space, and can only achieve sub-wavelength-level lateral parallel displacement of the line space.

[0006] At the same time, traditional different terahertz vector light fields need to rely on traditional space optical devices such as optical modulators, phase plates, diffraction elements, etc. to generate and control after beam splitting and combining. In addition, the method of generating terahertz vector light fields is difficult to jointly control two or more electromagnetic parameters, and it is also very difficult to independently design different wave components. Summary of the invention

[0007] To solve the above problems, the present invention provides a terahertz metadevice that can produce observable lateral displacement of optical spin Hall effect and its application method in two-dimensional control of optical spin Hall effect, which can simultaneously control core parameters such as phase and polarization of light waves in different channels and has broad application prospects.

[0008] To achieve the above-mentioned purpose, the present invention provides a terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement, comprising a plurality of periodically arranged superunits, the superunits being made of an all-silicon medium, and the superunits comprising a rectangular base and a petal-shaped column vertically formed at the top of the rectangular base, the petal-shaped column comprising two elliptical columns vertically intersecting each other.

[0009] Preferably, the arrangement period of the plurality of super units is P = 140 μm;

[0010] The height of the petal-shaped pillars is T = 200 μm, and the minor axis of the two elliptical pillars is W = 40 μm;

[0011] The height of the rectangular parallelepiped base is H=300 μm.

[0012] Preferably, the transmittance of the supercell is greater than 0.6.

[0013] A terahertz meta-device capable of generating observable lateral displacement of optical spin Hall effect and a method for applying the same in two-dimensional control of optical spin Hall effect, comprising the following steps:

[0014] S1. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed;

[0015] S2, introducing the vortex beam phase distribution function in the PB phase design, and introducing the spatial phase distribution function of the convergent beam in the RVB phase design;

[0016] S3. Design the phase of LCP based on the vortex beam phase distribution function and the spatial phase distribution function of the convergent beam. Phase of RCP And the phase of the initial surface of the metasurface light field

[0017] S4. Change the geometric structure parameters and rotation angle of the superunit structure, control the RVB and PB phases, and realize photon regulation of different spin states.

[0018] Preferably, step S1 specifically includes the following steps:

[0019] S11. According to the generalized Snell's theorem, the relationship between the incident angle θ1 and the extraordinary refraction angle θ2 is set as:

[0020]

[0021] In the formula, n1 and n2 represent the refractive index on both sides of the metasurface, λ represents the wavelength of the incident light, represents the phase gradient of the metasurface;

[0022] When the light beam is incident vertically, θ1 = 90°, and the refraction angle θ2 in the above formula (1) is expressed as:

[0023]

[0024] S12. Design of the phase of metasurface LCP Phase with RCP The refraction angles θ2 are opposite when LCP and RCP are incident, so that the transmitted light is laterally displaced, and the lateral displacement is positively correlated with the propagation distance z:

[0025] By rotating the superunit structure by an angle θ from 0 to 2π, and The phase gradient changes show a continuous opposite trend;

[0026] S13. The physical mechanism of the metasurface regulating the terahertz light field is analyzed by Jones matrix, and the linear transmission Jones matrix of a super unit is expressed as:

[0027]

[0028] In the formula, t xx represents the transmission coefficient of the electric field propagating along the x-axis, T xx , They represent the amplitude and phase of the electric field propagating along the x-axis respectively; t yy represents the transmission coefficient of the electric field propagating along the y-axis, T yy and They represent the amplitude and phase of the electric field propagating along the y-axis respectively;

[0029] Assume that the transmission amplitude T of the supercell xx =T yy= 1, the rotation angle of the supercell is θ, then the linear Jones matrix of the hypersurface J(θ) = M(θ) T ′T′M(θ), where is the optical rotation matrix, T represents the linear transmission Jones matrix;

[0030] S14. Combined with the analysis of the metasurface linear transmission Jones matrix under circular polarization, when the rotation angle of the super unit is θ, the metasurface linear transmission Jones matrix is ​​expressed as:

[0031]

[0032] In the formula, It represents the phase difference of the electric field propagating along the x and y axes. It represents the sum of the phases of the electric field propagating along the x and y axes. δ ± Dependent on the polarization state of the incident light, i is the imaginary unit;

