Terahertz fundamental and second harmonic coordinated phase modulation super surface, device and system

Abstract

The application provides a terahertz fundamental frequency and double frequency cooperative phase modulation super surface, device and system, relates to the generation and phase modulation technical field of terahertz waves, and the super surface comprises: a plurality of structure units, each structure unit is arranged on the same two-dimensional plane in a two-dimensional arrangement manner;The rotation angles of each structure unit relative to the same reference direction in the two-dimensional plane are not all the same;When the structure unit is subjected to circularly polarized terahertz wave incidence, the transmission end of the structure unit outputs terahertz waves comprising at least a fundamental frequency component and a second harmonic component, and the phases of the terahertz waves output by the structure units with different rotation angles are different.In the same super surface, a fundamental frequency signal with a frequency of a fundamental frequency and a second harmonic signal with a frequency of twice the fundamental frequency are generated, and the phases of the output terahertz waves are different by setting structure units with different rotation angles, so that the phase of the terahertz wave can be modulated while the frequency of the terahertz wave is doubled.

Description

Technical Field

[0001] This invention relates to the field of terahertz wave generation and phase modulation technology, and in particular to metasurfaces, devices and systems for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency. Background Technology

[0002] Terahertz (THz) waves typically refer to electromagnetic radiation with frequencies between 0.1 THz and 10 THz, and have significant applications in information communication, radar, and non-destructive testing. As terahertz wave generation systems develop towards higher integration and multi-functionality, traditional radiation sources that only output a single frequency and have a fixed wavefront are no longer sufficient to meet application requirements. Practical applications not only need to achieve frequency expansion but also require flexible control of the electromagnetic wave front to achieve complex field distributions such as beam deflection, focusing, and vortexing.

[0003] In terms of frequency extension, frequency doubling is a crucial means of achieving terahertz frequency conversion. However, traditional schemes based on nonlinear crystals rely on phase-matching conditions, and their harmonic phases are typically determined by material parameters, lacking controllability. Therefore, existing technologies usually require separating "harmonic generation" from "wavefront modulation," i.e., generating harmonics through nonlinear materials and then using devices such as lenses or prisms for wavefront modulation, resulting in high system complexity, large size, and increased losses. Furthermore, in the terahertz band, due to the extremely weak intrinsic second-order nonlinearity of materials and significant absorption losses, achieving efficient frequency doubling is already difficult, let alone achieving designable controllability of the harmonic phase during frequency conversion.

[0004] In recent years, through the design of subwavelength structural elements, metasurfaces have enabled precise control of the phase of electromagnetic waves and have been able to enhance the nonlinear response by strengthening the local field. However, existing technologies mainly focus on one of the following two directions: (1) only achieving the enhancement or generation of terahertz frequency doubling; (2) only performing phase modulation on a single frequency (usually the fundamental frequency or a certain harmonic). Therefore, how to use the same metasurface to simultaneously achieve terahertz wave frequency doubling and phase modulation has become an urgent technical problem to be solved. Summary of the Invention

[0005] This invention provides a metasurface, device, and system for coordinated phase modulation of terahertz fundamental frequency and second harmonic, which can modulate the phase of terahertz waves while achieving frequency doubling.

[0006] This invention provides a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, comprising: Multiple structural elements are arranged in a two-dimensional manner on the same two-dimensional plane; the rotation angles of each structural element relative to the same reference direction in the two-dimensional plane are not all the same; When the structural element is subjected to circularly polarized terahertz waves, the transmission end of the structural element outputs terahertz waves that include at least the fundamental frequency component and the second harmonic component, and the phase of the terahertz waves output by the structural element at different rotation angles is different.

[0007] According to the present invention, a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided, wherein the structural unit includes a micro / nano structure and a coupling structure, and the coupling structure is located within the near-field coupling range of the micro / nano structure; When the micro / nano structure is subjected to incident circularly polarized terahertz waves, it generates a localized enhanced electromagnetic field. Under the action of the localized enhanced electromagnetic field, the free carriers in the coupling structure generate nonlinear currents, and output terahertz waves from the transmission end, including at least the fundamental frequency component and the second harmonic component.

[0008] According to the present invention, a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided, wherein the micro / nano structure is disposed around the coupling structure on the two-dimensional plane, and the micro / nano structure has an opening on the two-dimensional plane.

