A multi-dimensional quasi-bic-mediated terahertz modulation device

Abstract

A kind of based on multi-dimension under quasi-BIC mediation terahertz regulating device, including substrate layer and multiple resonance rings, substrate layer surface is coated with photosensitive germanium layer, multiple resonance rings are arranged on photosensitive germanium layer, resonance ring includes mutually parallel and equal length first metal strip and second metal strip, the central position of first metal strip and second metal strip is connected by intermediate arm perpendicular to both, the end surface of first metal strip is vertically connected with the side arm extending towards second metal strip, the end surface of second metal strip is provided with the corresponding part corresponding with side arm and extending towards side arm, there is always gap between corresponding part end and side arm end, corresponding part length and side arm length are not equal and the sum of both is always equal to 86 μm;Or the end surface of second metal strip has no corresponding part, there is gap between side wall and second metal strip end and the length of side arm is equal to 86 μm, the present application, improves the flexibility of application and application flexibility.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of terahertz modulation devices, in particular to a terahertz modulation device based on quasi-BIC mediation under multi-dimension. BACKGROUND

[0002] The bound state in the continuum (BIC) is an open system at a frequency in the radiation continuum, which is still completely bound without any energy leakage. In practical applications, the mode (quasi-BICs) excited by BIC has the characteristics of super-high quality factor and strong local field, which is expected to provide significant application potential in solid-state lasers, nonlinear phenomena, high-performance sensing, chiral enhancement, and narrow-band filtering.

[0003] The electromagnetically induced transparency (EIT) phenomenon is an important quantum interference phenomenon existing in a three-level atomic system, which shows a sharp transmission window in the originally absorption region. In the electromagnetically induced transparency phenomenon, the traditional implementation method requires very strict conditions, such as ultra-low temperature and high-power laser devices, which are extremely difficult to obtain in reality. With the continuous research of metamaterials, in order to solve the problem of traditional experimental conditions in the process of realizing electromagnetically induced transparency, researchers have developed a method of using metamaterials to simulate in some simplified experimental environment to realize electromagnetically induced transparency. This physical phenomenon realized by using metamaterials is called electromagnetically induced transparency-like (EIT-like) because it is similar to electromagnetically induced transparency, but its sensitivity is limited. SUMMARY

[0004] In order to improve the adjustability and application flexibility of the modulation device, the present application provides a terahertz modulation device based on quasi-BIC mediation under multi-dimension, which improves the adjustability and application flexibility.

[0005] In order to achieve the above object, the specific scheme adopted by the present application is: a terahertz modulation device based on quasi-BIC mediation under multi-dimension, comprising a substrate layer and a plurality of resonance rings, the surface of the substrate layer is coated with a photosensitive germanium layer, the plurality of resonance rings are arranged on the photosensitive germanium layer, the resonance ring comprises a first metal strip and a second metal strip which are parallel to each other and have equal lengths, the center positions of the first metal strip and the second metal strip are connected by an intermediate arm perpendicular to both, and the end surface of the first metal strip is vertically connected with a side arm extending towards the second metal strip; the end surface of the second metal strip is provided with a corresponding part corresponding to the side arm and extending towards the side arm, there is a gap between the end of the corresponding part and the end of the side arm, the length of the corresponding part and the length of the side arm are not equal and the sum of the two is always equal to 86μm; or, the end surface of the second metal strip has no corresponding part, there is a gap between the end of the side arm and the end of the second metal strip, and the length of the side arm is equal to 86μm.

[0006] As an optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the width of the corresponding part is equal to the width of the side arm.

[0007] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the end surface of the corresponding part towards the first metal strip is higher than the edge of the second metal strip.

[0008] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the end surface of the corresponding part towards the first metal strip is lower than the edge of the second metal strip, so that a recess area is formed between the corresponding part and the second metal strip.

[0009] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the end surface of the side arm towards the second metal strip is provided with a first graphene strip, and the recess area is provided with a second graphene strip.

[0010] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the width of the first graphene strip and the second graphene strip is equal to the width of the side arm, and the end surface of the second graphene strip towards the side arm is flush with the edge of the second metal strip.

[0011] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the material of the resonance ring is gold, and the thickness of the resonance ring is 0.3μm.

[0012] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension: the material of the substrate layer is polytetrafluoroethylene.

[0013] As another optimization scheme of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension, a transparent protective film coated on the surface of the photosensitive germanium layer is arranged between the resonant ring and the photosensitive germanium layer.

[0014] A preparation method of the above-mentioned terahertz modulation device based on quasi-BIC mediation under multi-dimension, the substrate layer is surface treated; the photosensitive germanium is deposited on the surface of the treated substrate layer by chemical vapor deposition to form a photosensitive germanium layer; gold is deposited on the photosensitive germanium layer by chemical reaction in the gas phase to form a plurality of resonant rings arranged in an array.

