Liquid crystal cascade superstructure device for actively regulating vortex waves on upper surface of terahertz sheet

Abstract

The invention discloses a liquid crystal cascade superstructure device for actively regulating and controlling vortex waves on the upper surface of a terahertz sheet, and belongs to the technical field of terahertz devices. The device is composed of a dielectric spiral metasurface, a metal spiral metasurface, two liquid crystal layers, two graphene electrode layers and two glass substrates. The medium spiral metasurface excites free space vortexes locked in a spinning mode, the metal spiral metasurface constructs two on-chip channels, and the liquid crystal layer dynamically regulates and controls the spinning state. The device is based on spin-orbital angular momentum coupling and photon state superposition principles, and excitation of on-chip vortexes of spin decoupling is realized through a cascade strategy. In addition, by controlling the orientation of the two layers of liquid crystals, various on-chip vortex states can be achieved in the two channels respectively. Therefore, the device realizes eight vortex states and active switching thereof. The liquid crystal cascade super-structure device has the advantages of on-chip double channels, multi-state multiplexing and active adjustability, and has important application value in the fields of terahertz high-speed communication, information processing, substance detection and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz applications, and in particular relates to a liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip. Background Art

[0002] Terahertz waves are electromagnetic waves in the frequency range of 0.1-10THz between microwave and infrared bands. With the development of terahertz (THz) technology, terahertz wavefront work, especially the generation and adjustment of special beams, has important applications in sensing, radar monitoring and imaging. The angular momentum of electromagnetic waves includes spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is related to the spin state and can expand the number of independent channels, while OAM is related to the spiral wavefront. Since vortex electromagnetic waves with different modal values ​​are orthogonal to each other, the information capacity carried can be increased. Similar to free-space vortex electromagnetic waves, on-chip surface plasma vortex waves (abbreviated as on-chip surface vortex waves or on-chip vortices) have attracted widespread attention due to their potential in optical tweezers, quantum computing and communications. However, traditional on-chip vortex devices have problems such as limited modes, fixed functions and complex structures. Therefore, it is necessary to develop devices with more design freedom, more vortex modes and active regulation.

[0003] Artificial metasurfaces can manipulate the amplitude, phase, and polarization of electromagnetic waves on a subwavelength scale and have been widely used in the generation and regulation of vortex beams. The Pancharatnam-Berry (PB) metasurface composed of a half-wave plate with spatially varying axes has been reported to generate OAM in free space [Physical review letters, 2006, 96 (16): 163905]. In addition, slit-based metal metasurfaces provide a feasible way to excite near-field OAM. For example, Prinz et al. proposed a double-chain structure design with overall chiral symmetry based on a pair of segmented spirally arranged slits, which can excite on-chip surface vortex waves with arbitrary high-order vortex topological charge numbers [Nano Letters, 2021, 21 (9): 3941-3946]. However, the structures of these metasurfaces are relatively complex, and the vortex states generated under the incidence of a pair of conjugated spin states are mutually locked (the topological charges are opposite to each other), which limits the multifunctionality and channel multiplexing of the device. In order to remove the spin-locking restriction of the vortex beam, Kim et al. proposed a metasurface composed of a segmented spiral curved line structure to realize spin-decoupled on-chip surface vortex waves [Nano letters, 2010, 10 (2): 529-536]. However, the topological charge difference between the two on-chip vortices obtained based on this design strategy is constant at 2, and the mode of the on-chip vortex cannot be flexibly controlled. In addition, since the structure of most devices is predetermined according to demand, the on-chip vortex mode in each polarization channel is often fixed and cannot be actively transformed.

[0004] Liquid crystal (LC) has obvious uniaxial anisotropy in the terahertz region and can be flexibly manipulated by light, electricity or magnetic field. Studies have shown that liquid crystal integrated metasurfaces can perform wavefront modulation such as terahertz beam deflection. In addition, some work has demonstrated that LC is effective in modulating the wavefront of surface waves. For example, Wang et al. demonstrated an LC integrated on-chip broadband focusing device that can actively modulate on-chip focusing energy [Photonics Research, 2024, 12(10): 2148-2157]. However, there is still a need to develop devices that can actively manipulate on-chip surface vortex waves.