[0009] According to the present invention, a metasurface with coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided, wherein different components of the terahertz wave output by the structural element correspond to different carrier channels.

[0010] According to the present invention, a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided. The terahertz wave output by the structural unit includes: a fundamental frequency cross-circular polarization component, a first second harmonic component, and a second second harmonic component; the frequency of the fundamental frequency cross-circular polarization component is ω, and the circular polarization direction is opposite to the incident circular polarization direction; the frequency of the first second harmonic component is 2ω, and the circular polarization direction is opposite to the incident circular polarization direction; the frequency of the second second harmonic component is 2ω, and the circular polarization direction is the same as the incident circular polarization direction; ω is the frequency of the circularly polarized terahertz wave, and the incident circular polarization direction is the circular polarization direction of the circularly polarized terahertz wave.

[0011] According to the present invention, a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided, wherein the rotation angle of the structural unit is θ, the phase of the fundamental frequency cross-circular polarization component is 2σθ, the phase of the first second harmonic component is 3σθ, and the phase of the second second harmonic component is σθ, wherein σ represents the polarization state of the circularly polarized terahertz fundamental frequency wave, σ=+1 represents the left-hand circular polarization state, and σ=-1 represents the right-hand circular polarization state.

[0012] According to the present invention, a metasurface for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency is provided, wherein the phase of the components of each carrier channel is uniquely determined by the rotation angle of the structural element.

[0013] According to a metasurface for cooperative phase regulation of terahertz fundamental frequency and second harmonic provided by the present invention, the metasurface includes a plurality of periodic units, each of the periodic units includes N structural elements, and the N structural elements are arranged on the same straight line. The difference in the rotation angles of adjacent structural elements in the same periodic unit is α, and α×N = 360°.

[0014] According to a metasurface for cooperative phase regulation of terahertz fundamental frequency and second harmonic provided by the present invention, each of the structural elements forms a plurality of "mouth"-shaped units with different sizes. The plurality of "mouth"-shaped units are arranged in a nested manner centered on the coordinate origin of the metasurface. The rotation angle θ of the structural element with coordinates (x, y) is: ; where r is the distance between the structural element and the coordinate origin, k w is the wave vector of the fundamental frequency wave, f 0 is the focal length of the fundamental frequency wave.

[0015] The present invention also provides a metasurface device for cooperative phase regulation of terahertz fundamental frequency and second harmonic, which includes the metasurface for cooperative phase regulation of terahertz fundamental frequency and second harmonic described in any one of the present applications.

[0016] The present invention also provides a metasurface system for cooperative phase regulation of terahertz fundamental frequency and second harmonic, which includes the metasurface device for cooperative phase regulation of terahertz fundamental frequency and second harmonic described in any one of the present applications.

