Low-frequency light-driven terahertz radiation source and generation wave method, preparation method

Abstract

This invention provides a low-frequency optically driven terahertz radiation source and a method for generating and fabricating waves, relating to the field of terahertz optoelectronic devices. The source comprises, from top to bottom, a topological insulator thin film layer, a metal metaarray layer, and a substrate, with the three layers tightly bonded together. The overall outline of the metal metaarray layer is consistent with the shape of the substrate. The metal metaarray layer is configured with a metal open-loop resonant ring array component fabricated by micro-nano machining. The array's geometric cavity enables the low-frequency pump light incident beam to be focused into a high-density focused beam. The entire surface of the topological insulator thin film layer completely covers the metal metaarray layer, and under the influence of the high-density focused beam, it directionally radiates terahertz electromagnetic waves outward along the positive direction of the film surface normal. This invention uses low-frequency continuous light or low-energy pulsed light for driving, eliminating dependence on high-peak-power femtosecond laser systems. The enhanced metal metafield and the strong nonlinearity and spin-polarized current of the topological insulator work together to improve conversion efficiency.

Description

Technical Field

[0001] This invention relates to the field of terahertz optoelectronic device technology, and more specifically, to a low-frequency optically driven terahertz radiation source and a method for generating and preparing it. Background Technology

[0002] Terahertz (THz) waves typically refer to electromagnetic waves with frequencies between 0.1 THz and 10 THz. Their spectrum lies between microwaves and infrared light, possessing both electronic and photonic characteristics. They exhibit unique advantages such as low photon energy, non-ionization, high penetration, broadband spectral response, and material fingerprint spectral features, making them irreplaceable in fields such as 6G / terahertz communication, biomedical imaging, non-destructive testing of semiconductor chips, security and inspection, aerospace, and materials science. Achieving a room-temperature operating, high-efficiency, broadband, miniaturized, low-cost, and integrable terahertz radiation source is the core key to propelling terahertz technology from the laboratory to practical engineering applications.

[0003] Currently, the mainstream terahertz radiation generation technologies mainly include the following categories: 1) Photoconductive antenna technology: This technology uses femtosecond laser pulses to irradiate semiconductor materials (such as GaAs and InGaAs) to excite charge carriers, forming transient photocurrents under an applied bias electric field, which then radiate terahertz waves. This technology is mature, but it has significant drawbacks: it requires an external high-voltage electric field, has a large system size, narrow radiation bandwidth, low energy conversion efficiency, is difficult to integrate on a chip, and relies on high-peak-power femtosecond lasers, resulting in high equipment costs.

[0004] 2) Nonlinear optical crystal rectification technology: This technology utilizes the second-order nonlinear optical effects of nonlinear crystals such as LiNbO3, ZnTe, and GaP to convert the difference frequency of ultrashort pulse lasers into terahertz waves. This technique can achieve broadband terahertz output, but it requires strict phase-matching conditions; its essence lies in the conservation of momentum between the pump light and the terahertz wave. Collinear propagation requires a certain refractive index of the pump group. Terahertz refractive index satisfy ( It is the wavelength of the terahertz band. (The wavelength of the pump light), or by using tilted pulse leading-edge technology to meet the requirements. ( (Critical angle / Cherenkov angle), this condition is highly sensitive to parameters such as crystal orientation, temperature, and pump wavelength, with an extremely narrow matching window; the energy conversion efficiency is extremely low, typically only [value missing]. (0.001%-0.1%); High-efficiency solutions (such as LiNbO3) require complex pulse shaping and tilted pulse leading edge systems, resulting in large device size and poor integration.

[0005] 3) Air Plasma Terahertz Source: This technology utilizes a powerful femtosecond laser to ionize air and form plasma, generating terahertz waves through four-wave mixing and other effects. While this technology can achieve ultra-wideband terahertz radiation, it requires an extremely high peak power femtosecond laser, resulting in a complex system with poor stability, making it unsuitable for portable and integrated applications.

[0006] 4) Metamaterial / Metasurface Terahertz Sources: These sources achieve optical field localization and electric field enhancement based on artificial subwavelength structures, generating terahertz waves using the material's own nonlinear effects. While simple metallic metastructures can achieve field enhancement, the metal's second-order nonlinear polarization is extremely weak, making it difficult to obtain high-power output. Conventional semiconductor materials have limited nonlinear coefficients, making it difficult to effectively radiate terahertz waves under continuous light or low-power, low-frequency light excitation.

[0007] 5) Topological Insulator Terahertz Sources: Topological insulators are a class of quantum materials with topologically protected surface states. They are bulk insulators with high surface conductivity, possessing advantages such as strong spin-momentum locking, high mobility, significant second-order nonlinear optical effects, and no need for phase matching. Terahertz waves can be generated under low-power excitation through nonlinear optical rectification and spin-polarized current. However, single topological insulator thin films have limited light absorption, low optical field coupling efficiency, and the intensity and directionality of terahertz radiation are still difficult to meet the needs of practical applications.

