A tunable terahertz sensor dominated by multiple continuous domain bound states

Abstract

The present invention provides a tunable terahertz sensor dominated by multiple continuous domain bound states, which is composed of a plurality of unit cell structures arranged periodically. The unit cell structure includes: a substrate and a pair of slotted U-shaped resonant arms disposed above the substrate. The plurality of unit cell structures are arranged periodically to form a metasurface structure. The slotted U-shaped resonant arms are made of Dirac semimetal material or Si, and the substrate material is SiO2. Among them, the slotted U-shaped resonant arms are composed of two U-shaped structures symmetrically arranged. The bottom of the U-shaped structure has a slit, and the bottom slits of the two U-shaped structures are adjacent to each other. The distances between the two slits are represented by g1 and g2 respectively, and the distance that the two slotted U-shaped resonant arms move relative to the substrate is represented by d. By adjusting the parameter values of g1, g2, and d, three quasi-BICs, namely QBICs, are obtained. The technical solution of the present invention overcomes the problems of low sensitivity of terahertz sensors in the prior art and the inability to actively tune the sensitivity and sensing range.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz metamaterials, and particularly relates to a tunable terahertz sensor dominated by multiple bound states in the continuum. Background Art

[0002] In recent years, the research on BICs in the terahertz range has become very popular due to their high quality factor (i.e., Q factor). In a metasurface, a BIC is defined as a non-radiative eigen-solution of the wave equation above the light cone. Usually, two methods are used to obtain BICs, namely symmetry protection of the structure and mode cancellation interference of resonators (accidental BIC). A BIC is a dark mode and cannot be directly presented in the spectrum because of its infinite Q factor and infinitely narrow bandwidth. Therefore, by violating the implementation method of BIC and introducing perturbations to the structure, a QBIC with a converged Q factor can be obtained. The application of BICs has unique functions for spatially varying optical properties and optical responses. In sensitive sensing applications, the QBIC with a high Q factor generated in the spectrum can produce a small frequency shift when the analyte or environmental change is weak, which is very beneficial to enhancing the sensitivity of the sensor. However, researchers have verified the QBIC in metallic terahertz metasurfaces. Due to the limitation of non-radiative intrinsic losses (Ohmic losses) in metallic materials, its Q factor is relatively low, which is disadvantageous for the application of highly sensitive devices. At the same time, the emergence of all-dielectric metasurfaces overcomes this defect, and the Q factor has been significantly improved. However, only studying one BIC has a narrow application range of BICs in the frequency range and has certain limitations. Therefore, the research on generating multiple BICs in the same metasurface structure has great application potential. At the same time, terahertz sensors have attracted much attention due to the improvement of sensing performance caused by high Q factors. Currently, Fano resonance is well-known for its high Q factor. Researchers first fabricated sensors based on metallic resonators using the principle of Fano resonance. However, it is not sensitive to extremely low concentrations of molecules and has low sensitivity. Subsequently, researchers used high refractive index media to enhance the sensitivity of the sensor by utilizing the principle of ring dipole resonance. The sensors fabricated based on the above technologies cannot change their inherent properties, which limits the application of the devices. And He et al. achieved active tuning of the sensitivity based on a graphene metasurface using EIIT resonance. However, the graphene layer is too thin, resulting in weak light-matter interaction. Recently, the tuning properties of Dirac semimetal metamaterials have become a research hotspot. Among them, Dirac semimetal is a topological semimetal. As a new type of quantum material, Dirac semimetal has a band structure similar to that of graphene and is also known as "three-dimensional graphene". It can dynamically regulate the Fermi level by applying an external bias voltage to change its conductivity, enabling it to switch from the "dielectric" state to the "semi-metal" state and then to the "metal" state. Currently, the attention to sensing applications based on BICs and actively tuning the sensing range to modulate sensors is limited and requires further research and discussion. Therefore, there is a need for a terahertz sensor with relatively high sensitivity and whose sensitivity and sensing range can be actively tuned. Summary of the Invention

[0003] The main object of the present invention is to provide a tunable terahertz sensor dominated by multiple bound states in the continuum, so as to solve the problems of low sensitivity of terahertz sensors in the prior art and the inability to actively tune the sensitivity and sensing range.