[0015] Compared with the prior art, the present application has the following beneficial effects:

[0016] The present application provides a terahertz modulation device based on quasi-BIC mediation under multi-dimension, which can realize single-double valley switching in small amplitude modulation, and the maximum group delay in the double cavity can reach 24.904ps, and the maximum delay bandwidth product is 6.5248; the substrate layer is coated with a photosensitive germanium layer, and the transient photoconductivity of the photosensitive germanium layer is much shorter than the photon lifetime; the photosensitive layer restructures the EIT cavity to realize range regulation of transmission peak-valley amplitude 89.25%; at the same time, by realizing single-double transmission valley switching, the transmittance, absorptivity, frequency response and other characteristics in the terahertz band can be accurately regulated. This regulation capability has important application in the fields of terahertz communication, imaging, sensor, optical modulator, etc., and can improve the performance and functionality of the device, especially providing a new way for efficient terahertz wave modulation and frequency selective regulation; the sensitivity of the regulation device is high, which promotes its potential application in modulators, sensors, filters and dynamic imaging. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a schematic diagram of the resonant ring structure in the regulation device;

[0018] Figure 2 It is a three-dimensional schematic of the wave absorber product, Figure 2 (a) is a schematic diagram of the designed n*n periodic structure, the normal incidence is terahertz wave, and the oblique incidence is laser; (b) is a schematic diagram of each unit; (c) is the geometric parameter setting under the unit structure;

[0019] Figure 3 (a) is the evolution process of the regulation structure, (b) is the evolution process of the transmission spectrum; (c) is the transmission heat map; (d) is the transmission peak-valley amplitude δ T change condition;

[0020] Figure 4 It is the current distribution of the leaky state formed by the intrinsic state and quasi-BIC under (a)-(g) Modele 1-4;

[0021] Figure 5 Contribution of dipole for different resonant ring structures under multi-stage expansion, (a) Module 1; (b) Module 2; (c) Module 3; (d) Module 4;

[0022] Figure 6 (a) Equivalent circuit model according to the difference of the position of the electric field excitation under two modes Mode 1, Mode 3; (b) Resonant frequency change of Mode 1 and Mode 3 in the evolution process;

[0023] Figure 7 Phase and group delay under two kinds of double-resonance valley modules are measured. (a)-(b) Module 2; (c)-(d) Module 4;

[0024] Figure 8 For the amplitude modulation method of the transmission peak 1: use the evolution law between the resonant ring structures, respectively through shearing and supplementing M1 and M2, and changing the carrier concentration of graphene at two places from 1×10 11 cm 2 to 8×10 13 cm 2 . (a) Overall step schematic diagram; (b) Transmission peak from complete disappearance to highest amplitude under different values of M1 and M2; (c) δ T change heat map; (d) Transmission peak resonant frequency corresponding heat map;

[0025] Figure 9 For the amplitude modulation method of the transmission peak 2: under laser irradiation, the photoconductivity of photosensitive germanium is increased from 0 S / m (laser not excited) to 3500 S / m, and the change of transmission spectrum and transmission characteristics corresponding to different modules is observed. (a) Module 3 transmission spectrum; (b) Module 4 transmission spectrum; (c)-(d) Module 3 0-100 S / m, 100 S / m-3500 S / m corresponding transmission cloud picture; (e) Module 3 peak-valley amplitude change; (f)-(g) Module 4 0-100 S / m, 100 S / m-3500 S / m corresponding transmission cloud picture; (h) Module 4 peak-valley amplitude change;

[0026] Figure 10 Change of photoconductivity of photosensitive germanium under Module 4. (a) Phase change; (b) Group delay change;

[0027] Figure 11For practical application test: the sensing characteristics of Ge conductivity of 0 S / m and 400 S / m are analyzed under Module 4, respectively. The refractive index NA is 1.5, the thickness tA of the analyte is increased from 5 μm to 30 μm, and the transmission spectrum and the transmission peak frequency shift are observed. (a) Ge conductivity of 0 S / m; (b) Ge conductivity of 400 S / m; (c) micro-nano structure of the designed metamaterial sensor device; (d) transmission peak resonance frequency shift of 0 S / m; (e) transmission peak resonance frequency shift of 400 S / m;

[0028] Figure 12 For practical application test: the thickness tA of the analyte is 10 μm, the refractive index NA is changed from 1 to 2; 1 μm of polyethylene (n = 1.51) is used above and below to carry the refractive index analyte. The transmission spectrum, transmission cloud chart, and transmission peak resonance frequency change rule following the refractive index are observed, respectively. (a)-(c) Ge conductivity of 0 S / m; (d)-(f) Ge conductivity of 400 S / m.

[0029] The figure mark: 1, substrate layer, 2, photosensitive germanium layer, 3, first metal strip, 4, middle arm, 5, side arm, 6, second metal strip, 601, corresponding part, 7, gap. DETAILED DESCRIPTION

[0030] The technical solutions of the present application will be further described in detail below in combination with specific embodiments. The parts not described and disclosed in detail in the following embodiments of the present application should be understood as the prior art known or should be known by those skilled in the art.

[0031] Example 1

[0032] A terahertz modulation device based on multi-dimensional quasi-BIC mediation includes a substrate layer 1 and a plurality of resonant rings, wherein the material of the substrate layer 1 is polytetrafluoroethylene (PTFE), and the relative dielectric constant thereof is 2.05. In the present embodiment, the thickness of the substrate layer 1 is 1000 μm. The surface of the substrate layer 1 is coated with a photosensitive germanium layer 2 with a thickness of 0.3 μm, and a plurality of resonant rings are arranged on the photosensitive germanium layer 2. The material of the resonant ring is gold, the plasma frequency thereof is 1.37×10 16 rad / s, the collision frequency is 4.08×10 13 rad / s, and it has relatively high stability in the terahertz wave band. In the present embodiment, Au is selected as the metamaterial of the resonant ring, and here the Drude model can be used to obtain:

[0033]

[0034] wherein the plasma frequency ω p is 1.36659×10 16 rad / s, and the collision frequency ω c4.082×10 13 The gold layer has a thickness of 0.3 μm and a refractive index of n = 1.42. The loss tangent tanδ = 0.02, and the substrate layer 1 has a thickness of 25 μm. Without laser irradiation, the photosensitive germanium layer 2 has a dielectric constant ε = 16.3 in the 0.1–1.9 THz frequency band, and its photoconductivity varies with the intensity of the pump light.