[0017] In the metasurface, device and system for cooperative phase regulation of terahertz fundamental frequency and second harmonic provided by the present invention, the metasurface includes: a plurality of structural elements, and each of the structural elements is arranged in a two-dimensional arrangement on the same two-dimensional plane; the rotation angles of each of the structural elements relative to the same reference direction in the two-dimensional plane are not all the same; when the structural elements are irradiated by circularly polarized terahertz waves, the transmission end of the structural elements outputs terahertz waves including at least a fundamental frequency component and a second harmonic component, and the phases of the terahertz waves output by the structural elements with different rotation angles are different. In the same metasurface, a fundamental frequency signal (i.e., the fundamental frequency component) with a frequency of the fundamental frequency and a second harmonic signal (i.e., the second harmonic component) with a frequency of twice the fundamental frequency are generated, and by setting the structural elements with different rotation angles, the phases of the output terahertz waves are different, and the phase can be regulated while generating the terahertz wave frequency doubling. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is one of the structural schematic diagrams of the metasurface provided by the present invention, which features coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency. Figure 2a This is one of the structural schematic diagrams of the structural elements provided by the present invention; Figure 2b This is one of the structural schematic diagrams of the structural elements provided by the present invention; Figure 3 This is a schematic diagram showing the phase and frequency relationship between the incident and emitted light of the structural unit in this invention. Figure 4a This is the second schematic diagram of the metasurface structure for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency provided by the present invention; Figure 4b This is a schematic diagram of the right-hand circularly polarized fundamental frequency wave output when the right-hand circularly polarized terahertz wave is incident perpendicularly in this invention. Figure 4c This is a schematic diagram of the left-hand circularly polarized fundamental frequency wave output when a right-hand circularly polarized terahertz wave is incident perpendicularly in this invention. Figure 4d This is a schematic diagram of the right-hand circularly polarized second harmonic output when the right-hand circularly polarized terahertz wave is incident perpendicularly in this invention. Figure 4e This is a schematic diagram of the output of the left-hand circularly polarized second harmonic when the right-hand circularly polarized terahertz wave is incident perpendicularly in this invention. Figure 5 This is the third schematic diagram of the metasurface structure for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency provided by the present invention; Figure 6a This is a schematic diagram of the focusing of the right-hand circularly polarized terahertz fundamental frequency vortex light at different depths in this invention; Figure 6b This is a schematic diagram of the intensity distribution of the right-hand circularly polarized terahertz fundamental frequency vortex light on the focal plane in this invention; Figure 6c This is a schematic diagram of the phase distribution of the right-hand circularly polarized terahertz fundamental frequency vortex light on the focal plane in this invention; Figure 7a This is a schematic diagram of the focusing of right-handed circularly polarized terahertz second-harmonic vortex light at different depths in this invention; Figure 7bThis is a schematic diagram of the intensity distribution of right-handed circularly polarized terahertz second harmonic vortex light on the focal plane in this invention; Figure 7c This is a schematic diagram of the phase distribution of the right-hand circularly polarized terahertz second harmonic vortex light on the focal plane in this invention; Figure 8a This is a schematic diagram of the focusing of left-handed circularly polarized terahertz second-harmonic vortex light at different depths in this invention; Figure 8b This is a schematic diagram of the intensity distribution of the left-handed circularly polarized terahertz second-harmonic vortex light on the focal plane in this invention. Figure 8c This is a schematic diagram of the phase distribution of the left-handed circularly polarized terahertz second harmonic vortex light on the focal plane in this invention.

[0020] Figure label: 101: Structural unit; 1011: Coupled structure; 1012: Micro / nano structure; 10121: Opening. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] Existing terahertz nonlinear metasurfaces mainly suffer from the following shortcomings: (1) Terahertz second-order nonlinear response has low efficiency, and the source of nonlinearity is limited by the material, resulting in low design freedom. It depends on the intrinsic χ of the material. (2) The effect is weak in the terahertz band, making it difficult to achieve efficient harmonic generation. Moreover, most solutions rely on specific nonlinear crystals or low-symmetry material systems, which are difficult to design and integrate flexibly.

[0023] (2) Separation of terahertz harmonic generation and control functions. Existing metasurfaces can usually only achieve harmonic enhancement or output, and lack the ability to precisely control the harmonic performance (phase, polarization).

[0024] (3) It is impossible to achieve multi-frequency coordinated control and multi-channel multiplexing. Most studies only target a single frequency (fundamental frequency or harmonic frequency), making it difficult to achieve coordinated functional design of terahertz fundamental frequency and harmonics in the same device. More importantly, existing schemes for terahertz waves are difficult to achieve joint control of multi-channel output (polarization + frequency + phase). When multiple channels are involved at the same time, different control strategies are often required, resulting in system complexity and difficulty in expansion.

[0025] To address at least one of the aforementioned problems, this invention provides a metasurface, device, and system for coordinated phase modulation of terahertz fundamental and second harmonic frequencies, which will be discussed below. Figures 1 to 8c Please provide a detailed explanation.

[0026] Figure 1 This is one of the structural schematic diagrams of the metasurface with coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency provided by the present invention, such as... Figure 1 As shown, the metasurface includes: Multiple structural elements 101 are arranged in a two-dimensional manner on the same two-dimensional plane; the rotation angles of each structural element 101 relative to the same reference direction in the two-dimensional plane are not all the same.

[0027] When the structural element 101 is subjected to circularly polarized terahertz waves, the transmission end of the structural element 101 outputs terahertz waves that include at least the fundamental frequency component and the second harmonic component, and the phase of the terahertz waves output by the structural element 101 is different for different rotation angles.

[0028] Structural element 101 is subwavelength scale, meaning its characteristic size is smaller than the wavelength of the incident circularly polarized terahertz wave. The number of structural elements 101 in the metasurface can be customized according to actual conditions. Figure 1 The above is just an example. The two-dimensional arrangement of the structural elements 101 in the metasurface can also be set according to actual requirements. For example, it can be set to a matrix form or a spiral form, but it needs to be ensured that they are on the same two-dimensional plane.