[0004] To achieve the above object, the present invention provides a tunable terahertz sensor dominated by multiple bound states in the continuum, which is composed of a plurality of unit cell structures arranged periodically. The unit cell structure includes: a substrate and a pair of slotted U-shaped resonant arms disposed above the substrate. The plurality of unit cell structures are arranged periodically to form a metasurface structure. The slotted U-shaped resonant arms are made of Dirac semimetal material or Si, and the substrate material is SiO2;

[0005] Among them, the slotted U-shaped resonant arm is composed of two U-shaped structures symmetrically arranged. The bottom of the U-shaped structure has a slit, and the bottom slits of the two U-shaped structures are adjacent to each other;

[0006] The distances between the two slits are represented by g1 and g2 respectively, the distance between the two slotted U-shaped resonant arms is represented by g0, the height of the resonant arm is represented by t2, and the moving distance of the two slotted U-shaped resonant arms relative to the substrate is represented by d; the thickness of the analyte is represented by h; by adjusting the parameter values of g1, g2 and d, three quasi-BICs, namely QBICs, are obtained.

[0007] Furthermore, the initial values of the slit distances are g1 = g2 = 1 μm, the lengths P of the unit cell in the x-axis direction and the y-axis direction are both = 40 μm, the heights of the substrate and the resonant arm are t1 = 15 μm and t2 = 5 μm respectively, the initial value of the distance between the two resonant arms is g0 = 4 μm, the width of the resonant arm is w = 4 μm, and the two slotted U-shaped resonant arms are located in a cube with a side length a = 32 μm.

[0008] Furthermore, in order to obtain sharp quasi-bound states in the continuum QBICs, the parameter values of g1, g2 and d are adjusted. When g1 = g2 = 5.2 μm and d = 2 μm, three QBICs appear simultaneously, namely one accidental QBIC and two symmetry-protected QBICs.

[0009] Furthermore, the analyte to be detected is placed above the metasurface structure, and the range of the thickness h of the analyte is 0 ≤ h ≤ 20 μm. The range of the thickness h of the analyte is 0 ≤ h ≤ 20 μm.

[0010] Furthermore, when the resonance active tuning function of QBIC needs to be realized, a Dirac semimetal material is selected as the material of the slotted U-shaped resonant arm. One pole of the bias voltage is connected to the upper end of the metasurface structure, and the other pole of the bias voltage is connected to the lower end of the substrate. The range of the Fermi level of the bias voltage is 0.1 eV to 0.3 eV. The structural parameters of the metasurface are selected as g1 = g2 = 1 μm, g0 = 2 μm, d = 0 μm, the height of the resonant arm t2 = 0.5 μm, and the initial Fermi level Ef = 100 meV.

[0011] The present invention has the following advantages:

[0012] 1. Different types of BIC degeneracy can be manipulated by changing the structural parameters, and the frequency range for sensing applications is wider;

[0013] 2. The QBIC sensitive to the incident angle, i.e., SP-QBIC, can adjust the sensing range by changing the angle of the incident THz wave, achieving the tuning of the sensing range without changing the metasurface structure. For THz devices, this is a very flexible and practical technical means;

[0014] 3. The sensitivity and sensing range of the sensor based on the Dirac semimetal metasurface can be actively tuned, and the sensing range is expanded, which is very beneficial to the applications of devices such as narrowband filters, switches, and lasers, provides options for the development of THz wave manipulation, and promotes the development of sensor devices. Description of the Drawings

[0015] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. In the drawings:

[0016] Figure 1 Shows the structural diagram of a sensing system composed of a tunable terahertz sensor dominated by multiple bound states in the continuum according to the present invention;