[0035] The resonant ring is a symmetrically broken double-opening structure. Specifically, the resonant ring includes a first metal strip 3 and a second metal strip 6 that are parallel to each other and of equal length. The centers of the first metal strip 3 and the second metal strip 6 are connected by an intermediate arm 4 perpendicular to both of them. That is, one end of the intermediate arm 4 is integrally connected to the first metal strip 3, and the other end of the intermediate arm 4 is integrally connected to the second metal strip 6. The intermediate arm 4, the first metal strip 3, and the second metal strip 6 form an "I" shaped structure. Each end face of the first metal strip 3 is perpendicularly connected to a side arm 5 extending toward the second metal strip 6, such as... Figure 1 As shown, the edge of the side arm 5 is integrally connected to the end face of the first metal strip 3. The top end face of the side arm 5 is flush with the outer edge of the first metal strip 3, and the bottom end face of the side arm 5 faces the second metal strip 6. The end face of the second metal strip 6 is provided with a corresponding part 601 that corresponds to the side arm 5 and extends toward the side arm 5. The corresponding part 601 is integrally connected to the end face of the second metal strip 6. The bottom end face of the corresponding part 601 is flush with the outer edge of the second metal strip 6, and the top end face of the corresponding part 601 faces the side arm 5. The width of the corresponding part 601 is equal to the width of the side arm 5, that is, the outer edge of the corresponding part 601 is flush with the outer edge of the side arm 5.

[0036] A gap 7 is always present between the end of the corresponding part 601 and the end of the side arm 5. The lengths of the corresponding part 601 and the side arm 5 are unequal, and their sum is always equal to 86 μm. In this embodiment, the end face of the corresponding part 601 facing the first metal strip 3 is lower than the edge of the second metal strip 6, forming a recessed area between the corresponding part 601 and the second metal strip 6. This structure is as follows: Figure 2 As shown in (c), the length of the side arm 5 is 80 μm, the length of the corresponding part 601 is 6 μm, and the gap 7 between the corresponding part 601 and the side arm 5 is 10 μm. The specific dimensions are shown in Table 1.

[0037] Table 1 Structural parameters of the resonant ring in this embodiment

[0038]

[0039] In this embodiment, a transparent protective film coated on the surface of the photosensitive germanium layer 2 is provided between the resonant ring and the photosensitive germanium layer 2. The transparent protective film is made of aluminum oxide, silicon nitride, or organic polymer. The transparent protective film can prevent the penetration of external substances (such as moisture and oxygen) without affecting the light transmission performance.

[0040] Example 2

[0041] This embodiment is an improved scheme based on Example 1, and the main structure is the same as Example 1. The improvement point is that the end surface of the corresponding part 601 is higher than the edge of the second metal strip 6, as shown in the following figure: Figure 3 (a) The structure of Module 1, Module 2 and Module 3, wherein the length of the side arm 5 in Module 1 is 58 μm, and the length of the corresponding part 601 is 28 μm; the length of the side arm 5 in Module 2 is 73.5 μm, and the length of the corresponding part 601 is 12.5 μm; the length of the side arm 5 in Module 3 is 76.75 μm, and the length of the corresponding part 601 is 9.25 μm.

[0042] Example 3

[0043] This embodiment is an improved scheme based on Example 1, and the main structure is the same as Example 1. The improvement point is that the end surface of the second metal strip 6 has no corresponding part 601, and there is a gap 7 between the side arm 5 and the end of the second metal strip 6, and the length of the side arm 5 is equal to 86 μm, as shown in the following figure: Figure 3 The structure of End in (a).

[0044] Example 4

[0045] This embodiment is an improved scheme based on Example 1, and the main structure is the same as Example 1. The improvement point is that the side arm 5 is provided with a first graphene strip towards the end surface of the second metal strip 6, and a second graphene strip is provided in the recessed area. Specifically, the length of one end of the side arm 5 towards the second metal strip 6 is discarded by 3.75 μm, and a first graphene strip M1 with a length of 3.75 μm is fixed thereon. A second graphene strip M2 with a length of 3.75 μm is provided in the recessed area. The outer edges of the first and second graphene strips are flush with the outer edge of the side arm 5, and the inner edges of the first and second graphene strips are flush with the inner edge of the side arm 5. In this embodiment, the width of the first and second graphene strips is 6 μm, and the first and second graphene strips can be independently controlled by a gate voltage to control the carrier concentration of each other.