[0008] In summary, existing terahertz radiation sources generally suffer from the following common bottleneck problems: (1) Low energy conversion efficiency makes it difficult to achieve high-power terahertz output; (2) It relies on high peak power femtosecond laser pumping, which has harsh driving conditions, high system cost, and large size; (3) The structure is complex, the phase matching conditions are strict, and the tunability is poor; (4) It is difficult to achieve miniaturization, on-chip integration and flexibility; (5) Terahertz waves have poor directionality and low signal collection efficiency. Summary of the Invention

[0009] Therefore, the purpose of this invention is to address the problems of low conversion efficiency, demanding pumping conditions, and difficulty in integration of existing terahertz radiation sources. It proposes a high-efficiency, broadband, directional, compact, easily integrated, and low-cost terahertz radiation source that can be driven by low-frequency continuous light or low-frequency pulsed light. This source combines a topological insulator with a metallic metaarray and utilizes low-frequency near-infrared or mid-infrared light for driving. Terahertz waves are generated through nonlinear optical rectification and spin-polarized current, achieving stable terahertz output with low-frequency light driving, high efficiency, wide spectrum, tunable polarization, and on-chip integration. This solves the difficulties of traditional terahertz sources, such as low conversion efficiency, reliance on high-peak-power femtosecond lasers, large structural volume, difficulty in integration, and untunable spectrum and polarization. It achieves high-efficiency, wide-spectrum, tunable, integrable, and room-temperature stable terahertz radiation driven by near-infrared / mid-infrared low-frequency light.

[0010] This invention provides a low-frequency optically driven terahertz radiation source, comprising, from top to bottom, a topological insulator thin film layer, a metal metaarray layer, and a substrate, which are assembled into an integrated component by tightly bonding the three layers one by one; the overall outline of the metal metaarray layer is consistent with the shape of the substrate; the metal metaarray layer is configured with a metal open resonant ring array component fabricated by micro-nano machining, which uses the array geometric cavity to focus the low-frequency pump light incident light to form a high-density focused beam; the entire surface of the topological insulator thin film layer completely covers the metal metaarray layer, and under the action of the high-density focused beam, it radiates terahertz electromagnetic waves outward in the positive direction of the normal to the thin film surface.

[0011] Metallic metamaterials are composite structures formed by the periodic arrangement of subwavelength artificial structures. They can excite localized surface plasmon resonances in specific wavelength bands, achieving strong localization of the optical field and order-of-magnitude enhancement of the electric field, thus significantly improving the intensity of light-matter interaction. Topological insulators are a class of quantum materials with special topological electronic states. They are insulators within their bulk, while their surfaces or boundaries possess topologically protected high-mobility conductive states. They also possess excellent properties such as spin-momentum locking, strong second-order nonlinear optical effects, and ultrafast carrier dynamics. Under low-power optical excitation, they can simultaneously generate second-order nonlinear polarization and spin-polarized transient currents, providing a novel physical mechanism for achieving efficient terahertz emission under low-pump conditions. This invention combines topological insulators with metallic metamaterial arrays, utilizing the field enhancement effect of the metastructure to amplify the effective optical field acting on the topological insulator. Under low-frequency continuous light or low-energy pulsed light driving, it can achieve synergistic enhancement and coherent superposition of nonlinear optical rectification and spin-polarized current effects, thereby significantly improving terahertz emission efficiency, power, bandwidth, and directionality.

[0012] Specifically, the subwavelength in this invention refers to the terahertz characteristic wavelength of the device operation. Typical terahertz wavelengths cover 30μm-3mm, and the resonant ring period of this invention is 20-100μm. The overall structural size is significantly smaller than the terahertz operating wavelength, falling within the typical subwavelength structure category. The metal metaarray layer employs a periodic subwavelength open resonant ring structure, which excites local surface plasmon resonances under low-frequency pump light irradiation. A topological insulator thin film layer completely covers the metal metaarray layer and, under the influence of an enhanced optical field, generates broadband terahertz waves through a second-order nonlinear optical rectification effect and a spin-polarized transient current in synergy. The topological insulator thin film is at least one of Bi₂Se₃, Bi₂Te₃, Sb₂Te₃, MnBi₂Te₄, van der Waals heterojunctions of the above materials, or multi-doped systems of the above materials, with a preferred thickness of 1 nm-50 nm. This thickness range is determined by both theoretical calculations and experimental testing. First-principles band structure simulations are used to clarify the critical film thickness and the range for maintaining topological characteristics. Simultaneously, combined with experimental characterization results of thin film deposition, the surface transport properties and photoresponse capability of the material are ensured, avoiding situations where excessive thickness prevents continuous film formation or excessive thickness introduces excessive bulk losses.

[0013] The substrate serves as the support base and can be either a rigid plate or a flexible film material to ensure the flatness of the substrate during subsequent coating and array processing.

[0014] Preferably, the wavelength range of the low-frequency pump light is 700nm-3000nm, which is near-infrared, mid-infrared, or long-wave visible light. The optical driving method is low-energy pulsed light driving or continuous light driving, and the optical power is 1mW-1W, eliminating the need for high peak power femtosecond lasers. This wavelength range is determined based on the material's spectral absorption characteristics and bandgap matching calculations. This band can effectively excite photogenerated carriers, enabling dynamic control of the metamaterial's resonant characteristics. The optical power range is defined through gradient experimental testing. The lower limit meets the effective optical control threshold, while the upper limit avoids thin-film thermal damage and structural deformation failure caused by high thermal power, ensuring stable device operation.

[0015] Furthermore, the metal open resonant ring array component is a periodic array structure integrally formed on a single metal sheet. The periodic array structure is uniformly arranged in a two-dimensional rectangular array on the metal sheet, with rows and columns aligned and evenly spaced. All resonant rings within the same array have the same size.

[0016] Specifically, the metal open-loop resonant ring array component is composed of two-dimensional periodically arranged metal units, which can be any of the following: ring, rod, cross, open-loop resonant ring, square lattice, or nano-gap array, with unit size on the micrometer / submicrometer scale; a typical unit is a square patch. The material of the metal metaarray layer is at least one of gold, silver, aluminum, titanium / gold composite layer, or alloy of the above materials.