[0005] In summary, firstly, the structure of the spin-decoupled on-chip surface vortex device is relatively complex and the design freedom is limited; secondly, the development of on-chip vortex meta-devices still needs to stimulate more modes; thirdly, due to the fixed structure of the metasurface, the device function is often fixed or only allows passive regulation, so actively adjustable devices have research value. Therefore, there is an urgent need for a terahertz on-chip surface vortex wave meta-device with multiple design freedoms, multiple spin decoupling states, and active regulation. Summary of the invention

[0006] The purpose of the present invention is to provide a liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip, so as to solve key technical problems in the background technology of the surface vortex metastructure device on the chip, such as limited structural design freedom, small number of modes, lack of active conversion mechanism, etc.

[0007] The technical solution of the invention is that the liquid crystal cascade metastructure device for actively regulating the surface vortex wave on the terahertz plate is composed of a dielectric spiral metasurface, a metal spiral metasurface, two liquid crystal layers, two graphene electrode layers and two glass substrates. The two liquid crystal layers act as half-wave plates, and the dynamic switching between the spin-holding state and the spin-flipping state is realized by controlling the orientation of the liquid crystal molecules in the plane of the metal layer (along the X-axis) or perpendicular to the plane (along the Z-axis). The basic structural unit of the dielectric spiral metasurface is composed of a silicon substrate and a rectangular silicon column standing upright above it, and is considered as a whole with the upper liquid crystal layer, and is designed as a Pancharatnam-Berry (PB) metasurface. The geometric phase is controlled by matching the anisotropic phase shift of the liquid crystal, thereby determining the size of the rectangular column and the thickness of the upper liquid crystal layer, and at the same time, the spatial spiral phase distribution is constructed by controlling the orientation of the rectangular column, thereby exciting the spin-locked free space vortex. The metal spiral metasurface is composed of orthogonal slit pairs distributed in a spiral on a metal film. The orientation angle and radial position of the unit introduce geometric phase and propagation phase to obtain spin-decoupled on-chip surface vortices. Based on the principles of spin-orbital angular momentum coupling and photon state superposition, the metasurface cascade scheme enables the on-chip surface vortex to achieve more states in two independent channels. By applying an external electric field to adjust the orientation of the two layers of liquid crystal, the device achieves the excitation and active regulation of 8 on-chip surface vortex states.

[0008] The liquid crystal cascade metastructure device for actively regulating the surface vortex wave of a terahertz chip comprises a front glass substrate (1), a graphene front electrode layer (2), an upper liquid crystal layer (3), a dielectric spiral metasurface (4), a graphene back electrode layer (5), a lower liquid crystal layer (6), a rear glass substrate (7), and a structured metal layer (8). The dielectric column metasurface (4) has a rectangular column (10) as a basic structure, and together with the upper liquid crystal layer (3) forms a liquid crystal-dielectric spiral metasurface (12), and the rear glass substrate (7) and the structured metal layer (8) form a metal spiral metasurface. The front glass substrate (1) and the back glass substrate (7) are both non-doped fused silica optical glass with a thickness of h1=h6=250μm-350μm; a graphene front electrode layer (2) and a graphene back electrode layer (5) with high conductivity and high transmittance to terahertz waves are laid on the lower surfaces of the front glass substrate (1) and the dielectric spiral metasurface (4), respectively, and a structured metal layer (8) is used as another front electrode layer, so that by applying an external electric field with an adjustable range of 0-40V / mm, it is ensured that the liquid crystal molecules in the upper liquid crystal layer (3) and the lower liquid crystal layer (6) can be independently oriented in the XZ plane. The liquid crystal material is a large birefringence nematic phase liquid crystal, and its birefringence coefficient in the terahertz band is 0.25-0.35. The thickness of the upper liquid crystal layer (3) h2 and the thickness of the lower liquid crystal layer (6) h5 are 1000-1250 μm, so that when the frequency f0=0.4-0.5 THz, a phase difference of πrad is obtained between the orthogonal polarization components. The dielectric spiral metasurface (4) is obtained by photolithography and silicon deep etching on a high-resistance silicon substrate with a thickness h4 of 1 mm; the period p of the PB unit of the liquid crystal-dielectric spiral metasurface is 500 μm, the height h3 of the rectangular column is 500 μm, the long side dimension is 320 μm, the width b is 50-150 μm, and its long axis orientation angle increases with the azimuth angle φ d The structured metal layer (8) is obtained by laser direct writing on a gold film with a thickness h7 of 300 to 500 nm by ion sputtering, and its overall structure includes m=1 to 5 turns of spiral structure (13); each turn of spiral structure (13) is composed of m,n and radial position r m,n The arranged n=50 to 120 orthogonal slit pairs (11) are composed of a plurality of orthogonal slit pairs, and the radial initial position is R m =2000~5000μm; each orthogonal slit pair unit (11) includes two slits (9) with long axis orientation angles θ1 and θ2 of 90° and 0° respectively, the slits have a large aspect ratio, generally greater than 2:1, the long side dimension q ranges from 160~180μm, the short side dimension w ranges from 40~60μm, and the relative distance d of the geometric center is 300~400μm.