[0046] In order to verify the effectiveness of the scheme, the following test data are further explained:

[0047] The surface current is obtained by integrating the current density on the surface of the metal microstructure in the simulation, and then the surface current of the resonant ring is decomposed. The contribution rate of different multipole components of the induced current density is calculated in the Cartesian coordinate system. Numerical calculation is carried out by using three-dimensional finite difference time domain (FDTD) solving software. Periodic boundary conditions are used in the x and y directions, and a perfectly matched layer (PML) absorbing boundary condition is used in the z direction of the free space. Adaptive mesh refinement is used for the resonant ring and the photosensitive germanium layer 2, and the refinement accuracy is 0.5 μm. The structure parameters of the resonant ring are modified, and the dynamic adjustment and sensing characteristics of the transmission type metamaterial device are realized according to the change of the structure parameters and the optical properties of the material. The frequency shift phenomenon of Mode1 and Mode3 in the system is studied according to the circuit model, and the electric field intensity distribution and the corresponding equivalent circuit model are shown in detail:

[0048] Single and double valley switching: Figure 3 (a) describes the change process of the structure under different parameters of the resonant ring. The initial module (XY / C2) represents a double-open resonant ring with mirror symmetry about the xy axis, that is, the length of the side arm 5 and the corresponding part 601 is completely the same (W1=W3=43 μm). At this time, the two-dimensional wave vector k of the momentum space of the C2 symmetric structure has a rotational symmetry of π at the Γ point. The coupling between the bound mode of the symmetric structure and the radiation mode of other symmetry classes is prohibited, and there is no radiation channel, forming a symmetric protection type BIC with infinite quality factor, as shown in Figure 3 (b) Bottom, which only produces an intrinsic mode Mode1 based on the ring itself.

[0049] Increase the length of the side arm 5 and reduce the length of the corresponding part 601, keep the total length of the side arm 5 and the corresponding part 601 as 86 μm, that is, W1+W3=86 μm. It should be noted that W2 changes with W1, and W2 will not be described in the following. The even-symmetric bound state leaks into the odd-symmetric continuous domain in the optical waveguide, and the light escapes into the adjacent waveguide and then diffuses into the continuous domain. BIC-I is a low-order quasi-BIC, and Mode2 is excited at the moment of destruction; BIC-II is a high-order mode, and there is a gap between the two resonant cavities at this time. The interference radiation channel needs higher radiation energy. Guided resonance phenomenon will be coupled with the radiation channel to some extent, and Mode3 is fully excited. When W1=58 μm, it can be seen that the Mode2 radiation channel is completely opened, the line width is increased, and it is reduced to the ordinary resonance level; at this time, Mode3 still has a strong radiation lifetime, and the line width is relatively small.

[0050] With the increase of the degree of symmetry breaking, the eigenmode Mode1 is blue-shifted, while the quasi-BIC mode Mode3 is red-shifted. When W1=73.5 μm, the two resonant valleys meet on the opposite side, showing an EIT-like mode; when W1=76.25 μm, the two modes are degenerate, showing a single valley mode with a wide linewidth; then the two modes move away from each other, Mode3 and Mode1 continue to red-shift and blue-shift respectively, and when W1=80 μm, the two modes are almost completely separated, returning to an EIT-like mode. When the short end metal arm completely disappears, Mode3 also disappears.

[0051] In the above system, Mode1 and Mode3 are taken as the research objects, and the switching of the valleys is achieved from single-double (BIC-II excited)-single (two-mode constructive interference)-double (coupling broken)-single (BIC-II completely disappeared). Figure 3 (c) The process can be visually observed. The δ T The concept of representing the transmission peak-valley amplitude is as follows: Figure 3 (d), it can be seen that δ T In the process of the phase shift of the two resonances from meeting to separation, it first decreases and then increases, and when W1=76.25 μm, it decreases to 0, forming a topologically protected BIC cavity.

[0052] Four special structural parameter settings are introduced to represent four modules respectively:

[0053] Module1 (W1=58 μm & W3=28 μm): double bright mode, quasi-BIC excited;

[0054] Module2 (W1=73.5 μm & W3=12.5 μm): double bright mode, EIT-like formed before the two resonant states meet;

[0055] Module3 (W1=76.75 μm & W3=9.25 μm): single bright mode, two modes interfere constructively and destructively;

[0056] Module4 (W1=80 μm & W3=6 μm): double bright mode, EIT-like window restored after the two modes meet.

[0057] The surface current distributions of the resonant modes existing in the four modules are shown in Figure 4 (a)-(g), first Figure 4(a)-(b) are Module 1, compared with the eigenmodes at 1.272 THz, quasi-BIC excited at 1.762 THz obviously shows stronger electromagnetic response. This is because the radiation energy of the leaky resonance state after the interaction of the closed resonant cavity with the radiation continuum is much larger than that of the ordinary vibration mode at the lowest energy state. The current of Mode 1 mainly gathers in the corresponding part 601, almost maintaining the left-right positive and negative dipole symmetry; the current of Mode 3 appears on the entire metal surface (resonant ring), and the current of the side arm 5 is relatively stronger, showing the resonance characteristics of four dipoles. After evolving into Module 2, the light-matter interaction of Mode 1 excited by quasi-BIC causes the current to strengthen; while Mode 3 gradually degenerates into an ordinary resonance, and the current of the corresponding part 601 basically disappears. Under Module 3, the wave vectors of the two modes match, and Figure 4 (e) It can be seen that the current between the two modes simultaneously gathers at 1.667 THz, and the current intensity of quasi-BIC returns to that of the eigenstate, and the wave vector is almost the same. At this time, the BIC at the non-Γ point is obtained by adjusting the wave vector parameter, producing a new phase matching, the wave vector is degenerate, the energy coupled to the open channel of radiation disappears, and a resonant coupled BIC (topologically protected BIC) is generated. Under Module 4, the resonance frequency of Mode 1 blue shifts to 1.708 THz, and the resonance frequency of Mode 3 red shifts to 1.622 THz, and the EIT-like effect is re-excited, and the current intensity of the two modes continuously strengthens after constructive interference.