[0017] The period, size, duty cycle, and aperture orientation of the metallic metaarray structure are matched with the terahertz emission peak position of the topological insulator, achieving a synergistic improvement in local field enhancement and terahertz radiation efficiency. The main effects of the metallic metaarray include: (1) Local optical field enhancement: Excite local surface plasmon resonance to achieve 10²–10³ times local electric field enhancement, providing efficient low-energy pump for topological insulators; (2) Directional radiation: In the 6–15 μm terahertz band, by optimizing the structural parameters, positive outgoing directional radiation with a divergence angle of <30° can be achieved, thereby improving the terahertz signal collection efficiency; (3) Frequency and polarization control: By adjusting the period and unit size, the terahertz center frequency can be controlled in the range of 0.3–1.0 THz to adapt to different polarization pump light and maximize radiation intensity.

[0018] Furthermore, each individual unit (metal unit) in the metal open resonant ring array component is an annular frame with an opening on one side; the annular frame is integrally formed by metal ribs of equal width (the width of the ribs is uniform), and the interior of the annular frame forms a hollow hollow area, with the hollow being a single unit cavity.

[0019] Preferably, the outer contour side length of the single unit (metal unit) is 0.5P-0.8μm (determining the equivalent resonant frequency), the array period P is 20-100μm (determining the array lattice constant), the metal thickness t is 30-100nm to ensure electrical conductivity and mechanical stability, and the duty cycle f is 0.25-0.64 to determine the metal filling ratio. These parameters were optimized based on electromagnetic simulation calculations. The optimal value range was determined through multi-parameter scanning simulations of the resonant frequency, electromagnetic field localization enhancement effect, and terahertz modulation performance. Simultaneously, the feasibility of micro / nano fabrication technology and experimental data from existing similar devices were considered to balance device performance and fabrication feasibility.

[0020] Preferably, the center frequency, radiation intensity, polarization state, and radiation direction of terahertz waves can be controlled by adjusting the period, size, duty cycle, topological insulator crystal orientation, and pump light polarization of the metal metaarray. Accurate control of terahertz radiation intensity, center frequency, polarization direction, and radiation directionality can be achieved by adjusting the period, size, gap, opening direction, and rotation angle of the metal metastructure.

[0021] Specifically, by adjusting the period P, unit size a, and duty cycle f of the metallic metaarray layer, precise control of terahertz radiation intensity, center frequency, and directivity can be achieved. The quantification relationship is as follows: (1) Center frequency modulation: The resonant frequency of the metallic metastructure satisfies ,in, L , C Determined by the element size 'a'; Terahertz center frequency fc With array period P satisfy Increasing the period or unit size can shift the center frequency to a lower frequency.

[0022] (2) Radiation intensity control: Terahertz radiation intensity Where γ is the local electric field enhancement factor, Q The resonant quality factor. Duty cycle; optimizing the duty cycle can enhance the local field enhancement effect, thereby increasing the radiation intensity.

[0023] (3) Directional control: direction of the terahertz radiation main lobe Satisfy the diffraction formula The directional radiation angle can be controlled by adjusting the period P.

[0024] Furthermore, the single-sided opening is located at the middle of any side of the annular frame, the opening width is consistent with the width of the rib, and the opening penetrates through the side wall of the annular frame, so that the closed square ring forms a discontinuous opening configuration.

[0025] All resonant ring units within the array have completely uniform opening widths and ring rib thicknesses, and all units in the array have consistent specifications with no irregularly shaped rings.

[0026] Furthermore, an interface control layer is sandwiched between the topological insulator layer and the metal metaarray layer. The interface control layer is an ultra-thin insulating pad used to isolate the two components, the topological insulator layer and the metal metaarray layer, forming an assembly gap between the two layers, reducing light energy loss, and thus optimizing field enhancement and terahertz emission efficiency.

[0027] The interface modulation layer is an insulating layer or an optical matching layer, and the material includes at least one of Al2O3, SiO2, and MgF2, with a preferred thickness of 1 nm-10 nm. This thickness is determined based on interface energy level modulation, carrier transport simulation calculations, and experimental rules for the fabrication of ultrathin dielectric / functional thin films. This thickness range can effectively achieve interface charge modulation and bandgap optimization. The thickness is controllable and the process is mature, avoiding the problem of excessive layer thickness hindering carrier coupling and excessive layer thickness losing the interface modulation effect.

[0028] Furthermore, the two-dimensional rectangular array area of ​​the periodic array structure is provided with an effective working surface of the resonant ring structure in the middle. The edge of the two-dimensional rectangular array area is reserved with a blank sealing edge (preferably 10-50μm) for assembly, bonding and positioning. The blank sealing edge has no resonant ring structure, which facilitates alignment and bonding with the interface control layer.

[0029] Furthermore, the substrate is at least one of SiO2, sapphire, silicon, quartz, PI or PDMS, and has a thickness of 100μm-1000μm.

[0030] This thickness was determined by combining optical loss simulation, mechanical structural characteristics, and general engineering specifications. This thickness range allows for controllable transmission loss in both the pump light and terahertz bands, while also meeting the mechanical strength requirements for micro / nano fabrication and device packaging. It represents a conventional and mature thickness range for infrared and terahertz functional devices.

[0031] Taking a square-shaped single unit (metal unit) as an example, the main geometric parameters of the low-frequency optically driven terahertz radiation source of this invention are as follows: Table 1

[0032] The low-frequency optically driven terahertz radiation source of the present invention operates stably at room temperature and in atmospheric environment, without the need for vacuum, low temperature, or high pressure bias, and has strong environmental adaptability and long service life.