[0033] S15, Phase and transmittance control of cross-polarization channels and co-polarization channels:

[0034] According to formula (4), the metasurface linear transmission Jones matrix expression is divided into two parts, where the first part is the co-polarization part, and its phase Transmittance The second part is the cross-polarization part, whose phase is related to the rotation angle of the super unit, that is, its phase Transmittance By super-element The design of the rotation angle θ allows independent control of the phase and transmittance of the cross-polarization channel and the same-polarization channel;

[0035] S16. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed. The linear transmission Jones matrix of the metasurface is obtained by formula (4) as follows:

[0036]

[0037] In the formula, J co (θ), J cross (θ) represents the Jones matrices of the same polarization channel and the cross-polarization channel, respectively;

[0038] S17, in order to improve the transmission efficiency, At this time, in formula (5), the transmittance of the same polarization channel component is zero, and the transmittance of the cross-polarization channel component is 100%. Formula (5) is simplified as follows:

[0039]

[0040] When the incident terahertz light is LCP, the polarization state of the transmitted field is converted to RCP, and the Jones vector of the transmitted field is:

[0041]

[0042] In the formula, E out1 represents the Jones vector of the transmitted field when the LCP terahertz light is incident; E LCP represents the Jones vector of LCP; E RCP The Jones vector represents the RCP;

[0043] When the incident terahertz light is RCP, the polarization state of the transmitted field is converted to LCP, and the Jones vector of the transmitted field is:

[0044]

[0045] In the formula, E out2 represents the transmission field when the RCP terahertz light is incident;

[0046] Combined phase difference At a known phase In the case of The relationship between and θ is expressed as follows:

[0047]

[0048] According to formula (9), Determine the design with the rotation angle θ phase.

[0049] Preferably, in step S2, according to the vortex phase plate, the vortex beam phase distribution function is set to:

[0050]

[0051] In the formula, l represents the topological charge number of the OAM carried by the vortex beam; X and Y represent the coordinate positions with the center of the metasurface as the origin; the vortex phase with opposite topological charge is added to the PB phase design in different spin states, so that the left-handed beam and the right-handed beam have opposite spiral wavefronts;

[0052] The spatial phase distribution function expression of the converged beam in step S2 is as follows:

[0053]

[0054] In the formula, f represents the focal position perpendicular to the propagation direction of the metasurface, so as to focus photons of different rotation states at different longitudinal positions.

[0055] Preferably, in step S3, according to the description of formula (10) and formula (11), And the phase of the initial surface of the metasurface light field The design is as follows:

[0056]

[0057] In the formula, f1 represents the focal length, and β represents the opening angle corresponding to the lateral displacement;

[0058] The longitudinal focus of the LCP wave component terahertz light is set to The longitudinal focus of the RCP wave component terahertz light is set to Among them, f1=6mm, β=0°, 20°, 40°, 60° or 80°, corresponding to lateral displacement d=0mm, 2.12mm, 4.37mm, 6.93mm or 10.07mm respectively.

[0059] Preferably, in step S4, the function of the superunit structure is configured as follows: when a linearly polarized terahertz beam is incident, the orthogonal polarization components of the transmission field are respectively converted into vortex beams focused at different focal points and having opposite spiral phase wavefronts, and the spin state of the beam will change, that is, when LCP is incident, the polarization state of the transmission field is converted into RCP; when RCP is incident, the polarization state of the transmission field is converted into LCP.

[0060] Preferably, the frequency of the incident linearly polarized terahertz beam is 1 THz.

[0061] The present invention has the following beneficial effects:

[0062] 1. It can achieve efficient SHEL two-dimensional control under the irradiation of a terahertz beam with a frequency of 1THz. The metasurface controls the RVB and PB phases by changing the geometric structure parameters and rotation angle of the superunit structure, thereby achieving photon regulation of different spin states;

[0063] 2. The spin-related components are regulated by manipulating the dynamical phase and geometric phase of the initial surface of the light field. The RVB geometric phase is controlled by changing the size and shape of the superunit; the PB geometric phase is controlled by rotating the superunit.