[0058] BIC originates from the decoupling of bound states and extended states. Based on the symmetry of the lattice and unit structure, the radiation field is expressed in the form of spherical harmonic vector summation, thereby giving the existence condition of the bound state in the optical system. In order to quantitatively analyze the physical mechanism of the resonant coupling of different modes, the scattering power (I) of different multipole moments in the Cartesian coordinate system, such as magnetic dipole (MD), electric quadrupole (EQ), magnetic quadrupole (MQ), electric dipole (ED) and toroidal dipole (Torid), is calculated. Figure 5 The trend of the multipole contribution rate of Module 1 to Module 4 with frequency change is shown. From Module 1, Figure 5 (a), the eigenstate is relatively weak, mainly in MD and EQ modes; while the maximum multipole contribution rate of quasi-BIC state reaches 0.264 μm 2 , showing strong resonant mode, in which the MQ mode accounts for a large proportion. Subsequently in Module 2 Figure 5 (b), different dipoles of the two modes meet, and the maximum contribution rate is still the MQ mode brought by quasi-BIC, slightly decreasing to 0.245 μm 2 , still maintaining a relatively strong resonant response. However, in Module 3 Figure 5 (c), the maximum contribution value suddenly drops to 0.043 μm 2This indicates that for the resonance phenomenon to achieve wave vector matching, energy needs to be unidirectionally converted from the pump light to the signal light. If the matching is not achieved, energy will circulate between the two light fields, preventing unidirectional energy transfer and thus causing a significant attenuation of the quasi-BIC. Finally, in Module 4 ( Figure 5 In (d), the multipole expansion recovers somewhat, with the maximum value rising back to 0.242 μm. 2 It is close to the level of Module 2.

[0059] The frequency shift phenomenon of Mode1 and Mode3 in the system was studied based on the circuit model. Figure 6 (a) The electric field intensity distribution and corresponding equivalent circuit models are shown respectively. Different field distribution characteristics can be observed in the electric field distribution diagrams of Mode 1 and Mode 3, indicating that these two modes correspond to different forms of electric field oscillation. Based on the field distribution, the equivalent circuit model is drawn. The local resonance characteristics of the system are composed of the upper metal arm resonant structure (inductor L1, capacitor C1) and the upper metal arm resonant structure (inductor L2, capacitor C2), respectively. Inductor L3 and capacitor Cg are related to the coupling of the two structures. The resonant frequency of the oscillation circuit is:

[0060]

[0061] Since the two resonant modes are distributed on the side arm 5 and the corresponding part 601 along the y-axis, respectively, and the gap 7 between the side arm 5 and the corresponding part 601 is not changed during the process, the capacitance remains unchanged. The relationship between the inductance and the side arm 5 is utilized:

[0062]

[0063] When l (length of side arm 5) is greater than w (width of side arm 5), the increase in l causes the inductance value L to show a near-linear growth trend. Therefore, throughout the process, as the side wall W1 increases and W3 decreases, L1 gradually decreases and L3 gradually increases, which also causes the resonant frequency of Mode1 to blue shift and the resonant frequency of Mode2 to red shift. Figure 6 (b) The resonant frequency variation trends of Mode1 and Mode3 can be clearly seen.

[0064] This invention is applied in terahertz modulators. By changing the electronic and optical properties of materials or structures, switching between single and double valleys can improve modulation efficiency. In particular, the double transmission valley effect may lead to a steeper transmission change, thus making the modulation process of terahertz waves more efficient and enabling a larger modulation depth with a smaller external control signal.

[0065] Selective absorption or transmission of terahertz waves at specific frequencies or wavelengths can be achieved. For example, in certain semiconductors or two-dimensional materials (such as graphene, transition metal dichalcogenides, etc.), by changing the electronic structure of the material, the generation of single or double valleys can be adjusted, thereby affecting the propagation characteristics of terahertz waves. This adjustment capability enables the device to more flexibly selectively filter or pass terahertz waves in a specific frequency range when in operation.

[0066] In certain nonlinear optical materials, the switching of single and double transmission valleys can enhance the nonlinear interaction between light and the material, thereby improving the nonlinear effects of terahertz waves, such as the generation, frequency doubling, or frequency modulation of terahertz waves. This provides new means of regulation for the generation and processing of terahertz waves.

[0067] Due to the influence of single and double valley switching on the absorption and transmission characteristics of terahertz waves by the material, more efficient terahertz wave detectors and imaging systems can be designed through this effect. For example, by regulating the depth and position of the transmission valley of the material, different terahertz wave signal responses can be obtained, thereby improving the imaging resolution or detection sensitivity.