[0033] The present invention also provides a method for generating waves using a low-frequency optically driven terahertz radiation source, based on the low-frequency optically driven terahertz radiation source as described above, comprising: The metallic metaarray layer forms a locally enhanced optical field under illumination by circularly or linearly polarized low-frequency pump light. This locally enhanced optical field acts on the topological insulator thin film layer, generating a spin-polarized transient current based on the second-order nonlinear optical rectification effect dominated by the surface states of the topological insulator thin film layer through its second-order nonlinear polarization response. This transient terahertz current is formed through spin-charge conversion, thus merging two low-frequency pump light beams of different frequencies. , Difference frequency converted to frequency Terahertz waves satisfy the law of conservation of energy. .

[0034] Through the synergistic effect of second-order nonlinear optical rectification and spin-polarized transient current, the spin-polarized current contributes an additional terahertz radiation source (and is sensitive to low-energy pump light), significantly enhancing the output intensity. The metallic metaarray layer excites and generates an enhanced optical field, which, in the case of low-energy pump light... High efficiency is achieved by generating coherent terahertz radiation.

[0035] Furthermore, the process of forming a transient terahertz current through spin-charge conversion includes: The enhanced optical field generated by the metal metaarray layer excites the surface state electrons of the topological insulator, generating spin-polarized charge carriers (spin direction locked to optical polarization). The spin-polarized charge carriers diffuse rapidly in the surface states and are converted into transient charge currents through spin-orbit coupling effects. ; transient current With nonlinear optical rectified current Coherent superposition results in high-intensity terahertz radiation.

[0036] Furthermore, the enhanced optical field generated by the metal metaarray layer excites the surface state electrons of the topological insulator to generate spin-polarized charge carriers, comprising the following steps: The metallic meta array layer locally amplifies the pump light field by 10. 2 -10 3 The effect is multiplied, acting on the topological insulator thin film layer; The non-centrosymmetric lattice structure of the topological insulator thin film layer breaks the inversion symmetry, resulting in second-order nonlinear polarization. ,in, It is the second-order nonlinear polarization intensity; It is the time-varying electric field of the pump light incident on the nonlinear medium; It is a second-order nonlinear polarizability. The magnitude of the radiation directly determines the intensity of the terahertz radiation. The larger the value, the more it produces under the same pump light. The stronger the terahertz power, the higher the output power; at the same time, The tensor components determine the polarization state and phase-matching condition of the terahertz wave. The second-order nonlinear polarizability of the topological insulator... It is significantly higher than ordinary metals and does not require strict phase matching, allowing it to operate efficiently under low-energy pumping.

[0037] The second-order nonlinear polarization intensity as a function of time Excite alternating current It radiates terahertz electromagnetic waves; among them, Transient current density; is the partial derivative of the transient current density with respect to time, and represents the rate of change of polarization intensity over time.

[0038] This invention also provides a method for preparing a low-frequency optically driven terahertz radiation source, which includes the following steps: Substrate cleaning: Select SiO2, sapphire, silicon, quartz, PI or PDMS as the substrate, and clean it with acetone, ethanol and deionized water in sequence using ultrasonic cleaning, and then dry it with nitrogen. Metal layer deposition: Deposit a thin metal film on the substrate surface using electron beam evaporation, magnetron sputtering or thermal evaporation. The material is Au, Ag, Al or Ti / Au composite layer with a thickness of 30nm-200nm. Metaarray fabrication: Periodic subwavelength metal metaarrays are fabricated using electron beam lithography, ultraviolet lithography, nanoimprinting, or lift-off processes. Interface control layer preparation: An Al2O3 or SiO2 thin layer is deposited on a metal metaarray by atomic layer deposition or plasma deposition to obtain the interface control layer; Topological insulator thin film growth: Topological insulator thin films are grown on top of a metal metaarray by molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, solution method or mechanical exfoliation. Annealing and Packaging: After low-temperature annealing to optimize interface coupling, terahertz radiation source devices are obtained after packaging.

[0039] This invention relates to a low-frequency optically driven terahertz radiation source capable of outputting stable terahertz waves under the drive of near-infrared or mid-infrared low-frequency continuous light or low-frequency pulsed light. The terahertz waves generated by the low-frequency optically driven terahertz radiation source have a frequency band of 0.1 THz-5 THz, a divergence angle ≤30°, and an energy conversion efficiency ≥1×10⁻⁻⁻⁶. 5 Specifically, the polarization, spectrum, and radiation direction of terahertz waves can be controlled by the parameters of the metal metastructure, the crystal orientation of the topological insulator, and the polarization of the pump light.

[0040] To verify the effectiveness of the present invention, the key structural features (including structural parameters) of the present invention were tested. The testing process is as follows: The substrate is SiO2 / Si with a thickness of 500 μm; the metal metaarray layer is an Au square patch array with a period of 50 μm, a unit size of 30 μm, and a thickness of 50 nm; the topological insulator thin film layer is a Bi2Se3 thin film with a total thickness of 10 nm, which is directly covered on the metal metaarray layer. Pumping conditions are: Pump light: 1064nm continuous fiber laser, average power 500mW Polarization: linear polarization, along the edge direction of the metallic metaunit.

[0041] The terahertz radiation parameters are shown in Table 2: Table 2

[0042] Removing the topological insulator thin film layer (using only the metal metaarray layer): the terahertz signal intensity drops to below 15%, proving that second-order nonlinear effects dominate; Changing the pump light polarization to circular polarization increases the terahertz intensity by 20%, proving that the spin polarization current makes a significant contribution. Adjusting the period of the metal metastructure to 60 μm: the center frequency dropped to 0.50 THz, verifying the ability of structural parameters to control the frequency.