[0009] The device works in such a way that when the incident wave is a left-handed circularly polarized wave (LCP) or a right-handed circularly polarized wave (RCP) incident along the -Z axis, the device excites an on-chip surface vortex wave, whose channel is defined according to the spin direction of the incident wave, that is, the device has two on-chip vortex channels, |zL> and |zR>. If the numbers 1 / 0 are used to represent whether an external electric field is applied to a certain liquid crystal layer, the orientation of the liquid crystal is along the Z axis / X axis respectively. If a two-digit number is used to describe the power-on scheme of the device, the first and second digits represent the power-on conditions of the upper and lower liquid crystal layers, that is, "whether an electric field is applied to the upper liquid crystal" + "whether an electric field is applied to the lower liquid crystal", then the device has four different power-on schemes, namely 00, 01, 10, and 11. Therefore, by actively controlling different power-on schemes, the device can excite a total of eight spin-decoupled on-chip surface vortex waves, and active switching of four on-chip vortex states can be achieved in each channel.

[0010] The beneficial effects and advantages of the present invention are:

[0011] 1. The device realizes a spin-decoupled on-chip vortex by cascading a dielectric spiral metasurface and a metal spiral metasurface based on the principles of spin-orbital angular momentum coupling and photon state superposition. The structural parameters of the dielectric metasurface introduced by this cascade scheme increase the design freedom of the on-chip vortex device.

[0012] 2. The device introduces liquid crystal as an active control material, which has the advantages of being easy to manufacture and having excellent dielectric tunable performance when working in the terahertz frequency band, which helps to achieve active modulation of on-chip vortices.

[0013] 3. The device integrates a double layer of liquid crystal, each layer of which acts as a half-wave plate, and controls the switching of spin states by applying an external bias electric field. A variety of power-on schemes can be used to obtain a variety of on-chip vortex states, and active switching between vortex states can be achieved.

[0014] 4. The overall size of the device is only 2cm×2cm×(<5mm). By designing the dielectric metasurface and liquid crystal as a whole, the liquid crystal can be directly filled between the dielectric metasurface and the glass substrate, making the device structure more compact. The device integration and flexibility are greatly improved compared with traditional terahertz components, which is conducive to the miniaturization and integration of terahertz application systems.

[0015] 5. This liquid crystal cascade metadevice has the advantages of on-chip dual channels, multi-state multiplexing, and active adjustable vortex state, and has important application value in the fields of terahertz high-speed communication, information processing, and material detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1(a) is a schematic diagram of the three-dimensional structure of the liquid crystal cascade metadevice that actively regulates the surface vortex waves on the terahertz chip;

[0017] Figure 1 (b) is a side view of the device;

[0018] Figure 2 (a) is a schematic diagram of the structure of the structured metal layer;

[0019] Figure 2 (b) is a schematic diagram of the design details of the structured metal layer;

[0020] Figure 2 (c) is a schematic diagram of the structure of the orthogonal slit pair unit;

[0021] Figure 3 (a) is a schematic diagram of the structure of the dielectric spiral metasurface;

[0022] Figure 3 (b) is a schematic diagram of the design details of the dielectric spiral metasurface;