[0068] Slow light effect: In the classical quantum EIT effect, the transparency phenomenon suppresses the absorption of light by introducing a coherent superposition of quantum states in the medium, forming a narrow transmission window accompanied by a corresponding enhancement of dispersion, thereby greatly reducing the group velocity of light within this frequency window, i.e., the so-called "slow light effect." In general, the slow light effect can be described by the group delay:

[0069]

[0070] where is the transmission phase shift, and ω is the angular frequency of the incident wave. Before and after the formation of the Friedrich-Wintgen BIC, two EIT-like windows are obtained. As shown in Figure 7 (a) and (c) show the phase shift of the two modules, and the slope of the phase shift represents the strength of the dispersion. It can be seen that, for both Module 2 and Module 4, the phase shift slope near Mode 2 is smaller, while there is a larger phase shift slope near the transparent window, and the dispersion is relatively strong. Then, the group delay under the two modules is calculated, and the group delay of Module 2 and Module 4 reaches a peak at 1.6497 THz and 1.6535 THz, respectively, with maximum values of 23.234 and 24.904, respectively, corresponding to a 7.4712 mm distance delay for light propagating in free space. Considering practical applications, the delay-bandwidth product (DBP) maximum value of the maximum group delay and bandwidth product is calculated to be 6.5248.

[0071] Transmission peak regulation method: the evolution law between different structures of resonant ring, from the process of EIT-like effect to single resonance mode, the transmission peak shows good regulation characteristics, and successfully realizes δ T from 0.932 (double valley) to 0 (single valley), which means that the generation and annihilation of the transmission peak are almost within the controllable range.

[0072] Therefore, two ways are designed to effectively regulate the transmission peak. The first way is as shown in Figure 8 (a), using the influence of parameter change in evolution on the transmission peak, when the structure of the resonant ring is the structure described in embodiment 4, the addition of the first graphene strip and the second graphene strip can realize the regulation of the transmission peak in multiple dimensions. First, discard the metal strip with a side wall of 3.75 μm, and add a first graphene strip M1 with a length and width of 3.75 μm and 6 μm; then add a second graphene strip M2 with the same specifications in the recessed area, both of which can be controlled by independent gate voltages to control the carrier concentration of each other. When M1 = 2 × 10 12 cm 2 and M2 = 1 × 10 13 cm 2 , the transmission peak is almost completely annihilated; when M1 = 8 × 10 13 cm 2 and M2 = 1 × 10 11 cm 2 , the EIT-like effect is excited, as shown in Figure 8 (b) can clearly see the transition from single valley to double valley. The change of δ T and resonance frequency of the transmission peak when M1 and M2 take different values is measured, and it can be found that δ T can be modulated from 0.005 to 0.875, and in this process, the resonance frequency can be shifted by up to 0.022 THz, which can almost realize the modulation of the transmission peak at the same phase.

[0073] The near-field interaction of the coupling mode always causes strong local field constraints sensitive to environmental changes. If the local charge density near the resonant cavity gap is high enough, the low-level light carriers generated in the "hot spot" will introduce non-radiative loss. Therefore, a conductive perturbation experiment is carried out on an amorphous germanium film with a thickness of 200 nm, which verifies that the transmission peak in the coupling mode is largely determined by the non-radiative loss between the two modes. As shown in Figure 9(a)-(b) show the transmittance of Module 3 and Module 4 as a function of frequency at different photoconductivities, respectively. Due to the introduction of a new refractive index material at the interface between the two media, a sudden phase shift on the wavelength scale is introduced in the optical path, which leads to a red shift of the resonance frequency. In the figure, as the photoconductivity increases (from 0 S / m to 3500 S / m), the transmittance peak of Module 3 will disappear within a change of 100 S / m, while the transmittance peak of Module 3 will approach 0 within a change of 400 S / m. Combined with Figure 9 (c)-(d) and (f)-(g); it is found that in the low photoconductivity range, the transmittance peak frequency dependence of the two cases is very large, indicating that the non-radiative loss of the module under the photoconductivity perturbation is more obvious, and has a significant impact on the overall optical properties. When the photoconductivity increases to a certain extent, the transmittance peak approaches to disappear, showing broadband characteristics, and the influence of the increase of photoconductivity on the transmittance gradually weakens, that is, the sensitivity of Ge photoconductivity to transmittance decreases with its increase. Figure 9 (e) and (h) can better see this, the δ T from 0.165 at 0 S / m to 0.011 at 0 S / m, the amplitude modulation is only 0.154. The δ T from 0.893 at 0 S / m to 0.005 at 400 S / m, the amplitude modulation is up to 0.8925. The two curves show similar rapid downward trends, but Module 4 has higher sensitivity to the transmittance of the photoconductivity than Module 3. These results show that the amorphous germanium film can be added under Module 4, and the Ge photoconductivity can be more effectively used to design and control the optical properties of terahertz devices.

[0074] Finally, the phase shift and group delay of Ge at different conductivities under Module 4 are calculated. As shown in Figure 10 (b), when the conductivity of Ge is 0 S / m, the maximum value of the group delay is 25.29 ps, corresponding to a delay of 7.587 mm distance in free space. As the conductivity increases, the group delay gradually decreases. When the conductivity of Ge is 400 S / m, the transmission amplitude and group delay approach to 0. This phenomenon means that the slow light effect almost disappears. The delay bandwidth product (DBP) is calculated during the process, and the maximum value of DBP is 7.182 when the conductivity of Ge is 0 S / m.

[0075] Table 2 shows the transmission amplitude, group delay and DBP at different conductivities. The above analysis shows that under Module 4, by adjusting the conductivity of Ge, the slow light effect of the proposed EIT-like metamaterial can be better dynamically controlled.