[0043] The test parameters, their quantitative relationship, the test process, and the test results are as follows: Bi₂Se₃ powder was weighed, and a topological insulator film with a thickness of 10 nm was prepared by molecular beam epitaxy. A gold square unit array metallic metastructure was prepared on the surface of the film with the following parameters: period P = 50 µm, unit size a = 30 µm, and duty cycle f = 0.36. Pumped by 1064 nm continuous-wave light, the detected terahertz center frequency was 0.60 THz, the radiation intensity was the reference value of 1.00, and the main lobe radiation direction was 0° (normal emission).

[0044] The period of the metal metaarray layer was adjusted to P=60µm, and the other parameters were the same as in Example 1. The detected terahertz center frequency was 0.50THz, the radiation intensity was 0.85, and the main lobe radiation direction was 0° (normal emission).

[0045] The metal unit size of the metal metaarray layer was adjusted to a=35µm, and the other parameters were the same as in Example 1. The detected terahertz center frequency was 0.52THz, the radiation intensity was 1.20, and the main lobe radiation direction was 0° (normal emission).

[0046] The metal unit size of the metal metaarray layer was adjusted to a=25µm, and the other parameters were the same as in Example 1. The detected terahertz center frequency was 0.65THz, the radiation intensity was 0.70, and the main lobe radiation direction was 0° (normal emission).

[0047] The period of the metal metaarray layer was adjusted to P=60µm, the unit size was adjusted to a=35µm, the duty cycle was f=0.34, and the other parameters were the same as in Example 1. The detected terahertz center frequency was 0.48THz, the radiation intensity was 1.10, and the main lobe radiation direction was ±15° (bidirectional radiation).

[0048] The technical solution of this invention has formed a mature, stable, mass-producible terahertz radiation source device suitable for integrated systems.

[0049] The present invention also provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method for generating waves using a low-frequency optically driven terahertz radiation source as described above.

[0050] The present invention also provides a computer device, the computer device including a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor executes the program to implement the steps of the method for generating waves using a low-frequency optically driven terahertz radiation source as described above.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a low-frequency optically driven terahertz radiation source and its fabrication and wave generation methods. It can be driven by low-frequency continuous light or low-energy pulsed light, eliminating dependence on high-peak-power femtosecond laser systems, significantly simplifying the driving conditions, and reducing system cost, size, and complexity. The combined effect of metallic metafield enhancement (10²-10³ times) and the strong nonlinearity and spin polarization current of the topological insulator results in an energy conversion efficiency far exceeding that of a single topological insulator or a single metastructure, significantly improving the conversion efficiency. The output frequency band covers 0.1THz-5THz. By adjusting the parameters of the metastructure, the terahertz center frequency, radiation intensity, and directionality can be flexibly changed; through the diffraction modulation of the periodic metaarray, narrow beam and positive emission directional radiation of terahertz waves can be achieved, greatly improving signal collection efficiency; the overall structure is a planar thin film, which is thin and can be directly integrated with chips, optical fibers, and imaging systems, making it suitable for portable and wearable devices; it can be fabricated using mature micro-nano fabrication processes such as magnetron sputtering, electron beam evaporation, photolithography, molecular beam epitaxy (MBE), and chemical vapor deposition (CVD), making it suitable for large-scale production and applicable to a wide range of scenarios. Attached Figure Description

[0052] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention.

[0053] In the attached diagram: Figure 1 A schematic diagram of the overall layered structure of a low-frequency optically driven terahertz radiation source according to an embodiment of the present invention; Figure 2 Axonometric schematic diagram of the composite structure of a topological insulator thin film layer and a metal metaarray layer according to an embodiment of the present invention; Figure 3 A top view of a metal open-loop resonant ring array component according to an embodiment of the present invention; Figure 4 A schematic diagram illustrating the terahertz radiation mechanism generated by the composite structure of a topological insulator thin film layer and a metal metaarray layer in an embodiment of the present invention; Figure 5 Terahertz time-domain response curves of embodiments of the present invention; Figure 6 Terahertz spectral intensity distribution curves of this invention embodiment; Figure 7 A schematic diagram of the structure in an embodiment of the present invention showing that the topological insulator thin film layer completely covers the metal metaarray layer.

[0054] The markings in the attached figure are as follows: 1. Topological insulator thin film layer, 2. Metal metaarray layer, 3. Substrate, 4. Metal open resonant ring array component. Detailed Implementation

[0055] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. Rather, they are merely examples of apparatuses and products consistent with some aspects of this disclosure as detailed in the appended claims.

[0056] The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. The singular forms “a,” “the,” and “the” as used in this disclosure and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0057] It should be understood that although the terms first, second, third, etc., may be used in this disclosure to describe various information, such information should not be limited to these terms. These terms are used only to distinguish information of the same type from one another. For example, without departing from the scope of this disclosure, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0058] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0059] Example This invention provides a low-frequency optically driven terahertz radiation source with a composite structure, such as... Figure 1 , Figure 2 The diagram shows a topological insulator thin film layer 1, a metal metaarray layer 2, and a substrate 3 arranged sequentially from top to bottom. These three layers are tightly bonded together to form an integrated assembly. The overall outline of the metal metaarray layer 2 is consistent with the shape of the substrate 3. The metal metaarray layer 2 is equipped with a metal open-loop resonant ring array component 4 (such as...) fabricated using micro / nano machining. Figure 3As shown, the metal open-ended resonant ring array component 4 is a periodic array structure integrally formed on a single sheet of thin metal. The periodic array structure is uniformly arranged in a two-dimensional rectangular array on the thin metal sheet, with rows and columns aligned and evenly spaced. All resonant ring units within the same array have the same size. Each unit (metal element) in the metal open-ended resonant ring array component 4 is a ring frame with an opening on one side (preferably a four-sided rectangular ring with a notch on one side). The opening is located at the middle of any side of the ring frame, and the width of the opening is consistent with the width of the rib. The opening penetrates the side wall of the ring frame, creating a discontinuous opening configuration for the closed square ring. The effective working surface of the resonant ring structure is arranged in the middle of the two-dimensional rectangular array area of ​​the periodic array structure. The edges of the two-dimensional rectangular array area are reserved with blank sealing edges for assembly, bonding, and positioning. The blank sealing edges do not have resonant ring structures, facilitating alignment and bonding with the interface control layer. The ring frame is integrally formed from metal ribs of equal width, and the interior of the ring frame forms a hollow hollow area, with the hollow being a single-unit cavity. The low-frequency pump light incident beam is focused into a high-density focused beam by relying on the arrayed geometric cavity; the entire surface of the topological insulator thin film layer 1 completely covers the metal metaarray layer 2 (e.g., Figure 7 As shown, terahertz electromagnetic waves are radiated outward in the positive direction of the normal to the thin film surface under the action of high-density focused light beams.