[0023] Figure 4 (a) is the transmission spectrum of the metal slit metasurface;

[0024] Figure 4 (b) is the intensity and phase distribution diagram of the vortex on the metal spiral metasurface excitation sheet when the incident light is LCP;

[0025] Figure 4 (c) is the intensity and phase distribution diagram of the vortex on the metal spiral metasurface excitation sheet when the incident light is RCP;

[0026] Figure 5 (a) is the phase delay spectrum of the supercell in the PB metasurface composed of the liquid crystal layer and the dielectric spiral metasurface when the upper liquid crystal is oriented along the Z axis;

[0027] Figure 5 (b) is the intensity and phase distribution diagram of the free space vortex excited by the liquid crystal-medium spiral metasurface when the incident light is LCP and the upper liquid crystal is oriented along the Z axis and X axis respectively;

[0028] Figure 5 (c) is the intensity and phase distribution diagram of the free space vortex excited by the liquid crystal-medium spiral metasurface when the incident light is RCP and the upper liquid crystal is oriented along the Z axis and X axis respectively;

[0029] Figure 6 (a) is a working principle diagram of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the power-on scheme of the double-layer liquid crystal layer is 00;

[0030] Figure 6(b) is a working principle diagram of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the power-on scheme for the double-layer liquid crystal layer is 01;

[0031] Figure 6 (c) is a working principle diagram of the liquid crystal cascade meta-device that actively regulates the surface vortex waves on the terahertz chip when the power-on scheme for the double-layer liquid crystal layer is 10;

[0032] Figure 6 (d) is a working principle diagram of the liquid crystal cascade meta-device that actively regulates the surface vortex waves on the terahertz chip when the power-on scheme for the double-layer liquid crystal layer is 11;

[0033] Figure 7 (a) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zL> channel by the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is LCP and the power-on schemes of the double-layer liquid crystal layer are 0 and 0 respectively;

[0034] Figure 7 (b) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zL> channel by the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is LCP and the power-on schemes of the double-layer liquid crystal layer are 0 and 1 respectively;

[0035] Figure 7 (c) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zL> channel of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is LCP and the power-on schemes of the double-layer liquid crystal layer are 10 respectively;

[0036] Figure 7 (d) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zL> channel by the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is LCP and the power-on schemes of the double-layer liquid crystal layer are 11 respectively;

[0037] Figure 8 (a) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zR> channel of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is RCP and the power-on schemes of the double-layer liquid crystal layer are 0 and 0 respectively;

[0038] Figure 8 (b) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zR> channel of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is RCP and the power-on schemes of the double-layer liquid crystal layer are 01 respectively;

[0039] Figure 8(c) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zR> channel of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is RCP and the power-on schemes of the double-layer liquid crystal layer are 10 respectively;

[0040] Figure 8 (d) is the intensity and phase distribution diagram of the on-chip vortex excited in the |zR> channel of the liquid crystal cascade meta-device that actively regulates the surface vortex wave on the terahertz chip when the incident light is RCP and the power-on schemes of the double-layer liquid crystal layer are 11 respectively; DETAILED DESCRIPTION

[0041] Combined with the drawings of the specification, the present invention is described in detail for the liquid crystal cascade metastructure device for actively regulating the surface vortex wave on the terahertz chip. It should be noted that the present invention can be implemented with a variety of different specific structural parameters, including but not limited to the following specific examples.