[0076] Table 2 Transmission amplitude, group delay and DBPS at different conductivities

[0077]

[0078] Table 3 Performance comparison of slow light devices based on the present application and full light devices in the prior art. The comparison results show that the slow light device based on the present application has excellent slow light effect.

[0079] Table 3 Comparison results of slow light devices based on the present application and slow light devices in the prior art

[0080]

[0081] Note: [1] in Table 4: Liang D, Chen T. Optical modulated graphene metamaterial based on plasmon-induced transparency in the terahertz band: application for sensing [J]. Diamond and Related Materials, 2023, 131: 109613.

[0082] [2]: Sarker D, Nakti P P, Zubair A. Graphene metamaterials-based plasmon-induced terahertz modulator for high-performance multiband filtering and slow light applications [J]. Optics Express, 2024, 32(6): 9442-9455.

[0083] [3]: Huang W, Lin Y S. Dynamically Tunable Electric Split-Ring Resonators Based on 300 nm Gold Films on Silica for Reconfigurable Slow-Light Application [J]. ACS Applied Nano Materials, 2023, 6(24): 23532-23541.

[0084] [4]: Chen M M, Yang X X, Gao S. Tunable electromagnetically induced transparency in a metal-perovskite hybrid metamaterial and its sensing performance [J]. IEEE Sensors Journal, 2023, 23(5): 4802-4808.

[0085] [5]: Liu C, Liu P, Yang C, et al. Analogue of dual-controlled electromagnetically induced transparency based on a graphene metamaterial [J]. Carbon, 2019, 142: 354-362.

[0086] [6]: Wang B X, Duan G, Lv W, et al. Design and experimental realization of triple-band electromagnetically induced transparency terahertz metamaterials employing two big-bright modes for sensing applications [J]. Nanoscale, 2023, 15(45): 18435-18446.

[0087] Sensing property study: High degree of freedom of transmission peak control is achieved by changing the Ge photoconductivity, i.e. effective conversion from double narrow band to single wide band. Figure 11 (c) shows the structural schematic diagram for terahertz sensing property study, which reflects the layered structure of the entire experimental setup, specifically, this structure consists of the following key parts: analyte layer (Analyte Layer) for simulating the influence of analyte (sample) with different thickness (tA) and refractive index (NA) on the optical properties of the entire system; PE layer (polyethylene) is below the analyte layer with a thickness of 2 μm. It provides support and helps stabilize the analyte layer, ensuring that different differential frequency shift phenomena can be accurately measured.

[0088] First, eliminate the influence of analyte thickness on sensing sensitivity, the refractive index of the analyte is 1.5, and the conductivity of Ge is 0 S / m or 400 S / m. Figure 11(a)-(b) show the transmission spectra of different thickness analyte. The increase of the equivalent optical path length due to the increase of the analyte layer thickness, the longer optical path means that the terahertz wave needs to pass through more medium, so that the phase delay increases, the transmission peak frequency red shifts with the increase of the analyte thickness. When the conductivity of Ge is 0 S / m, the transparent window is narrow, and when it increases to 400 S / m, the amplitude from the transparent window to the resonance valley is small, which shows the broadband frequency shift characteristics. Figure 11 (d)-(e) show the relationship between the transmission peak frequency and the thickness of the analyte. When the thickness of the analyte is greater than 10 μm, the frequency moves relatively slowly.

[0089] The thickness of the analyte is fixed at 10 μm, and the sensing performance is studied. The transmission spectrum of the analyte with different photoconductivity (0 S / m and 400 S / m) as the refractive index NA changes is shown in Figure 12 (a)(d). With the increase of NA, the resonance peak position in the two transmission spectra gradually moves to the low frequency (red shift) direction. Compared with 0 S / m, the resonance peak depth is reduced when the photoconductivity is 400 S / m, and at the same time, the overall shape of the transmission spectrum becomes smoother, and the broadband characteristics are obvious. This is because the increase of the refractive index causes the increase of the equivalent phase delay, that is, with the increase of NA, the propagation speed of light in the medium decreases, resulting in the decrease of the resonance frequency. In addition, the increase of the photoconductivity will introduce additional loss in the structure, so that the transmission peak is not as large in amplitude range as 0 S / m. Figure 12 (b) can clearly see the trajectory of the high frequency peak frequency position moving, while Figure 12 (e) the color gradient changes relatively gently. The fitting relationship between the transmission peak resonance frequency and the refractive index difference ΔNA under the two conditions is plotted as Figure 11 (c) and Figure 11 (f), the fitting formulas are calculated as y = 1.5731-0.3117x and y = 1.5775-0.3201x, and the sensitivity is defined as:

[0090]

[0091] where Δf is the frequency shift of the transparent window, and Δn is the change of the refractive index. Therefore, the sensitivity is also the fitting slope, which is 0.3117 THz / RIU and 0.3201 THz / RIU respectively. From the sensitivity, it can be seen that the increase of the photoconductivity has a certain influence on the change of the resonance frequency, but the change is very small, and the influence of the refractive index change on the resonance frequency is relatively consistent, that is, the switching between narrowband sensing and broadband sensing is realized at a fixed frequency point. The sensitivity of the sensor based on the present application is compared with that of the sensor in the prior art, and the comparison result is shown in Table 4. The results show that the sensor based on the present application has excellent sensitivity in the terahertz wave band, and at the same time, the appropriate photoconductivity regulation can switch the response mode of the sensor, which has higher freedom.