[0060] The metal metaarray layer 2 employs a periodic subwavelength open-loop resonant ring structure. The subwavelength unit size is much smaller than the pump light wavelength (1064nm / 1550nm), avoiding diffraction loss and achieving localization of the optical field. The two-dimensional periodic array ensures the uniformity of the enhanced optical field, facilitating large-area fabrication and large-scale application. Optimized unit size and topological insulator film thickness ensure efficient penetration of the enhanced optical field to the topological insulator surface. Localized surface plasmon resonance is excited under low-frequency pump light irradiation. The topological insulator film layer 1 completely covers the metal metaarray layer 2 and, under the action of the enhanced optical field, generates broadband terahertz waves through the synergistic effect of second-order nonlinear optical rectification and spin-polarized transient current.

[0061] This invention combines a topological insulator with a metallic metaarray (e.g., Figure 4 As shown, by utilizing the field enhancement effect of the metastructure to amplify the effective optical field acting on the topological insulator, the synergistic enhancement and coherent superposition of the nonlinear optical rectification effect and the spin polarization current effect can be achieved under the drive of low-frequency continuous light or low-energy pulsed light, thereby significantly improving the terahertz emission efficiency, power, bandwidth and directivity.

[0062] An interface control layer is sandwiched between the topological insulator layer and the metal metaarray layer 2. The interface control layer is an ultra-thin insulating pad used to isolate the two components, the topological insulator layer and the metal metaarray layer 2, forming an assembly gap between the two layers, reducing light energy loss, and thus optimizing field enhancement and terahertz emission efficiency.

[0063] The working mechanism of the low-frequency optically driven terahertz radiation source in this invention is as follows: enhancement by a metallic meta-field, second-order nonlinear optical rectification by a topological insulator, the combined effect of spin-polarized transient current, and coherent superposition, as detailed below: (1) Low-frequency pump light (700nm-3000nm) is incident on the metal metaarray layer 2 to excite local surface plasmon resonance, and the light field is highly localized in the gaps and surfaces of the metal structure to achieve 10²-10³ times electric field enhancement. (2) Enhanced light field acting on topological insulator thin films excites two terahertz radiation mechanisms: ① Second-order nonlinear optical rectification effect: The non-centrosymmetric lattice structure of the topological insulator breaks the inversion symmetry and generates strong second-order nonlinear polarization, which converts the pump light difference frequency into a terahertz wave without strict phase matching. ② Spin-polarized transient current: Spin-polarized charge carriers on the surface state of a topological insulator are photo-excited and form an ultrafast transient terahertz current source through spin-charge conversion; (3) The terahertz signals generated by the two terahertz radiation mechanisms are coherently superimposed to form a high-intensity, broadband, directional terahertz wave that radiates outward.

[0064] The performance parameters of the low-frequency optically driven terahertz radiation source in this embodiment are as follows: ① Driving light source: near-infrared / mid-infrared low-frequency continuous light or low-frequency pulsed light, wavelength 700nm-3000nm, optical power 1mW-1W; ② Output terahertz frequency band: 0.1THz-5THz; ③ Energy conversion efficiency: ≥1×10⁻ 5 Up to 5×10⁻ 5 ; ④ Radiation directionality: The main lobe is positively emitted, and the divergence angle is ≤30°, preferably ≤10°; ⑤ Operating conditions: Stable operation at room temperature and atmospheric conditions, no vacuum or low temperature required; ⑥ Adjustability: The terahertz center frequency, intensity, polarization, and radiation direction can be controlled by adjusting the period, size, duty cycle, crystal orientation of the topological insulator, and polarization of the pump light.

[0065] This invention also provides a method for generating waves using a low-frequency optically driven terahertz radiation source, based on the low-frequency optically driven terahertz radiation source as described above, including: When the metal metaarray layer 2 is irradiated by circularly or linearly polarized low-frequency pump light, a localized enhanced optical field is formed. This localized enhanced optical field acts on the topological insulator thin film layer 1. Through the second-order nonlinear polarization response of the topological insulator thin film layer 1, and based on the second-order nonlinear optical rectification effect dominated by the surface states of the topological insulator thin film layer 1, a spin-polarized transient current is generated. This transient terahertz current is formed through spin-charge conversion, converting the difference frequency of the two low-frequency pump lights of different frequencies into a terahertz wave of a certain frequency, thus satisfying the energy conservation.