[0042] The structure of the device is as follows Figure 1 (a) and Figure 1 As shown in (b), the liquid crystal cascade metastructure device for actively regulating the surface vortex wave of the terahertz chip includes: a front glass substrate (1), a graphene front electrode layer (2), an upper liquid crystal layer (3), a dielectric spiral metasurface (4), a graphene back electrode layer (5), a lower liquid crystal layer (6), a rear glass substrate (7), and a structured metal layer (8). Among them, the dielectric column metasurface (4) has a rectangular column (10) as a basic structure, and together with the upper liquid crystal layer (3) constitutes a liquid crystal-dielectric spiral metasurface (12), and the rear glass substrate (7) and the structured metal layer (8) together constitute a metal spiral metasurface. The front glass substrate (1) and the back glass substrate (7) are both non-doped fused silica optical glass with a thickness of h1=h6=300 μm; a graphene front electrode layer (2) and a graphene back electrode layer (5) with high conductivity and high transmittance to terahertz waves are laid on the lower surfaces of the front glass substrate (1) and the dielectric spiral metasurface (4), respectively, and a structured metal layer (8) is used as another front electrode layer, so that by not applying and applying an external electric field of 30 V / mm, it is ensured that the liquid crystal molecules in the upper liquid crystal layer (3) and the lower liquid crystal layer (6) can be independently oriented along the X axis and the Z axis respectively. The liquid crystal material is a large birefringence nematic phase liquid crystal, and its birefringence coefficient in the terahertz band is 0.3. The thickness h2 of the upper liquid crystal layer (3) and the thickness h5 of the lower liquid crystal layer (6) are both 1150 μm, so that at a frequency f0=0.4 THz, a phase difference of π rad is obtained between orthogonal polarization components. The structured metal layer (8) is obtained by laser direct writing on a gold film with a thickness h7 of 300 nm by ion sputtering, such as Figure 2 As shown in (a), its overall structure includes three turns of helical structure (13); Figure 2As shown in (b), the m=1st, 2nd, and 3rd turn helical structures (13) are formed according to the orientation angle φ m,n and radial position r m,n The arranged n=65, 85, 95 orthogonal slit pairs (11) are composed, and the corresponding radial initial position is R m =2944μm, 3680μm, 4416μm; Figure 2 As shown in (c), each orthogonal slit pair unit (11) includes two slits (9) with long axis orientation angles θ1 and θ2 of 90° and 0° respectively, with a size q of 180 μm, a short side size w of 50 μm, and a relative distance d of geometric centers of 368 μm. The dielectric spiral metasurface (4) is obtained on a high-resistance silicon substrate with a thickness h4 of 1 mm by photolithography and silicon deep etching. Its structure is shown in FIG. Figure 3 (a) as shown; Figure 3 As shown in (b), the period p of the PB unit of the liquid crystal-medium spiral metasurface is 500 μm, the height h3 of the rectangular column is 500 μm, the long side dimension a is 320 μm, the width b is 90 μm, and the long axis orientation angle is φ d equal.

[0043] The basic working principle of the device is as follows: the subwavelength slit is the basic element of the metal metasurface, and the near-field response to the incident component with polarization perpendicular to the long side can be regarded as a dipole resonance response, which excites surface waves propagating along the metal surface near the center frequency [Laser & Photonics Reviews, 2023, 17(6): 2200948]. Figure 4 As shown in (a), the metal metasurface consists of slits with a size of 180μm×50μm, and its operating frequency is 0.4THz. The two slits in the orthogonal slit pair are oriented orthogonally to ensure the response to the incident spin photon states |L> and |R>, and to make the initial phase parity symmetric. In addition, in each turn of the spiral structure, the orientation of the orthogonal slit pair is related to the azimuth angle, which introduces a spin-related geometric phase, and the on-chip distance difference from the slit pair to the center introduces the propagation phase gradient of the surface wave. When the geometric phase shift and propagation phase shift are 2πσrad and -2πrad, respectively, the surface wave electric field at the center O of the spiral structure can be expressed as:

[0044]

[0045] At this time, the metal metasurface has two on-chip vortex channels, |zL> and |zR>, and the order of the excited on-chip vortex can be expressed as:

[0046] l M =σ-1 (2) In the formula, C is the coefficient; λ SP is the surface wave wavelength; l Mrepresents the topological charge of the vortex on the chip; σ represents the spin state, and when σ=+1 or σ=-1, it corresponds to LCP and RCP. Figure 4 (b) and Figure 4 As shown in (c), when the incident photon states are |L> and |R>, the metallic metasurface generates 0th-order and -2nd-order on-chip vortices in the |zL> and |zR> channels respectively through spin-orbit angular momentum coupling.