[0064] 3. The vortex phases with opposite topological charges are added to the PB phase design of different spin states, so that the left-handed and right-handed beams have opposite spiral wavefronts;

[0065] 4. A converging beam phase distribution function is introduced into the spin-independent dynamic phase to focus photons of different rotation states at different longitudinal positions.

[0066] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 It is the result diagram of the simulation experiment of the present invention; wherein, Figure 1 (a) is the super unit simulation structure diagram; Figure 1 (b) is the amplitude screening result diagram of 15 structures in the unit library along the x direction; Figure 1 (c) is the phase screening result diagram of 15 structures in the unit library along the x direction; Figure 1 (d) Transmission amplitude and phase diagram of 15 structures in the unit library;

[0068] Figure 2 A flow chart of a terahertz meta-device capable of producing observable optical spin Hall effect lateral displacement and its application method in two-dimensional control of optical spin Hall effect of the present invention;

[0069] Figure 3 A schematic diagram of the function of the terahertz meta-device capable of producing observable optical spin Hall effect lateral displacement in the optical spin Hall effect lateral displacement control metasurface of the present invention;

[0070] Figure 4 is a diagram of the initial surface phase distribution of the metasurface light field at different deflection angles of the spin-related component of an embodiment of the present invention; wherein Figure 4 (a) is the initial surface phase distribution diagram of the metasurface light field when the deflection angle is 0°; Figure 4 (b) is the initial surface phase distribution diagram of the metasurface light field when the deflection angle is 20°; Figure 4 (c) is the initial surface phase distribution diagram of the metasurface light field when the deflection angle is 40°; Figure 4 (d) is the initial surface phase distribution diagram of the metasurface light field when the deflection angle is 60°; Figure 4 (e) is the initial surface phase distribution diagram of the metasurface light field when the deflection angle is 80°;

[0071] Figure 5 This is a simulation result diagram of SHEL lateral separation by a supersurface according to an embodiment of the present invention; wherein, Figure 5 (a)- Figure 5 (e) Schematic diagram of the metasurface function when the deflection angles are 0°, 20°, 40°, 60°, and 80° respectively; Figure 5 (f)- Figure 5 (j) The transmission field xoz surface E when the deflection angle is 0°, 20°, 40°, 60°, and 80° respectively x Intensity distribution diagram;

[0072] Figure 6 This is a simulation result diagram of SHEL lateral separation by a supersurface according to an embodiment of the present invention; wherein, Figure 6 (a)- Figure 6(d) Stokes S3 parameters of the transmission field xoy plane corresponding to the polarization angles of the spin-related component 0°, 20°, 40°, 60°, and 80°, respectively; Figure 6 (e)- Figure 6 (h) Local phase diagram of the transmission field xoy plane corresponding to the polarization angles of the spin-related component of 0°, 20°, 40°, 60°, and 80°, respectively;

[0073] Figure 6 (i)- Figure 6 (l) Normalized intensity diagram of the electric field on the xoy surface of the transmission field corresponding to the polarization angles of the spin-related component of 0°, 20°, 40°, 60°, and 80°, respectively;

[0074] Figure 7 The intensity and phase distribution diagram of the vector beam coherently synthesized by the metasurface structure vortex beam of an embodiment of the present invention; wherein, Figure 7 (a) The electric field intensity distribution diagram of xoy surface in the transmission field; Figure 7 (b) Phase distribution diagram of the transmission field xoy plane along the x direction; Figure 7 (c), Figure 7 (d) are the transmission field xoy surface E x 、E y Intensity distribution diagram.

[0075] Among them, 1. Rectangular base; 2. Petal-shaped columns. DETAILED DESCRIPTION

[0076] In order to make the purpose, technical scheme and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions.

[0077] It should be noted that the terms "include", "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or devices.

[0078] Like reference numerals and letters denote similar items in the following drawings, and thus, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0079] In order to realize the two-dimensional modulation of SHEL, a terahertz metadevice capable of generating observable lateral displacement of optical spin Hall effect is designed, including multiple superunits arranged periodically, the superunits are made of all-silicon medium, and the superunits include a rectangular base 1 and a petal-shaped column 2 vertically formed on the top of the rectangular base 1, and the petal-shaped column 2 includes two elliptical columns intersecting vertically. The arrangement period of multiple superunits P = 140μm; the height of the petal-shaped column 2 T = 200μm, the short axis of the two elliptical columns W = 40μm; the height of the rectangular base 1 H = 300μm. And the transmittance of the superunit is greater than 0.6 (the amplitude of the incident light is 1).