[0066] The process of forming transient terahertz current through spin-charge conversion includes: The enhanced optical field generated by the metal metaarray layer 2 excites the surface state electrons of the topological insulator, generating spin-polarized charge carriers. Spin-polarized charge carriers diffuse rapidly in the surface states and are converted into transient charge currents through spin-orbit coupling effects. The transient current and the nonlinear optical rectified current coherently superimpose to form high-intensity terahertz radiation.

[0067] The process of generating spin-polarized charge carriers by exciting surface state electrons in a topological insulator through an enhanced optical field generated by the metal metaarray layer 2 includes the following steps: The metal meta array layer 2 amplifies the pump light field locally by 10² to 10³ times, acting on the topological insulator thin film layer 1. The non-centrosymmetric lattice structure of the topological insulator thin film layer 1 breaks the inversion symmetry and generates second-order nonlinear polarization, where is the second-order nonlinear polarization intensity; is the time-varying electric field of the pump light incident on the nonlinear medium; is the second-order nonlinear polarizability, and the magnitude of directly determines the intensity of terahertz radiation. The larger the value, the stronger the terahertz output power under the same pump light; simultaneously, the tensor component of determines the polarization state and phase matching condition of the terahertz wave. The second-order nonlinear polarizability of topological insulators is significantly higher than that of ordinary metals, and strict phase matching is not required, enabling efficient operation under low-energy pumping.

[0068] The time-varying second-order nonlinear polarization intensity excites an alternating current, which radiates terahertz electromagnetic waves; where is the transient current density; is the partial derivative of the transient current density with respect to time, representing the time-varying rate of polarization intensity.

[0069] The intensity, center frequency, and directivity of terahertz radiation can be controlled by adjusting the period P, unit size a, and duty cycle f of the metallic metaarray layer 2, as detailed below: (1) Center frequency control: The resonant frequency of the metal metastructure satisfies the condition that L and C are determined by the unit size a; the terahertz center frequency fc satisfies the condition that the array period P. Increasing the period or the unit size can shift the center frequency to a lower frequency.

[0070] (2) Radiation intensity regulation: Terahertz radiation intensity, where γ is the local electric field enhancement factor, Q is the resonant quality factor, and is the duty cycle; optimizing the duty cycle can enhance the local field enhancement effect, thereby increasing the radiation intensity.

[0071] (3) Directional control: The direction of the main lobe of terahertz radiation satisfies the diffraction formula, and the directional radiation angle can be controlled by adjusting the period P.

[0072] Figure 5 The terahertz time-domain response curves of an embodiment of the present invention are shown; Figure 6 The terahertz spectral intensity distribution curve of an embodiment of the present invention is shown.

[0073] This invention also provides a method for preparing a low-frequency optically driven terahertz radiation source, which includes the following steps: Step 1: Substrate 3 selection and pretreatment: Select a rigid or flexible substrate 3, and ultrasonically clean it in sequence with acetone, ethanol, and deionized water, dry it with nitrogen, and perform oxygen plasma surface modification to improve surface smoothness and adhesion.

[0074] Step 2: Fabrication of the metal metaarray: Metapatterns are defined using photolithography, electron beam lithography, or nanoimprinting processes. Metal thin films are deposited by electron beam evaporation or magnetron sputtering, and periodic subwavelength metal metaarrays are formed by peeling or etching.

[0075] Step 3: Preparation of the interface control layer: 1nm-10nm insulating matching layers are prepared by atomic layer deposition (ALD) or chemical vapor deposition (CVD).

[0076] Step 4: Preparation of topological insulator thin films: Molecular beam epitaxy (MBE), magnetron sputtering, chemical vapor deposition (CVD), or pulsed laser deposition (PLD) are used to completely cover and grow 1nm-50nm topological insulator films on metal metaarrays. The growth temperature, beam current ratio, and deposition rate are controlled to ensure crystal quality.

[0077] Step 5: Post-processing and packaging: Vacuum annealing optimizes crystallization performance, and inert gas encapsulation enhances device stability and lifespan.

[0078] Application examples The low-frequency optically driven terahertz radiation source of this invention can be applied to a variety of scenarios. Several typical application scenarios are listed below.

[0079] Application Example 1: Substrate 3 is made of double-sided polished sapphire with a thickness of 430μm; The metal metaarray layer 2 is an Au open-loop resonant ring array with a period of 2μm, a linewidth of 200nm, and a thickness of 50nm. The topological insulator thin film layer 1 is a Bi2Se3 thin film with a thickness of 10nm, which completely covers the metaarray. The working method is as follows: 1550nm continuous laser pumping with an optical power of 50mW is used to significantly enhance the electric field at two locations in the metal metaarray layer; the Bi2Se3 thin film generates second-order nonlinear polarization and spin polarization currents under the enhanced optical field, radiating broadband terahertz waves of 0.1-3THz.

[0080] Application Example 2: Substrate 3 is a flexible PI film with a thickness of 100 μm; The metal metaarray layer 2 is an Ag nanorod array with a period of 3 μm, a length of 1.5 μm, a width of 200 nm, and a thickness of 40 nm. Topological insulator thin film layer 1 is a Bi2Te3 thin film with a thickness of 8 nm; The operating mode is as follows: driven by 800nm ​​low-frequency pulsed light, the device can still stably output terahertz waves under the condition of a bending radius of 5mm, and is suitable for wearable terahertz sensing and flexible imaging systems.

[0081] The low-frequency optically driven terahertz radiation source of this invention can be widely used in the following scenarios: Terahertz imaging: biomedical imaging, security inspection imaging, non-destructive testing of semiconductor chips, and flexible electronics inspection; Terahertz spectroscopy detection: material fingerprint spectral recognition, gas sensing, biomolecule detection, and pharmaceutical quality testing; Terahertz communication: 6G ultra-wideband communication, short-range high-speed wireless communication, inter-satellite communication; Integrated optoelectronic devices: on-chip terahertz sources, terahertz modulators, terahertz detectors, and terahertz sensing chips.