[0047] The integration of liquid crystals provides an effective strategy for active metasurface devices. The liquid crystal layer has a phase delay of πrad near the center frequency of 0.4THz, which plays a role in switching between spin states. The coefficient Q is defined to describe the preservation and flipping of the spin state, and the subscripts U and D represent the upper and lower layers respectively. When an external electric field of 30V / mm is applied, the liquid crystal molecules in this layer are oriented along the Z axis, and the spin state does not change at this time, that is, Q U =1 and / or Q D =1; when no external electric field is applied, the liquid crystal molecules are oriented along the X axis, and the spin state is converted to the spin flip state, i.e., Q U = -1 and / or Q D =-1.

[0048] The dielectric spiral metasurface and the upper liquid crystal layer are considered as a whole and designed as a PB geometric phase metasurface. The rectangular silicon pillars in the PB supercell control the geometric phase by matching the anisotropy of the liquid crystal, such as Figure 5 As shown in (a), the PB supercell achieves a phase delay of πrad, and at the same time constructs a spatial spiral phase distribution by controlling the orientation of the rectangular columns. Therefore, the transmission matrix of the liquid crystal-medium spiral metasurface can be expressed as [Advanced Optical Materials, 2019, 7(20): 1900594]:

[0049]

[0050] At this time, when circularly polarized light is incident normally, the order of the spin-coupled free-space vortex can be expressed as:

[0051] l D =2Q U σ (4) Figure 5 As shown in (b), when LCP is incident, if the liquid crystal molecules are along the Z axis, that is, σ = 1 and Q U =1, the generation order is l D = 2 free space vortex; if the liquid crystal molecules are along the X axis, that is, σ = 1 and Q U =-1, then the order is l D = -2 free space vortex; such as Figure 5 As shown in (c), when RCP is incident, if the liquid crystal molecules are along the Z axis, that is, σ = -1 and QU =1, the generation order is l D = -2 free space vortex; if the liquid crystal molecules are along the X axis, that is, σ = -1 and Q U =-1, then the order is l D =2 free space vortex.

[0052] Considering the device as a whole, the cascade between the liquid crystal-dielectric spiral metasurface, the lower liquid crystal layer, and the metal metasurface can realize spin-orbital angular momentum coupling and photon state superposition, so this double-layer liquid crystal integrated device can greatly increase the number of vortex states in the on-chip channel. The orientation of the upper and lower liquid crystal layers can be independently controlled by applying an external electric field. When incident light with a spin state of σ enters the device, if the upper and lower liquid crystal molecules are arranged along the X-axis direction, the outgoing wave passing through the dielectric metasurface is a free-space vortex with a spin state of σ and a topological charge of -2σ. Then, when it passes through the lower liquid crystal layer and enters the metal metasurface, its spin state flips to -σ, and the topological charge of the vortex on the chip is excited to be -2σ+(-σ-1)=-3σ-1. If the upper and lower liquid crystal molecules are arranged along the X-axis and Z-axis directions respectively, the spin state of the free-space vortex excited by the dielectric metasurface is σ, and the topological charge is -2σ. Then, after passing through the lower liquid crystal layer, its spin state remains σ, and the topological charge of the vortex on the chip is excited to be The charge is -2σ+(σ-1)=-σ-1; if the upper and lower liquid crystal molecules are arranged along the Z-axis and X-axis directions respectively, the spin state of the free-space vortex excited by the dielectric metasurface is -σ, and the topological charge is 2σ, and then after passing through the lower liquid crystal layer, its spin state flips to σ, and the topological charge of the vortex excited on the chip is 2σ+(σ-1)=3σ-1; if the upper and lower liquid crystal molecules are arranged along the Z-axis direction, the spin state of the free-space vortex excited by the dielectric metasurface is -σ, and the topological charge is 2σ, and then after passing through the lower liquid crystal layer, its spin state remains -σ, and the topological charge of the vortex excited on the chip is 2σ+(-σ-1)=σ-1. Therefore, the order of on-chip vortices excited by the liquid crystal cascade metastructure device that actively controls the surface vortex waves on the terahertz chip can be expressed as:

[0053] l=2Q U σ-Q U Q D σ-1=Q U σ(2-Q D )-1 (5)