[0029] Structural element 101 can generate a locally enhanced electromagnetic field under the excitation of incident circularly polarized terahertz waves, and induce an equivalent second-order nonlinear response at the structural level, thereby generating nonlinear harmonics with frequencies that are integer multiples of the circularly polarized terahertz wave frequency. The fundamental mode frequency is equal to the frequency of the incident circularly polarized terahertz wave, and is the terahertz wave directly radiated by structural element 101 at the fundamental frequency of its electron motion. The harmonic mode is the terahertz wave radiated by structural element 101 at integer multiples of the fundamental frequency using the nonlinear effect of electron motion; for example, the second harmonic frequency is twice the fundamental frequency. The terahertz wave output from the transmission end of structural element 101 is a composite beam and can include multiple components. Specifically, the output terahertz wave includes at least two frequencies; the wave with the fundamental frequency is called the fundamental frequency component, and the wave with a frequency twice the fundamental frequency is called the second harmonic component.

[0030] The rotation angle of structural element 101 refers to the angle between the reference direction of structural element 101 and the reference direction of the two-dimensional plane. For example, such as... Figure 2aAs shown, the coordinate system of the two-dimensional plane is called the experimental coordinate system, which includes the x-axis and y-axis; the coordinate system of structural element 101 is called the local coordinate system, which includes the u-axis and v-axis. The rotation angle of structural element 101 can be the angle θ1 between the v-axis of structural element 101 and the y-axis of the two-dimensional plane, or the rotation angle of structural element 101 can be the angle θ2 between the u-axis of structural element 101 and the x-axis of the two-dimensional plane. Wherein, the v-axis of structural element 101 is perpendicular to the u-axis, the y-axis of the two-dimensional plane is perpendicular to the x-axis, and θ1 = θ2. The phase of the terahertz wave output by structural element 101 is determined by the rotation angle of structural element 101; different rotation angles result in different phases in the terahertz waves output by structural element 101. The rotation angle of each structural element 101 can be set according to the actual phase modulation requirements of the output component waves.

[0031] In this embodiment of the invention, a fundamental frequency signal (i.e., fundamental frequency component) and a second harmonic signal (i.e., second harmonic component) with a frequency twice the fundamental frequency are generated in the same metasurface. Furthermore, by setting structural elements with different rotation angles, the phase of the output terahertz wave is different, which enables the phase of the terahertz wave to be controlled while achieving frequency doubling.

[0032] exist Figure 1 Based on the illustrated embodiment, the structural element 101 includes a micro / nano structure and a coupling structure, wherein the coupling structure is located within the near-field coupling range of the micro / nano structure; thus, the coupling structure can generate a second harmonic under near-field coupling. See also: Figure 2b The micro / nano structure 1012 is disposed around the coupling structure 1011 on the two-dimensional plane, and the micro / nano structure 1012 has an opening 10121 on the two-dimensional plane. Thus, the micro / nano structure 1012 forms an open ring on the two-dimensional plane and surrounds the coupling structure 1011.

[0033] When the micro / nano structure 1012 is subjected to incident circularly polarized terahertz waves, it generates a localized enhanced electromagnetic field. Under the action of the localized enhanced electromagnetic field, the free carriers in the coupling structure 1011 generate nonlinear currents, and output terahertz waves from the transmission end, including at least the fundamental frequency component and the second harmonic component.

[0034] The micro / nano structure 1012 can be fabricated from metallic materials such as gold, silver, aluminum, and copper; the coupling structure 1011 can be fabricated from semiconductor materials such as silicon, germanium, and gallium arsenide, or Dirac half-metals such as graphene and cadmium arsenide, or topological insulators such as bismuth selenide. The micro / nano structure 1012 resonates under the excitation of circularly polarized terahertz waves (with a frequency of ω), significantly enhancing the local terahertz electric and magnetic fields. The coupling structure 1011 provides a large number of free carriers with high mobility. Near-field coupling occurs between the two parts. Under the influence of the local field generated by the micro / nano structure 1012, the free carriers in the coupling structure 1011 undergo nonlinear drift motion driven by the Lorentz force, forming a nonlinear current. This oscillating nonlinear current acts like a miniature antenna, radiating electromagnetic waves, and further radiating a fundamental frequency wave (corresponding to the fundamental frequency component) with a frequency of ω and a second harmonic wave (corresponding to the second harmonic component) with a frequency of 2ω.