[0017] Figure 2 Shows the three-dimensional structural schematic diagram of a symmetric unit cell of a tunable terahertz sensor dominated by multiple bound states in the continuum according to the present invention;

[0018] Figure 3 Shows the two-dimensional structural schematic diagram of a symmetric unit cell of a tunable terahertz sensor dominated by multiple bound states in the continuum according to the present invention;

[0019] Figure 4Shows the variation curve of the real part of the dielectric constant of the Dirac semimetal metamaterial in the THz range with respect to the Fermi level;

[0020] Figure 5 Shows the variation curve of the imaginary part of the dielectric constant of the Dirac semimetal metamaterial in the THz range with respect to the Fermi level;

[0021] Figure 6 Shows Figure 2 The functional relationship between the slit spacings g1, g2 and the THz wave frequency and the transmission coefficient;

[0022] Figure 7 Shows Figure 2 The schematic structural diagram of the asymmetric unit cell of

[0023] Figure 8 Shows Figure 7 The discrete transmission spectrum of the asymmetric metasurface of varying with the slit spacings g1, g2 and the moving distance d of the resonant arms;

[0024] Figure 9 Shows Figure 7 The functional relationship between the displacement magnitude d of the asymmetric metasurface of and the THz wave frequency and the transmission coefficient;

[0025] Figure 10 Shows the functional relationship between the Q factor of the SP - BIC and the displacement magnitude d of the resonant arm;

[0026] Figure 11 Shows the transmission spectrum of the all - dielectric metasurface when g1 = g2 = 6μm and d = 2μm;

[0027] Figure 12 Shows the normalized scattering energy diagram of the multipole in Cartesian coordinates;

[0028] Figure 13 Shows Figure 7 The schematic three - dimensional structure diagram of the unit cell after adding the analyte of

[0029] Figure 14 Shows the functional relationship between the frequency shifts of three QBICs and the thickness h of the analyte;

[0030] Figure 15 Shows the functional relationship between the frequency shifts of three QBICs and the refractive index of the analyte when h = 20μm;

[0031] Figure 16 Shows the functional relationship between the frequency shifts of three QBICs and the refractive index of the analyte when h = 2μm;

[0032] Figure 17 Shows the FoM of the accidental QBIC;

[0033] Figure 18 Shows the angular dependence of QBIC on the incident wave;

[0034] Figure 19 Shows the unit cell structure diagram of the Dirac semimetal metasurface;

[0035] Figure 20 Shows the accidental BIC that appears in the Dirac semimetal metasurface;

[0036] Figure 21 Shows the functional relationship between the transmission spectrum of the Dirac semimetal metasurface without analyte and frequency and Fermi level;

[0037] Figure 22 Shows the functional relationship between the transmission spectrum of the Dirac semimetal metasurface with analyte and frequency and Fermi level;

[0038] Figure 23 Shows the frequency difference between the same Fermi levels before and after adding the analyte and the frequency shift relationship diagram of different Fermi levels for the reference transmission spectrum after adding the analyte. Detailed implementation mode

[0039] Next, the technical solution of the present invention will be clearly and completely described in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative work shall fall within the protection scope of the present invention.

[0040] As Figure 1 shown, the sensing system composed of the sensor proposed by the present invention is arranged from top to bottom as analyte, metasurface structure and substrate in sequence. As Figure 2 and Figure 3 shown, the unit cell structure of the sensor of the present invention includes: a substrate and a pair of slotted U-shaped resonant arms arranged above the substrate. A plurality of unit cell structures are arranged periodically to form the metasurface structure. The slotted U-shaped resonant arms are made of Dirac semimetal material or Si, and the substrate material is SiO2;

[0041] Among them, the slotted U-shaped resonant arm is composed of two U-shaped structures symmetrically arranged. The bottom of the U-shaped structure has a slit, and the bottom slits of the two U-shaped structures are adjacent to each other;

[0042] The distances between the two slits are represented by g1 and g2 respectively, the distance between the two slotted U-shaped resonant arms is represented by g0, the width of the resonant arm is represented by w, the heights of the substrate and the resonant arm are represented by t1 and t2 respectively, and the distance that the two slotted U-shaped resonant arms move relative to the substrate is represented by d. By adjusting the parameter values of g1, g2 and d, three quasi-BICs, namely QBICs, are obtained.