[0080] Simulation experiment

[0081] The long axes of the two elliptical columns of the metasurface are set to dx and dy respectively; the scanning function of the time domain solver of the simulation software is used to perform parameter scanning of the dx and dy of the metasurface structure from 40 to 140 μm with a step size of 2 μm. Figure 1 The scanning results shown in FIG. 15 are selected from 2500 scanning results that meet the requirements of formula (9), and the selected 15 super-unit structures form a unit library, with π / 8 as the phase gradient, Realize and Full coverage from -π to π. And by Figure 1 (d) It can be seen that the phase difference, amplitude and phase gradient between different supercells of the 15 structures in the cell library all meet the design requirements.

[0082] like Figure 2 As shown, a terahertz meta-device capable of generating observable lateral displacement of optical spin Hall effect and an application method thereof in two-dimensional control of optical spin Hall effect include the following steps:

[0083] S1. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed;

[0084] Step S1 specifically includes the following steps:

[0085] S11. According to the generalized Snell's theorem, the relationship between the incident angle θ1 and the extraordinary refraction angle θ2 is set as:

[0086]

[0087] In the formula, n1 and n2 represent the refractive index on both sides of the metasurface, λ represents the wavelength of the incident light, represents the phase gradient of the metasurface;

[0088] When the light beam is incident vertically, θ1 = 90°, and the refraction angle θ2 in the above formula (1) is expressed as:

[0089]

[0090] S12. Linearly polarized light is considered to be composed of LCP and RCP. When a beam of linearly polarized light is incident vertically on the metasurface, it can be regarded as two orthogonal beams of circularly polarized light penetrating the metasurface. Therefore, the phase of the LCP (left-handed polarized light) of the metasurface is designed. and the phase of RCP (right-hand polarized light) The refraction angles θ2 are opposite when LCP and RCP are incident, so that the transmitted light is laterally displaced (to achieve the lateral separation of the left-handed component and the right-handed component of SHEL), and the lateral displacement is positively correlated with the propagation distance z, that is, the longer the propagation distance, the greater the lateral separation:

[0091] By rotating the superunit structure by an angle θ from 0 to 2π, and The phase gradient changes show a continuous opposite trend;

[0092] S13. The physical mechanism of the metasurface regulating the terahertz light field is analyzed by Jones matrix, so that a general theoretical model can be used to analyze the Jones matrix expression and transmission field Jones vector of the proposed metasurface structure, and the linear transmission Jones matrix of a super unit is expressed as:

[0093]

[0094] In the formula, t xx represents the transmission coefficient of the electric field propagating along the x-axis, T xx , They represent the amplitude and phase of the electric field propagating along the x-axis respectively; t yy represents the transmission coefficient of the electric field propagating along the y-axis, T yy and They represent the amplitude and phase of the electric field propagating along the y-axis respectively;

[0095] Assume that the transmission amplitude T of the supercell xx =T yy = 1, the rotation angle of the supercell is θ, then the linear Jones matrix of the hypersurface J(θ) = M(θ) T ′T′M(θ), where is the optical rotation matrix, T represents the linear transmission Jones matrix;

[0096] S14. Combined with the analysis of the metasurface linear transmission Jones matrix under circular polarization, when the rotation angle of the super unit is θ, the metasurface linear transmission Jones matrix is ​​expressed as:

[0097]

[0098] In the formula, It represents the phase difference of the electric field propagating along the x and y axes. It represents the sum of the phases of the electric field propagating along the x and y axes. δ ± Dependent on the polarization state of the incident light, i is the imaginary unit;

[0099] S15, Phase and transmittance control of cross-polarization channels and co-polarization channels:

[0100] According to formula (4), the metasurface linear transmission Jones matrix expression is divided into two parts, where the first part is the co-polarization part, and its phase Transmittance The second part is the cross-polarization part, whose phase is related to the rotation angle of the super unit, that is, its phase Transmittance By super-element The design of the rotation angle θ allows independent control of the phase and transmittance of the cross-polarization channel and the same-polarization channel;