[0082] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

[0083] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-frequency optically driven terahertz radiation source, characterized in that, The assembly comprises, from top to bottom, a topological insulator thin film layer, a metal metaarray layer, and a substrate, which are tightly bonded together to form an integrated component. The overall outline of the metal metaarray layer is consistent with the shape of the substrate. The metal metaarray layer is equipped with a metal open resonant ring array component fabricated by micro-nano machining, which uses the array geometric cavity to focus the incident low-frequency pump light into a high-density focused beam. The entire surface of the topological insulator thin film layer completely covers the metal metaarray layer, and under the action of the high-density focused beam, it radiates terahertz electromagnetic waves outward in the positive direction of the normal to the thin film surface.

2. The low-frequency optically driven terahertz radiation source according to claim 1, characterized in that, The metal open resonant ring array component is a periodic array structure integrally formed on a single metal sheet. The periodic array structure is uniformly arranged in a two-dimensional rectangular array on the metal sheet, with rows and columns aligned and evenly spaced. All resonant rings within the same array have the same size.

3. The low-frequency optically driven terahertz radiation source according to claim 2, characterized in that, Each unit in the metal open resonant ring array component is an annular frame with an opening on one side; the annular frame is integrally formed from metal ribs of equal width, and the interior of the annular frame forms a hollow area.

4. The low-frequency optically driven terahertz radiation source according to claim 3, characterized in that, The single-sided opening is located at the middle of any side of the annular frame, and the opening width is consistent with the width of the rib. The opening penetrates the side wall of the annular frame, so that the closed square ring forms a discontinuous opening configuration.

5. The low-frequency optically driven terahertz radiation source according to claim 1, characterized in that, An interface control layer is sandwiched between the topological insulator layer and the metal metaarray layer. The interface control layer is an ultra-thin insulating pad used to isolate the two components, the topological insulator layer and the metal metaarray layer, forming an assembly gap between the two layers.

6. The low-frequency optically driven terahertz radiation source according to claim 2, characterized in that, The two-dimensional rectangular array area of ​​the periodic array structure has an effective working surface for the resonant ring structure in the middle; the edges of the two-dimensional rectangular array area are reserved with blank sealing edges for assembly, bonding and positioning.

7. A method for generating waves using a low-frequency optically driven terahertz radiation source, based on the low-frequency optically driven terahertz radiation source as described in any one of claims 1-6, characterized in that, include: The metallic metaarray layer forms a locally enhanced optical field under illumination by circularly or linearly polarized low-frequency pump light. This enhanced optical field acts on the topological insulator thin film layer, generating a spin-polarized transient current based on the second-order nonlinear optical rectification effect dominated by the surface states of the topological insulator thin film layer through its second-order nonlinear polarization response. This transient terahertz current is formed through spin-charge conversion, thus converting two low-frequency pump light beams of different frequencies. , Difference frequency converted to frequency Terahertz waves satisfy the law of conservation of energy. .

8. The method for generating waves using a low-frequency optically driven terahertz radiation source according to claim 7, characterized in that, The process of forming transient terahertz current through spin-charge conversion includes: The enhanced optical field generated by the metal metaarray layer excites the surface state electrons of the topological insulator, generating spin-polarized charge carriers. The spin-polarized charge carriers diffuse rapidly in the surface states and are converted into transient charge currents through spin-orbit coupling effects. ; transient current With nonlinear optical rectified current Coherent superposition results in high-intensity terahertz radiation.

9. The method for generating waves using a low-frequency optically driven terahertz radiation source according to claim 8, characterized in that, The enhanced optical field generated by the metal metaarray layer excites surface state electrons in the topological insulator to generate spin-polarized charge carriers, comprising the following steps: The metallic meta array layer locally amplifies the pump light field by 10. 2 -10 3 The effect is multiplied, acting on the topological insulator thin film layer; The non-centrosymmetric lattice structure of the topological insulator thin film layer breaks the inversion symmetry, resulting in second-order nonlinear polarization. ,in, It is the second-order nonlinear polarization intensity; It is the time-varying electric field of the pump light incident on the nonlinear medium; It is a second-order nonlinear polarizability. The magnitude of the radiation directly determines the intensity of the terahertz radiation. The second-order nonlinear polarization intensity as a function of time Excite alternating current ; radiates terahertz electromagnetic waves; among which, Transient current density; is the partial derivative of the transient current density with respect to time, and represents the rate of change of polarization intensity over time.

10. A method for preparing a low-frequency optically driven terahertz radiation source, used to prepare the low-frequency optically driven terahertz radiation source as described in any one of claims 1-6, characterized in that, Includes the following steps: SiO2, sapphire, silicon, quartz, PI or PDMS are selected as substrates, and ultrasonic cleaning is performed sequentially with acetone, ethanol and deionized water, followed by nitrogen drying. Metal thin films are deposited on the substrate surface by electron beam evaporation, magnetron sputtering or thermal evaporation. The materials are Au, Ag, Al or Ti / Au composite layers with a thickness of 30nm-200nm. Periodic subwavelength metal metaarrays are fabricated using electron beam lithography, ultraviolet lithography, nanoimprinting, or lift-off processes. A thin layer of Al2O3 or SiO2 is deposited on a metal metaarray by atomic layer deposition or plasma deposition to obtain an interface control layer. A topological insulator film is grown to completely cover a metal metaarray using molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, solution method, or mechanical exfoliation. After low-temperature annealing to optimize interface coupling, a terahertz radiation source device is obtained after encapsulation.

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