[0043] Due to the slot waveguide effect, the interaction between the local electromagnetic field and the surrounding medium can be enhanced. Therefore, a slit is opened in the middle part between the two U-shaped loop resonant arms. As Figure 2 and Figure 3 shown in the unit cell structure diagram, where the initial values of the slit spacing are g1 = g2 = 1 μm, the period of the metasurface structure is P = 40 μm, the spacing between the two resonant arms is g0 = 4 μm, the width of the resonant arms is g = 4 μm, the two resonant arms are located in a cube with a side length a = 32 μm, and the heights of the substrate and the resonant arms are t1 = 15 μm and t2 = 5 μm respectively. The refractive indices of Si and SiO2 are 3.9 and 1.45 respectively. The THz wave is incident vertically along the y polarization direction.

[0044] According to the random phase approximation theory, in the case of low temperature limit T≥E F , the dynamic conductivity of the Dirac semimetal metamaterial using the Kubo formula 3D is expressed as:

[0045]

[0046]

[0047] where e is the electron charge, is the reduced Planck constant, is the Fermi momentum, E F is the Fermi level, is the relaxation time, μ = 3×10 4 cm 2 V -1 s -1 , θ is the Riemann–Siegel function, v F = 10 6 m / s is the Fermi velocity.

[0048] Preferably, AlCuFe quasicrystal is used as the 3D Dirac semimetal metamaterial, and g = 40 is the degeneracy factor. The complex relative permittivity of the 3D Dirac semimetal metamaterial can be expressed as

[0049] ε = ε b + iσ / ωε0 (3)

[0050] where ε b = 1, and ε0 is the vacuum permittivity.

[0051] By changing the bias voltage of the Dirac semimetal metasurface through an external circuit, the Fermi level of the material can be changed, and thus the conductivity of the material can be adjusted. Figure 4 、 Figure 5The real and imaginary parts of the complex permittivity of a Dirac semimetal metasurface with different Fermi levels (EF) are shown respectively as functions of the incident wave frequency. For the same Fermi level, the real part of the permittivity increases with increasing frequency, while the imaginary part decreases sharply. When the frequency is kept constant, the real part of the permittivity increases slowly with increasing Fermi level. Conversely, when the Fermi level increases uniformly, the imaginary part decreases slowly. Considering the change in the Fermi level as a perturbation of the Dirac semimetal metamaterial, the shift in the resonance frequency δf of the QBIC can be estimated as

[0052]

[0053] where and represent the changes in the permittivity and permeability of the Dirac semimetal metamaterial respectively, and represent the unperturbed electric and magnetic fields respectively, and represent the electric and magnetic fields under perturbation respectively, f0 represents the resonance frequency before material perturbation, and δf represents the change in the resonance frequency, which is proportional to the dot product of the change in the material permittivity and the electric field This provides the possibility for the Dirac semimetal metamaterial to achieve a tuning function.

[0054] Specifically, to obtain a sharp QBIC, while keeping the structure of the all-dielectric metasurface symmetric, by simultaneously adjusting the sizes of the slits g1 and g2 of the left and right resonant arms, the transmission spectrum of the metasurface is observed to explore the accidental BIC. The continuous transmission spectra of the metasurface varying with g1 and g2 at an incident frequency of 6.1 - 6.4 THz are shown in Figure 6 As shown, the BICs are marked with circles. Correspondingly, the bright modes that appear during the parameter adjustment process are the QBICs after the degradation of the BIC.