[0092] Table 4 Comparison of sensitivity of sensors based on the present application and sensors in the prior art

[0093]

[0094]

[0095] Note: [7] in the table: Zheng D, Lin Y S. Tunable Dual-Split-Disk Resonator with Electromagnetically Induced Transparency Characteristic [J]. Advanced Materials Technologies, 2020, 5(11): 2000584.

[0096] [8]: Yin W, Shen Z, Li S, et al. THz absorbers with an ultrahigh Q-factor empowered by the quasi-bound states in the continuum for sensing application [J]. Optics Express, 2022, 30(18): 32162-32173.

[0097] [9]: Huang W, Lin Y S. Dynamically Tunable Electric Split-Ring Resonators Based on 300 nm Gold Films on Silica for Reconfigurable Slow-Light Application [J]. ACS Applied Nano Materials, 2023, 6(24): 23532-23541.

[0098]

[10] : Sarkar R, Devi K M, Ghindani D, et al. Polarization independent double-band electromagnetically induced transparency effect in terahertz metamaterials [J]. Journal of Optics, 2020, 22(3): 035105.

[0099]

[11] Chen M M, Yang X X, Gao S. Tunable electromagnetically induced transparency in a metal-perovskite hybrid metamaterial and its sensing performance[J]. IEEE Sensors Journal, 2023, 23(5): 4802-4808.

[0100]

[12] Wang R, Xu L, Huang L, et al. Ultrasensitive Terahertz Biodetection Enabled by Quasi-BIC-Based Metasensors[J]. Small, 2023, 19(35): 2301165.

[0101] A preparation method of a multi-dimensional quasi-BIC-mediated terahertz modulation device, the substrate layer 1 is surface treated; the photosensitive germanium is deposited on the surface of the substrate layer 1 after treatment by using chemical vapor deposition method, to form a photosensitive germanium layer 2; gold is deposited on the photosensitive germanium layer 2 through chemical reaction in gas phase, to form a plurality of resonant rings arranged in an array.

[0102] The above description of disclosed embodiments enables those skilled in the art to carry out or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A terahertz modulation device based on multi-dimensional quasi-BIC-mediated modulation, comprising a substrate layer (1) and multiple resonant rings, characterized in that: A photosensitive germanium layer (2) is coated on the surface of the substrate layer (1). Multiple resonant rings are arranged in an array on the photosensitive germanium layer (2). The resonant rings include a first metal strip (3) and a second metal strip (6) that are parallel to each other and of equal length. The center positions of the first metal strip (3) and the second metal strip (6) are connected by an intermediate arm (4) perpendicular to both of them. The end face of the first metal strip (3) is perpendicularly connected to a side arm (5) extending toward the second metal strip (6). The end face of the second metal strip (6) is provided with a side arm (5) corresponding to and extending toward the side arm (5). The corresponding part (601) has a gap (7) between the end of the corresponding part (601) and the end of the side arm (5), the length of the corresponding part (601) and the length of the side arm (5) are not equal and their sum is always equal to 86μm; or, the end face of the second metal strip (6) has no corresponding part (601), there is a gap between the end of the side arm (5) and the end of the second metal strip (6) and the length of the side arm (5) is equal to 86μm, the width of the corresponding part (601) is equal to the width of the side arm (5), the material of the resonant ring is gold, and the thickness of the resonant ring is 0.3μm.

2. The terahertz modulation device based on quasi-BIC-mediated multidimensionality as described in claim 1, characterized in that: The end face of the corresponding part (601) facing the first metal strip (3) is higher than the edge of the second metal strip (6).

3. The terahertz modulation device based on quasi-BIC-mediated multidimensionality as described in claim 1, characterized in that: The end face of the corresponding part (601) facing the first metal strip (3) is lower than the edge of the second metal strip (6), so that a recessed area is formed between the corresponding part (601) and the second metal strip (6).

4. The terahertz modulation device based on quasi-BIC mediation in a multi-dimensional manner as described in claim 3, characterized in that: The side arm (5) is provided with a first graphene strip on the end face facing the second metal strip (6), and a second graphene strip is provided in the recessed area.

5. A terahertz modulation device based on quasi-BIC-mediated multidimensionality as described in claim 4, characterized in that: The width of the first graphene strip and the second graphene strip are both equal to the width of the side arm (5), and the end face of the second graphene strip facing the side arm (5) is flush with the edge of the second metal strip (6).

6. The terahertz modulation device based on quasi-BIC-mediated multidimensionality as described in claim 1, characterized in that: The substrate (1) is made of polytetrafluoroethylene.

7. The terahertz modulation device based on quasi-BIC-mediated multidimensionality as described in claim 1, characterized in that: A transparent protective film coated on the surface of the photosensitive germanium layer (2) is provided between the resonant ring and the photosensitive germanium layer (2).

8. A method for fabricating a terahertz modulation device based on quasi-BIC mediated in a multidimensional manner, as described in any one of claims 1-7, characterized in that: The substrate (1) is surface treated; photosensitive germanium is deposited on the surface of the substrate (1) after treatment by chemical vapor deposition to form a photosensitive germanium layer (2); gold is deposited onto the photosensitive germanium layer (2) through chemical reaction in the gas phase to form multiple resonant rings arranged in an array.

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