[0035] In this embodiment of the invention, by constructing a metasurface structure primitive with spatial inversion symmetry broken, and by utilizing the resonance-induced enhancement of the local terahertz electromagnetic field and the nonharmonic motion of charge carriers, an equivalent artificial second-order nonlinear response is achieved without relying on the intrinsic second-order nonlinearity of the material, thereby significantly improving the generation efficiency of terahertz second harmonics and breaking through the limitation of weak nonlinearity of traditional materials.

[0036] The anisotropy of structural element 101 results in a deterministic mapping relationship between the phases of the radiated fundamental and second harmonic waves and the rotation angle θ of structural element 101. For example... Figure 3 As shown, for structural element 101 with a rotation angle of θ, when a circularly polarized terahertz wave with a rotation direction of σ and a frequency of ω is incident perpendicularly, the fundamental frequency field (frequency ω) of the radiation is as follows: (1); Among them, E xout and E yout Let t represent the polarization components of the fundamental frequency wave along the x and y directions, respectively. u and t v Let represent the transmission coefficients of the circularly polarized terahertz wave along the u-axis and v-axis of structural element 101, respectively. σ represents the circular polarization direction of the circularly polarized terahertz wave, with σ = ±1 corresponding to left-handed and right-handed circular polarization, respectively. It can be seen that any circularly polarized terahertz wave with a single rotation direction incident on the metasurface will radiate a fundamental frequency field composed of both left-handed and right-handed circularly polarized waves. The component with the same rotation direction as the incident wave has no additional phase, while the component with the opposite rotation direction will have an additional 2σθ phase.

[0037] For structural element 101 with a rotation angle of θ, when a circularly polarized terahertz wave with a rotation direction of σ and a frequency of ω is incident perpendicularly, its second-order nonlinear dipole moment is: (2); in, The dipole moment of the second harmonic wave in the laboratory coordinate system has the same rotation direction as the incident circularly polarized terahertz wave. The dipole moment of the second harmonic wave, which has the opposite rotation direction to the incident circularly polarized terahertz wave, in the laboratory coordinate system. The dipole moment of the second harmonic wave, which has the same rotation direction as the incident circularly polarized terahertz wave in the local coordinate system, is represented. This represents the dipole moment of the second harmonic wave, which has the opposite rotation direction to the incident circularly polarized terahertz wave, in the local coordinate system. The experimental coordinate system is a two-dimensional plane coordinate system, while the local coordinate system is the coordinate system of the structural primitives.

[0038] Therefore, for second harmonics with the same and opposite rotation directions as the incident circularly polarized terahertz wave, the nonlinear polarizability tensor can be expressed as follows: (3); in, The nonlinear polarizability tensor represents the second harmonic wave with the same rotation direction as the incident circularly polarized terahertz wave. The nonlinear polarizability tensor represents the second harmonic with the opposite rotation direction to the incident circularly polarized terahertz wave.

[0039] When a circularly polarized terahertz fundamental wave of any single rotation direction is incident on a metasurface, the radiated second harmonic consists of two parts: a left-handed circularly polarized wave and a right-handed circularly polarized wave. The second harmonics with the same and opposite rotation directions as the fundamental wave introduce phases of σθ and 3σθ, respectively. By changing the rotation angle θ of the structural element, second harmonics with arbitrary phases can be obtained.

[0040] As can be seen from the above analysis, the phase of each component of the carrier channel is uniquely determined by the rotation angle of the structural element.

[0041] In this embodiment of the invention, a qualitative mapping relationship is established between the rotation angle of the structural element and the phase of the output terahertz wave. This allows the phase of the nonlinear terahertz harmonic wave to be no longer limited by material parameters or phase matching conditions, but to be continuously controlled through structural design. This significantly improves the design freedom of the nonlinear wavefront and avoids the need for multi-stage cascaded structures of nonlinear devices and wavefront control devices in traditional technologies. This effectively reduces system complexity and transmission loss, and meets the development needs of miniaturization and integration of terahertz devices.