[0055] The characteristics of the QBIC are usually quantitatively described by the Q factor, which is defined as:

[0056] Q = ω0 / Δω (5)

[0057] where ω0 is the resonance frequency and Δω is the full width at half maximum (FWHM) of the resonance intensity.

[0058] Breaking the C2 rotational symmetry of the metasurface structure (i.e., (x,y) → (-x,-y)) to obtain a symmetrically protected BIC (SP-BIC), the mechanism is to move the middle gap of the resonant arm and the whole left and right rods by a displacement d, as shown in Figure 7 As shown. Figure 8Shows the discrete transmission spectra of the all-dielectric metasurface varying with the slit sizes g1, g2, and the moving distance d of the resonant arms, with the corresponding structural parameters being g1 = g2 = 5.2 μm and d = 0 μm.

[0059] Figure 8 To facilitate the comparison of the simultaneous occurrence of accidental QBIC and SP-QBIC, the frequency band is changed to 5 - 6.5 THz. Maintaining the structural parameters of the accidental BIC as g1 = g2 = 4 μm, only the symmetry of structure C2 is broken, i.e., d = 2 μm, and the energy of the SP-BIC starts to leak, showing two sharp resonance line shapes, namely the QBIC after the degeneration of the SP-BIC. The accidental BIC not only diverges at the origin but also at non-high symmetry points. The symmetry breaking of the structure does not change the destructive interference of the accidental BIC mode.

[0060] Changing the structural parameters to g1 = g2 = 5.2 μm and d = 2 μm, three BICs appear. The reason is that when changing g1 and g2 under structural symmetry to degenerate the accidental BIC into QBIC, the modes are always in a state of phase mismatch and cannot disappear through destructive interference in the frequency spectrum. Coupled with the symmetry breaking of the structure, three QBICs appear simultaneously. At this time, in the same metasurface, three BICs corresponding to three frequency bands are realized by breaking the symmetry of the structure and adjusting the structural parameters, and the change of the structural parameters can control the appearance order of the BICs. As Figure 8 shown, in the range of 5 - 6.5 THz, if only g1 and g2 are changed, the accidental BIC can be manipulated to degenerate into QBIC; if only d is changed, only two SP-QBICs can be obtained. This provides the driving force for the development of multi-band sensing in the THz band, greatly facilitating the scientific research process when manipulating THz waves and providing a powerful technical means for it.

[0061] Figure 9 Shows the functional relationship between the frequency and the moving distance d and the transmission amplitude. We name them BIC low and BIC high respectively according to the frequency bands where the two SP-BICs are located. BIC low is the SP-BIC at the low frequency, and BIC high is the SP-BIC at the high frequency. The circle of BIC low is larger than the circle of BIC high , indicating that the sensitivities of BICs to the symmetry breaking of the structure are different. BIC high is more sensitive, and with a slight symmetry breaking of the structure, the BIC will degenerate into QBIC.

[0062] Figure 10 Shows the dependence of the Q factors of the two SP-QBICs on the degree of asymmetry, where the degree of asymmetry α is defined as:

[0063] α = d / l (6)

[0064] Where d is the moving distance of the resonant arm and l is the inner wall length of the original resonant arm. As α decreases, the Q factor gradually increases. It should be noted that when α = 0, the Q factors of the two BICs tend to infinity and the resonance line shape disappears in the spectrum. By fitting the Q factors of the two QBICs, both satisfy the inverse quadratic relationship with the asymmetry.

[0065] The generation mechanism of QBIC is understood by performing a multipole analysis on the metasurface. The multipole decomposition is carried out in the Cartesian coordinate system. Since the contribution of the higher-order multipole expansion is extremely small, only the expansion of five multipoles is considered. They include the electric dipole (ED), the electric quadrupole (EQ), the magnetic quadrupole (MD), and the toroidal dipole (TD).