[0101] S16. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed. The linear transmission Jones matrix of the metasurface is obtained by formula (4) as follows:

[0102]

[0103] In the formula, J co (θ), J cross (θ) represents the Jones matrices of the same polarization channel and the cross-polarization channel, respectively;

[0104] S17, in order to improve the transmission efficiency, At this time, in formula (5), the transmittance of the same polarization channel component is zero, and the transmittance of the cross-polarization channel component is 100%. Formula (5) is simplified as follows:

[0105]

[0106] When the incident terahertz light is LCP, the polarization state of the transmitted field is converted to RCP, and the Jones vector of the transmitted field is:

[0107]

[0108] In the formula, E out1 represents the Jones vector of the transmitted field when the LCP terahertz light is incident; E LCP represents the Jones vector of LCP; E RCP The Jones vector represents the RCP;

[0109] When the incident terahertz light is RCP, the polarization state of the transmitted field is converted to LCP, and the Jones vector of the transmitted field is:

[0110]

[0111] In the formula, E out2 represents the transmission field when the RCP terahertz light is incident;

[0112] Combined phase difference At a known phase In the case of The relationship between and θ is expressed as follows:

[0113]

[0114] According to formula (9), Determine the design with the rotation angle θ The phase of the super unit structure Different choices of and can satisfy any LCP and RCP phase design.

[0115] In step S2, according to the vortex phase plate, the vortex beam phase distribution function is set to:

[0116]

[0117] In the formula, l represents the topological charge number of the OAM carried by the vortex beam; X and Y represent the coordinate positions with the center of the metasurface as the origin; the vortex phase with the opposite topological charge is added to the PB phase design in different spin states, so that the left-handed beam and the right-handed beam have opposite spiral wavefronts; it can be seen that after the vortex beam is introduced, and By designing different topological charges for the phase, the difference between left-handed and right-handed rotation can be more intuitively reflected in the phase distribution.

[0118] S2, introducing the vortex beam phase distribution function in the PB phase design, and introducing the spatial phase distribution function of the convergent beam in the RVB phase design;

[0119] The spatial phase distribution function expression of the converged beam in step S2 is as follows:

[0120]

[0121] In the formula, f represents the focal position perpendicular to the propagation direction of the metasurface, so as to focus photons of different rotation states at different longitudinal positions.

[0122] S3. Design the phase of LCP based on the vortex beam phase distribution function and the spatial phase distribution function of the convergent beam. Phase of RCP And the phase of the initial surface of the metasurface light field

[0123] In step S3, according to the description of formula (10) and formula (11), And the phase of the initial surface of the metasurface light field The design is as follows:

[0124]

[0125] In the formula, f1 represents the focal length, and β represents the opening angle corresponding to the lateral displacement;

[0126] The longitudinal focus of the LCP wave component terahertz light is set to The longitudinal focus of the RCP wave component terahertz light is set to Among them, f1=6mm, β=0°, 20°, 40°, 60° or 80°, corresponding to lateral displacement d=0mm, 2.12mm, 4.37mm, 6.93mm or 10.07mm respectively.

[0127] S4. Change the geometric structure parameters and rotation angle of the superunit structure, control the RVB and PB phases, and realize photon regulation of different spin states.

[0128] like Figure 3 As shown, in step S4, the super unit structure function is configured as follows: when a linearly polarized terahertz beam is incident, the orthogonal polarization components of the transmission field are respectively converted into vortex beams focused at different focal points and having opposite spiral phase wavefronts, and the spin state of the beam will change, that is, when LCP is incident, the polarization state of the transmission field is converted to RCP; when RCP is incident, the polarization state of the transmission field is converted to LCP. And the frequency of the incident linearly polarized terahertz beam is 1THz.

[0129] Example

[0130] like Figure 4 As shown, in this embodiment, the key factor leading to the lateral separation of the left and right hand components SHEL is that the left and right hand components have opposite phase gradient distributions.