[0042] Different components of the terahertz wave output by the structural element have different frequencies or phases, so these different components can be used as different carriers for data transmission. In this case, the different components of the terahertz wave output by the structural element 101 correspond to different carrier channels. In one example, the terahertz wave output by the structural element 101 includes: a fundamental frequency cross-circularly polarized component, a first second harmonic component, and a second second harmonic component; the frequency of the fundamental frequency cross-circularly polarized component is ω, and its circular polarization direction is opposite to the incident circular polarization direction; the frequency of the first second harmonic component is 2ω, and its circular polarization direction is opposite to the incident circular polarization direction; the frequency of the second second harmonic component is 2ω, and its circular polarization direction is the same as the incident circular polarization direction; ω is the frequency of the circularly polarized terahertz wave, and the incident circular polarization direction is the circular polarization direction of the circularly polarized terahertz wave.

[0043] When the rotation angle of the structural element 101 is θ, in the terahertz wave output by the structural element 101: the phase of the fundamental frequency cross-circular polarization component is 2σθ, the phase of the first second harmonic component is 3σθ, and the phase of the second second harmonic component is σθ, where σ represents the polarization state of the circularly polarized terahertz fundamental frequency wave, σ=+1 represents the left-hand circular polarization state, and σ=-1 represents the right-hand circular polarization state.

[0044] In this embodiment of the invention, by adjusting the rotation angle θ of the structural element, the phase encoding relationship between different frequencies and polarization channels can be achieved: the phase of the fundamental frequency cross-circular polarization component is 2σθ, the phase of the second harmonic cross-circular polarization component is 3σθ, and the phase of the second harmonic co-rotational circular polarization component is σθ. This indicates that polarization-frequency-phase triple reuse control can be achieved in the same metasurface, enabling multi-channel information multiplexing and allowing different polarization states and frequencies to correspond to different phase responses, thereby significantly improving information capacity and functional integration.

[0045] To achieve integrated frequency conversion and wavefront deflection, in one possible implementation, the structural units 101 in the metasurface can be configured as follows: Figure 4a The metasurface is arranged as shown, comprising multiple periodic units, each periodic unit comprising N structural elements 101, and the N structural elements 101 in each periodic unit are arranged on the same straight line. The difference in rotation angle between adjacent structural elements 101 within the same periodic unit is α, and α × N = 360°. It can be understood that... Figure 4a The number of structural primitives 101 in each periodic unit is for illustrative purposes only; the actual number can be set according to the actual situation.

[0046] Taking α as 15° and N as 24 as an example, one periodic unit contains 24 structural primitives, such as Figure 4aAs shown, a row of structural elements 101 is a periodic unit. In one example, each period of the structural element 101 can be a square with a size of 36μm × 36μm. The entire metasurface can be obtained by splicing multiple periodic units.

[0047] The circularly polarized terahertz wave incident on the metasurface is right-handed circular polarization with a frequency of 1 THz. The rotation angle of the structural element 101 is θ, as Figure 4b shown, the additional phase of the right-handed fundamental wave output by the structural element 101 is 0, and the light beam does not deflect. As Figure 4c shown, the additional phase of the left-handed fundamental wave output by the structural element 101 is 2θ. When the structural element rotates through 360°, its phase change is 720°, and the deflection angle is: = arcsin[(2×300) / (24×36)] = 44°. As Figure 4d shown, the additional phase of the right-handed second harmonic wave output by the structural element 101 is θ. When the structural element rotates through 360°, its phase change is 360°, and the deflection angle is: = arcsin[150 / (24×36)] = 10°. As Figure 4e shown, the additional phase of the left-handed second harmonic wave output by the structural element 101 is 3θ. When the structural element rotates through 360°, its phase change is 1080°, and the deflection angle is: = arcsin[(3×150) / (24×36)] = 31°. By rotating the structural element by a specific angle, the gradient phase required for the fundamental wave and the second harmonic wave can be obtained, and then the specific emission angle deflection of the fundamental wave and the second harmonic wave in different polarization states can be achieved.

[0048] The emergence of emerging fields such as massive data transmission, cloud computing, and artificial intelligence has put forward higher requirements for channel capacity. In one possible implementation, the structural elements 101 in the metasurface can be arranged in the manner as Figure 5 shown. Each of the structural elements forms multiple "square" - shaped units with different sizes. Multiple "square" - shaped units are arranged in a nested manner centered on the coordinate origin of the metasurface. The rotation angle θ of the structural element with coordinates (x, y) is: (4); where r is the distance between the structural element and the coordinate origin, k w is the wave vector of the fundamental wave, f 0 is the focal length of the fundamental wave.