[0054] The specific working method of the device is as follows: Figure 6 The schematic diagram of the device is shown in Figure 1. The incident wave is a circularly polarized wave incident along the -Z axis. The applied electric field is defined as 1, and the non-applied electric field is defined as 0. The power-on scheme of the double-layer liquid crystal can be described as "whether an electric field is applied to the upper layer of liquid crystal" + "whether an electric field is applied to the lower layer of liquid crystal", that is, there are four cases: 00, 01, 10, and 11. Figure 6 As shown in (a) to 6(d), under the power-on scheme of 00, when LCP and RCP are incident, the device generates on-chip vortices of -4 order and +2 order, respectively; under the power-on scheme of 01, when LCP and RCP are incident, the device generates on-chip vortices of -2 order and 0 order, respectively; under the power-on scheme of 10, when LCP and RCP are incident, the device generates on-chip vortices of +2 order and -4 order, respectively; under the power-on scheme of 11, when LCP and RCP are incident, the device generates on-chip vortices of 0 order and -2 order, respectively.

[0055] Figure 7 These are the four switchable states of the on-chip vortex in the |zL> channel of the device. Figure 7 (a) is the intensity and phase distribution of the surface vortex wave excited by the device when the power-on scheme of the double-layer liquid crystal layer is 00. At this time, σ=1, Q U =Q D = -1, according to formula (5), the topological charge of the vortex on the chip can be calculated as l = -4; Figure 7 (b) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 01. At this time, σ=1, Q U =-1, Q D =1, according to formula (5), the topological charge of the vortex on the chip can be calculated as l = -2; Figure 7 (c) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 10. At this time, σ=1, Q U =1,Q D = -1, according to formula (5), the topological charge of the on-chip vortex can be calculated as l = 2; Figure 7 (d) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 11. At this time, σ = 1, Q U =Q D =1, according to formula (5), the topological charge of the on-chip vortex can be calculated as l=0.

[0056] Figure 8 These are the four switchable states of the on-chip vortex in the |zR> channel of the device. Figure 8 (a) is the intensity and phase distribution of the surface vortex wave excited by the device when the power-on scheme of the double-layer liquid crystal layer is 00. At this time, σ=-1, Q U =Q D = -1, according to formula (5), the topological charge of the on-chip vortex can be calculated as l = 2; Figure 8 (b) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 01. At this time, σ=-1, Q U =-1, Q D=1, according to formula (5), the topological charge of the vortex on the chip can be calculated as l = 0; Figure 8 (c) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 10. At this time, σ = -1, Q U =1,Q D = -1, according to formula (5), the topological charge of the vortex on the chip can be calculated as l = -4; Figure 8 (d) is the intensity and phase distribution of the on-chip vortex excited by the device when the power-on scheme of the double-layer liquid crystal layer is 11. At this time, σ = -1, Q U =Q D =1, according to formula (5), the topological charge of the on-chip vortex can be calculated as l=-2.

[0057] In summary, the device realizes spin-decoupled on-chip vortex by cascading dielectric spiral metasurface and metal spiral metasurface based on the principle of spin-orbit angular momentum coupling and photon state superposition. The device introduces large birefringence liquid crystal layers at the front end of the dielectric metasurface and the metal metasurface respectively. Each layer of liquid crystal acts as a half-wave plate. The switching of the spin state is controlled by applying an external bias electric field of 0-30V / mm, thereby realizing multiple on-chip vortex states in two channels respectively. The overall size of the device is only 2cm×2cm×3.4mm, which is conducive to system miniaturization and integration. Therefore, the device realizes 8 on-chip surface vortex states and their active switching. This liquid crystal cascade meta-structure device has the advantages of on-chip dual channels, multiple vortex state multiplexing, and active adjustability, and has important application value in the fields of terahertz high-speed communication, information processing, and material detection.

Claims

1. A liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip, characterized in that: The device is arranged in sequence as a front glass substrate (1), a graphene front electrode layer (2), an upper liquid crystal layer (3), a dielectric column supersurface (4), a graphene back electrode layer (5), a lower liquid crystal layer (6), a rear glass substrate (7), and a structured metal layer (8); wherein the dielectric column supersurface (4) has a rectangular column (10) as a basic structure and together with the upper liquid crystal layer (3) constitutes a liquid crystal-dielectric spiral supersurface (12); the rear glass substrate (7) and the structured metal layer (8) together constitute a metal spiral supersurface; the structured metal layer (8) is composed of a multi-turn spiral structure (13), and its periodic unit is an orthogonal slit pair unit (11) composed of two orthogonal slits (9); when the incident wave is a circularly polarized wave, the device establishes two on-chip surface vortex channels according to the spin direction of the incident wave; by controlling the orientation of the two layers of liquid crystal, four on-chip surface vortex states can be obtained in each channel; the device has the function of on-chip dual-channel and dynamic switching of multiple vortex states.

2. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: The front glass substrate (1) and the rear glass substrate (7) are both non-doped fused quartz optical glass; a graphene front electrode layer (2) and a graphene back electrode layer (5) with high conductivity and high transmittance to terahertz waves are laid on the lower surfaces of the front glass substrate (1) and the dielectric spiral metasurface (4), respectively; a structured metal layer (8) serves as another front electrode layer, ensuring that an external electric field can be independently applied to the upper liquid crystal layer (3) and the lower liquid crystal layer (6), respectively, and ensuring that the liquid crystal molecules in the upper liquid crystal layer (3) and the lower liquid crystal layer (6) rotate in a plane perpendicular to the metal layer.

3. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: The upper liquid crystal layer (3) and the lower liquid crystal layer (6) both play a role in dynamically switching the spin state. When the liquid crystal molecules are oriented in a direction perpendicular to or parallel to the plane of the metal layer, the spin state is a spin-hold state or a spin-flip state respectively. The thickness of the upper liquid crystal layer (3) and the lower liquid crystal layer (6) is determined according to the center frequency so that a phase difference of πrad is obtained between the orthogonal polarization components. The liquid crystal material is a large birefringence nematic phase liquid crystal, and its birefringence coefficient in the terahertz band is 0.25-0.

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4. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: The dielectric spiral metasurface (4) is obtained on a high-resistance silicon substrate by photolithography and deep etching processes; the liquid crystal-dielectric spiral metasurface (12) is designed as a Pancharatnam-Berry (PB) metasurface, and the geometric phase is controlled by matching the anisotropic phase shift of the liquid crystal, thereby determining the size of the rectangular column (10), and at the same time, the long axis orientation angle of the rectangular column (10) is changed with the azimuth angle to construct a spatial spiral phase distribution, thereby ensuring the excitation of a spin-locked free space vortex.

5. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: The structured metal layer (8) is obtained by laser direct writing process on the ion sputtered metal film; The slits (9) have a high aspect ratio to ensure efficient excitation of surface waves, and their geometric dimensions are determined according to the center frequency; two mutually perpendicular slits (9) form an orthogonal slit pair unit (11), and the relative distance between their geometric centers is 1 / 2 of the wavelength of the surface wave; in each turn of the spiral structure (13), the orientation angle and radial position of the orthogonal slit pair unit (11) introduce a geometric phase and a propagation phase to ensure that when left-handed and right-handed circular polarizations are excited, the metal spiral metasurface excites a spin-decoupled on-chip surface vortex through spin-orbit angular momentum coupling.

6. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: Based on the principles of spin-orbit angular momentum coupling and photon state superposition, the cascade scheme of dielectric spiral metasurface and metal spiral metasurface enables the topological charge of the free-space vortex to be assigned to the on-chip surface vortex, increasing the freedom of structural design; and by regulating the orientation of the upper and lower layers of liquid crystal, it is ensured that multiple states of the on-chip surface vortex waves can be obtained in two channels.

7. The liquid crystal cascade metastructure device for actively regulating surface vortex waves on a terahertz chip according to claim 1, characterized in that: The incident wave is a circularly polarized wave incident from the side of the front glass substrate (1). If the numbers 1 and 0 are used to represent whether an external electric field is applied or not, respectively, the orientation of the liquid crystal is in a direction perpendicular to or parallel to the plane of the metal layer. If a two-digit number is used to describe the power-on scheme, the first and second digits represent the power-on conditions of the upper liquid crystal layer (3) and the lower liquid crystal layer (6), respectively. Under the four different power-on schemes of 00, 01, 10, and 11, the device can obtain eight types of on-chip surface vortex waves, among which four types of actively switchable vortex states can be obtained in two channels, respectively.

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