[0131] In this embodiment, according to the design requirements of formula (12), an automatic layout algorithm is used to construct a set of 80×80 units with different lateral separations of the metasurface, and the metasurface is simulated in the time domain using simulation software. The simulation frequency is set to 1 THz, the boundary condition is set to open (addspace), and the focal length is f1 = 6 mm. Figure 5 As shown, the transmitted field E x The intensity shows a radial distribution, that is, the farther away from the center of the beam, the electric field intensity gradually decreases, and the light spot is bright-dark-bright distribution. This is because the phase structure of the vortex beam can be described as a rotating spiral, and its phase has rotational symmetry in space. At the center of the beam, the annular phase structure of the rotating spiral just cancels each other, resulting in zero electric field intensity. At a certain distance from the center of the beam, the phase is no longer completely canceled, and the electric field intensity begins to increase, forming a bright light spot. It can be seen that the simulation results are approximately equal to the theoretical results, demonstrating the ability of the metasurface described in this embodiment to regulate the lateral separation of the left-right hand components of SHEL.

[0132] On the other hand, the Stokes parameter S3 value is used to represent the polarization distribution at the focal position, and S3=-1 and +1 respectively represent that the polarization state distribution of the light here is LCP and RCP. Figure 6 (a)- Figure 6 (d) shows the polarization state distribution of the xoy focal plane at z = 6.13, 6.02, 5.87, and 5.77 mm, respectively. Due to the conversion function of the metasurface cross-polarization channel on the incident polarization state, the focus S3 = 1 on the positive half axis of the focus x is the red focal spot, and the corresponding polarization state distribution is RCP; the focus S3 = -1 on the negative half axis of the focus x is the blue focal spot, and the corresponding polarization state distribution is LCP. It can be seen that although the simulation results have a slight error with the theoretical design focal plane f = 6 mm, the polarization distribution at the focal plane is almost consistent with the theoretical design.

[0133] In addition, by designing different vortex OAM topological charges for the left-handed and right-handed components of SHEL, it can be known from formula (10) that the topological charges of the left-handed and right-handed vortex beams in the transmission field are l = +1, -1. Figure 6 (e)-( Figure 6 f) Phase distribution at the focus: It can be observed that the phase at the focus of the SHEL left-handed and right-handed beams presents a spiral distribution with counterclockwise and clockwise trends, respectively. Figure 6 (i)- Figure 6As shown in (l), the field intensity transmittance is greater than 0.7 within the separation angle β = 60° and the lateral displacement d = 6.8mm. After exceeding 60°, the electric field intensity and phase distribution are distorted due to the increase in the numerical aperture of the metasurface and the offset angle between the focusing plane and the xoy plane. The theoretical values ​​of the lateral displacement calculated according to the separation angle are d = 2.12, 4.37, 6.93, 10.07mm, which are slightly different from the simulation results of lateral displacement d = 2.10, 4.26, 6.80, 9.98mm, but the errors are all within an acceptable range.

[0134] From the above analysis, it can be seen that the metasurface described in this embodiment can almost achieve the control ability of the lateral separation of the terahertz SHEL to the metasurface size limit (metasurface size is: 11.2×11.2mm2), thereby demonstrating stronger control ability than traditional control methods.

[0135] In the metasurface structure, the left-handed and right-handed components of the transmitted field SHEL also carry spiral wavefronts with different and opposite OAM topological charges. According to the vortex beam coherent synthesis method, a left-handed polarized vortex beam with an angular quantum number of l=+1 and another right-handed polarized vortex beam with an angular quantum number of l=-1 are annihilated by the OAM carried by the coaxially combined vortex beam when the lateral displacement of the SHEL is zero, and are converted into a plane wavefront vector beam with a polarization order of m=1 and an initial polarization direction of α=0 degrees.

[0136] Figure 7 (a) shows the electric field intensity distribution and polarization state distribution of the radial vector beam coherently synthesized by left-handed and right-handed polarized vortex terahertz beams at the focal plane z = 6.13 mm. The intensity distribution of the vector light field is a uniformly distributed ring with a dark core in the center of the ring, which is the polarization singularity. Figure 7 (b) shows the phase diagram at the focal spot. There is an obvious singular line at the center of the focal phase distribution. On both sides of this line, the phase suddenly changes. Figure 7 (c) Figure 7 (d) shows the radial vector beam E x and E y From the intensity distribution, we can see that the electric field distribution along the x and y directions has an extinction dark line in the horizontal and vertical directions respectively, and the light spots on both sides of the extinction dark line are symmetrically distributed, with a number of 2m. The reason for the appearance of this dark line can be understood as the phase difference between the light fields on both sides at this location, resulting in destructive interference.