[0049] As Figure 5As shown, starting from the very center, the \(i\)-th "square" - shaped unit includes \(4(2i - 1)\) structural elements 101. For each structural element 101, the values of its \(x\) - coordinate and \(y\) - coordinate range from \(-i\) to \(i\) as integers and do not include 0. For example, the smallest "square" - shaped unit includes four structural elements 101. Starting from the first quadrant and in a clockwise direction, the coordinates of the four structural elements 101 are \((1,1)\), \((1, - 1)\), \(( - 1, - 1)\), and \(( - 1,1)\) respectively; the second - smallest "square" - shaped unit includes twelve structural elements 101. Starting from the structural element 101 with the smallest \(x\) - value in the first quadrant and in a clockwise direction, the coordinates of the twelve structural elements 101 are \((1,2)\), \((2,2)\), \((2,1)\), \((2, - 1)\), \((2, - 2)\), \((1, - 2)\), \(( - 1, - 2)\), \(( - 2, - 2)\), \(( - 2, - 1)\), \(( - 2,1)\), \(( - 2,2)\), and \(( - 1,2)\) respectively.

[0050] To increase the channel capacity of an optical communication system, the vortex beam carrying orbital angular momentum emerges in a new multiplexing manner, providing an effective means to solve the problems of rate and channel capacity in multiplexing communication. Vortex fundamental waves and second - harmonic waves with different helicities and carrying orbital angular momentum have great application potential in the fields of polarization multiplexing, frequency multiplexing, and phase multiplexing.

[0051] From equations (1) and (2), it can be obtained that the phase distribution of the output terahertz wave is: (5); where, is the phase of the output terahertz wave, l is the topological charge of the vortex light. Based on the Figure 5 shown metasurface, terahertz waves with different helicities and different frequencies emitted can be focused at different positions and have different vortex topological charges. Taking a left - hand circularly polarized Gaussian light as the incident fundamental wave to vertically excite the metasurface as an example: the topological charge of the right - hand circularly polarized terahertz fundamental - frequency vortex light (corresponding to the fundamental - frequency cross - circular polarization component) output by the metasurface is 2, and the focal length is f 0 = 90 mm. As shown in, for example, Figures 6a-6c where, Figure 6a is the focusing situation of the right - hand circularly polarized terahertz fundamental - frequency vortex light at different depths, Figure 6b is the intensity distribution of the right - hand circularly polarized terahertz fundamental - frequency vortex light on the focal plane, Figure 6c is the phase distribution of the right - hand circularly polarized terahertz fundamental - frequency vortex light on the focal plane. The topological charge of the right - hand circularly polarized terahertz second - harmonic - frequency vortex light (corresponding to the first second - harmonic component) radiated by the metasurface is 3, and the focal length is 4 / 3 f 0 = 120 mm. As shown in, for example, Figures 7a-7c where, Figure 7aThe focusing behavior of right-handed circularly polarized terahertz second-harmonic vortex light at different depths. Figure 7b This represents the intensity distribution of right-handed circularly polarized terahertz second-harmonic vortex light on the focal plane. Figure 7c The phase distribution of right-hand circularly polarized terahertz second-harmonic vortex light on the focal plane is given. The topological charge (second harmonic component) of left-hand circularly polarized terahertz second-harmonic vortex light radiated from the metasurface is 1, and the focal length is 4. f 0 = 360mm, for example Figures 8a-8c As shown, where, Figure 8a The focusing behavior of left-handed circularly polarized terahertz second-harmonic vortex light at different depths. Figure 8b This represents the intensity distribution of left-handed circularly polarized terahertz second-harmonic vortex light on the focal plane. Figure 8c This represents the phase distribution of left-handed circularly polarized terahertz second-harmonic vortex light on the focal plane. Using the designed metasurface, simultaneous radiation of the terahertz fundamental and second harmonic frequencies was achieved, and wavefronts of different frequencies and directions of rotation were simultaneously modulated, resulting in different focal lengths and vortex topological charges.

[0052] In this embodiment of the invention, by designing the spatial distribution of the rotation angle of the structural elements, a variety of wavefront modulation functions can be realized, including but not limited to beam deflection, focusing, and generation of vortex beams carrying orbital angular momentum. At the same time, different frequencies and polarization channels can correspond to different propagation directions, focal lengths, or topological charge numbers, thus having broad application prospects in terahertz communication, imaging, and information processing.