[0137] From the above analysis, it can be seen that the metasurface realizes the coherence of vortex beams carrying opposite topological charges into vector beams with m=1 and α=0.

[0138] It can be seen that the metasurface described in this embodiment can simultaneously control the core parameters of light waves such as phase and polarization in different channels, and therefore has great potential in the synthesis of complex structured light fields, and is expected to be used as a tool for manipulating the angular momentum of photons in the field of quantum information; in addition, it can be developed into a precise metrology tool, for example, to characterize the refractive index changes on a subwavelength scale; or to provide a sensitive way to study the physical properties in nanostructures; it also provides a more intuitive and convenient opportunity for measuring weak topological phenomena, and has great application prospects.

[0139] Therefore, the present invention adopts the above-mentioned terahertz metadevice that can produce observable lateral displacement of optical spin Hall effect and its application method in two-dimensional control of optical spin Hall effect, and uses the metasurface to amplify the light beam splitting, separate different spin states or different orbital angular momentum states, and can simultaneously control the core parameters of the light wave such as phase and polarization in different channels, which has broad application prospects.

[0140] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. Application of a terahertz metadevice capable of producing observable lateral displacement of optical spin Hall effect in two-dimensional control of optical spin Hall effect, characterized in that: The following steps are involved: S1. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed; Step S1 specifically includes the following steps: S11. According to the generalized Snell's theorem, the angle of incidence and the anomalous refraction angle The relationship is set as: (1) In the formula, , They represent the refractive index on both sides of the metasurface, λ represents the wavelength of the incident light, represents the phase gradient of the metasurface; When the beam is incident vertically , the refraction angle in the above formula (1) is It is expressed as: (2) S12. Design of the phase of metasurface LCP Phase with RCP , so that the refraction angle of LCP and RCP is is the opposite number, so that the transmitted light undergoes lateral displacement, and the lateral displacement is positively correlated with the propagation distance z: By rotating the superunit structure Rotate from 0 to 2π so that and The phase gradient changes show a continuous opposite trend; S13. The physical mechanism of the metasurface regulating the terahertz light field is analyzed by Jones matrix, and the linear transmission Jones matrix of a super unit is expressed as: (3) In the formula, represents the transmission coefficient of the electric field propagating along the x-axis, , , They represent the amplitude and phase of the electric field propagating along the x-axis respectively; represents the transmission coefficient of the electric field propagating along the y-axis, , and They represent the amplitude and phase of the electric field propagating along the y-axis respectively; Assume that the transmission amplitude of the supercell , the rotation angle of the superelement is , then the linear Jones matrix of the hypersurface is ,in is the optical rotation matrix, represents the linear transmission Jones matrix; S14. Combined with the analysis of the Jones matrix of linear transmission of the metasurface under circular polarization, when the rotation angle of the super unit is When , the metasurface linear transmission Jones matrix is ​​expressed as: (4) In the formula, It represents the phase difference of the electric field propagating along the x and y axes. ; It represents the sum of the phases of the electric field propagating along the x and y axes. ; Dependent on the polarization state of the incident light, , ; is an imaginary unit; S15, Phase and transmittance control of cross-polarization channels and co-polarization channels: According to formula (4), the metasurface linear transmission Jones matrix expression is divided into two parts, where the first part is the co-polarization part, and its phase , transmittance The second part is the cross-polarization part, whose phase is related to the rotation angle of the super unit, that is, its phase , transmittance ; By super-element , and rotation angle The design can independently control the phase and transmittance of the cross-polarization channel and the same polarization channel; S16. Combining the generalized Snell theorem and the metasurface Jones matrix analysis method, the RVB phase and PB phase of the metasurface are designed. The linear transmission Jones matrix of the metasurface is obtained by formula (4) as follows: (5) In the formula, , They represent the Jones matrices of the same polarization channel and the cross-polarization channel respectively; S17, in order to improve the transmission efficiency, , in formula (5), the transmittance of the same polarization channel component is zero, and the transmittance of the cross-polarization channel component is 100%, and formula (5) is simplified to: (6) When the incident terahertz light is LCP, the polarization state of the transmitted field is converted to RCP, and the Jones vector of the transmitted field is: (7) In the formula, represents the Jones vector of the transmitted field when the LCP terahertz light is incident; represents the Jones vector of LCP; Jones vector representing RCP; When the incident terahertz light is RCP, the polarization state of the transmitted field is converted to LCP, and the Jones vector of the transmitted field is: (8) In the formula, represents the transmission field when the RCP terahertz light is incident; Combined phase difference , at a known phase , In the case of , and The relationship is expressed as follows: (9) According to formula (9), , With rotation angle Determine the design , phase; S2, introducing the vortex beam phase distribution function in the PB phase design, and introducing the spatial phase distribution function of the convergent beam in the RVB phase design; S3. Design the phase of LCP based on the vortex beam phase distribution function and the spatial phase distribution function of the convergent beam. , RCP phase And the phase of the initial surface of the metasurface light field ; S4, changing the geometric structure parameters and rotation angle of the super unit structure, controlling the RVB and PB phases, and realizing photon regulation of different spin states; A terahertz metadevice capable of producing observable lateral displacement of optical spin Hall effect includes a plurality of periodically arranged superunits, the superunits being made of all-silicon medium, and the superunits including a rectangular base and a petal-shaped column vertically formed on the top of the rectangular base, the petal-shaped column including two elliptical columns vertically intersecting each other.

2. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 1 in two-dimensional control of optical spin Hall effect, characterized in that: Arrangement period of multiple superunits of a terahertz metadevice capable of producing observable lateral displacement of optical spin Hall effect ; Height of petal-shaped columns , the minor axes of the two elliptical columns ; Height of the rectangular base .

3. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 2 in two-dimensional control of optical spin Hall effect, characterized in that: The transmittance of the supercell is greater than 0.

6.

4. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 1 in two-dimensional control of optical spin Hall effect, characterized in that: In step S2, according to the vortex phase plate, the vortex beam phase distribution function is set to: (10) In the formula, Represents the topological charge of the OAM carried by the vortex beam; , They represent the coordinate positions with the center of the metasurface as the origin respectively; in the PB phase design of different spin states, the vortex phase with opposite topological charge is added, so that the left-handed light beam and the right-handed light beam have opposite spiral wavefronts; The spatial phase distribution function expression of the converged beam described in step S2 is as follows: (11) In the formula, represents the focal position perpendicular to the propagation direction of the metasurface, so as to focus photons of different rotation states at different longitudinal positions.

5. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 4 in two-dimensional control of optical spin Hall effect, characterized in that: In step S3, according to the description of formula (10) and formula (11), , And the phase of the initial surface of the metasurface light field The design is as follows: (12) In the formula, represents the focal length, represents the opening angle corresponding to the lateral displacement; The longitudinal focus of the LCP wave component terahertz light is set to ; The longitudinal focus of the RCP wave component terahertz light is set to ,in f 1 = 6 mm, β = 0°, 20°, 40°, 60° or 80°, corresponding to lateral displacement d = 0 mm, 2.12 mm, 4.37 mm, 6.93 mm or 10.07 mm, respectively.

6. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 5 in two-dimensional control of optical spin Hall effect, characterized in that: In step S4, the function of the superunit structure is configured as follows: when a linearly polarized terahertz beam is incident, the orthogonal polarization components of the transmitted field are respectively converted into vortex beams focused at different focal points and having opposite spiral phase wavefronts, and the spin state of the beam will change, that is, when LCP is incident, the polarization state of the transmitted field is converted into RCP; when RCP is incident, the polarization state of the transmitted field is converted into LCP.

7. The application of the terahertz metadevice capable of generating observable optical spin Hall effect lateral displacement according to claim 6 in two-dimensional control of optical spin Hall effect, characterized in that: The incident linearly polarized terahertz beam has a frequency of 1 THz.

Images