[0053] The present invention also provides a metasurface device for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, including any of the metasurfaces for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency described in this application.

[0054] Metasurface devices include encapsulated metasurfaces and may also include other structures to support the operation of the metasurface.

[0055] The present invention also provides a metasurface system for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, including any of the metasurface devices for coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency described in this application.

[0056] In addition to the metasurface device, the metasurface system may also include a light source and a controller. The controller is used to control the light source to generate a circularly polarized terahertz wave of a specified frequency and to incident the circularly polarized terahertz wave onto the metasurface in the metasurface device. Furthermore, when the terahertz wave output by the metasurface uses a special terahertz optical fiber as the transmission medium, the metasurface system may also include the optical fiber and its supporting structures.

[0057] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without any creative effort.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A metasurface with coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, characterized in that, Comprising: A plurality of structural elements, each of the structural elements being arranged on the same two-dimensional plane in a two-dimensional arrangement; the rotation angles of each of the structural elements with respect to the same reference direction in the two-dimensional plane are not all the same; When the structural element is irradiated by circularly polarized terahertz waves, the transmission end of the structural element outputs terahertz waves including at least a fundamental frequency component and a second harmonic component, and the phases of the terahertz waves output by the structural elements with different rotation angles are different.

2. The metasurface according to claim 1, characterized in that, The structural element includes a micro-nano structure and a coupling structure, and the coupling structure is within the near-field coupling range of the micro-nano structure; When the micro-nano structure is irradiated by circularly polarized terahertz waves, a locally enhanced electromagnetic field is generated, and free carriers in the coupling structure generate a non-linear current under the action of the locally enhanced electromagnetic field, and terahertz waves including at least a fundamental frequency component and a second harmonic component are output from the transmission end.

3. The metasurface according to claim 2, characterized in that, The micro-nano structure is arranged around the coupling structure on the two-dimensional plane, and the micro-nano structure has an opening on the two-dimensional plane.

4. The metasurface according to claim 1, characterized in that, Different components of the terahertz wave output by the structural element correspond to different carrier channels, and the phases of the components of each carrier channel are uniquely determined by the rotation angle of the structural element.

5. The metasurface according to claim 4, characterized in that, The terahertz wave output by the structural element includes: a fundamental frequency cross-circularly polarized component, a first second harmonic component, and a second second harmonic component; the frequency of the fundamental frequency cross-circularly polarized component is ω, and the circular polarization rotation direction is opposite to the incident circular polarization rotation direction; the frequency of the first second harmonic component is 2ω, and the circular polarization rotation direction is opposite to the incident circular polarization rotation direction; the frequency of the second second harmonic component is 2ω, and the circular polarization rotation direction is the same as the incident circular polarization rotation direction; ω is the frequency of the circularly polarized terahertz wave, and the incident circular polarization rotation direction is the circular polarization rotation direction of the circularly polarized terahertz wave.

6. The metasurface according to claim 5, characterized in that, The rotation angle of the structural element is θ, the phase of the fundamental frequency cross-circularly polarized component is 2σθ, the phase of the first second harmonic component is 3σθ, and the phase of the second second harmonic component is σθ, where σ represents the polarization state of the circularly polarized terahertz fundamental wave, σ = +1 represents the left-handed circular polarization state, and σ = -1 represents the right-handed circular polarization state.

7. The metasurface according to claim 1, characterized in that, The metasurface includes a plurality of periodic units, each periodic unit includes N structural elements, and the N structural elements are arranged on the same straight line. The difference in the rotation angles of adjacent structural elements in the same periodic unit is α, and α×N = 360°.

8. The metasurface according to claim 1, characterized in that, Each of the structural elements forms a plurality of "mouth"-shaped units of different sizes, and the plurality of "mouth"-shaped units are arranged in a nested manner centered on the coordinate origin of the metasurface. The rotation angle θ of the structural element with coordinates (x, y) is: ； Where r is the distance between the structural primitive and the origin of the coordinate system. k w The wave vector of the fundamental frequency wave, f 0 represents the focal length of the fundamental frequency wave.

9. A metasurface device with coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, characterized in that, Including the metasurface according to any one of claims 1-8.

10. A metasurface system with coordinated phase modulation of terahertz fundamental frequency and second harmonic frequency, characterized in that, Including: The metasurface device according to claim 9.

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