[0066] Under the illumination of the incident light wave, the multipole moment of the scatterer is determined by the polarization P(r) = ε0(ε r - ε d )E(r), where ε0, ε r and ε d are the permittivity of free space, the relative permittivity of the scatterer, and the relative permittivity of the surrounding medium, respectively; E(r) represents the total electric field inside the scatterer. According to the centroid of the scatterer, the multipole is located at the origin of the Cartesian coordinate system. Under this condition, the regular ED moment of the scatterer is given by

[0067] p = ∫ V P(r')dr' (7)

[0068] Where V is the volume of the scatterer and r' is the radius vector of the volume element inside the scatterer. The MD moment of the scatterer is expressed as

[0069]

[0070] In Equation (8), ω is the angular frequency and i is the imaginary unit.

[0071] The irreducible EQ tensor of the scatterer is

[0072]

[0073] Where is the 3×3 identity tensor, and the irreducible MQ tensor of the scatterer is

[0074]

[0075] The vector V is defined by the following expression:

[0076]

[0077] These tensors are symmetric and traceless.

[0078] The TD torque has a radiation pattern similar to that of ED and is defined as

[0079]

[0080] As Figure 12 shown, where the three frequency bands correspond to the frequency bands in Figure 11 They are respectively the frequency bands near the resonance points of three QBICs (from low frequency to high frequency are QBIClow, QBIChigh, and sporadic QBIC). They show the relationship between the transmission curve and the multipole analysis and the incident wave frequency. Although the contribution of MQ to QBIClow is not small, it is mainly controlled by EQ, MD plays a secondary role, and other multipoles are significantly suppressed. Even though MQ has a partial contribution to QBIChigh, the contribution of MD is one order of magnitude different from that of MQ. Therefore, MD dominates, and the remaining multipoles are almost suppressed. Relatively speaking, the sporadic QBIC is due to the multipole contributions of MD and EQ, and the contribution of MD is slightly higher and dominates.

[0081] Since the Q factor of the QBIC generated by the metasurface in the present invention is very high and the linewidth is very narrow, it is of great significance to study the sensing sensitivity based on the transmission peak of QBIC to represent the performance of sensing.

[0082] The sensor provided by the present invention is used to detect the refractive index of an analyte. A layer of photoresist material (refractive index n = 1.6) is added on the top layer of the all-dielectric metasurface as the analyte for sensing, and the structural parameters of the asymmetric metasurface are preferably g1 = g2 = 5.2 μm, d = 2 μm, as Figure 13 shown, where h is the thickness of the analyte. To study the sensitivity of the analyte thickness to the frequency shift, the frequency shift changes of the three QBICs are as Figure 14 shown. The frequency shift is defined as:[[]]END]]

[0083] f = f(h0) - f(h) (13)

[0084] Among them, f(h0) is the resonance frequency without the analyte, and f(h) is the resonance frequency of the QBIC when the thickness of the analyte is h. The variation curves of the frequency shifts of the three QBICs all show an increasing trend of power functions (the exponent is greater than zero and less than 1). Since the electric field of the all-dielectric metasurface is strongly localized in its gaps, when the thickness of the analyte is low, the interaction between the electric field and the analyte is stronger, resulting in a sharp increase in the amplitude of the frequency shift. When the thickness h is greater than 20 μm, the frequency shifts of the three QBICs all tend to be stable and the redshifts change little. This is because the analyte is located in the region where the edge electric field is mainly distributed, and the edge electric field disappears when it is at a certain height from the metasurface. Therefore, the frequency shift change is more sensitive to the analyte when the thickness h is in the range of 0 ≤ h ≤ 20 μm. The results show that when changing the unit thickness, due to the specific curve change, the frequency shift changes of the three QBICs are more sensitive to thin analytes. Among the frequency shift amplitudes of the three QBICs, the accidental QBIC is more sensitive to the change in thickness.

[0085] Generally, the THz band is more suitable for the detection of biomolecules, and they have strong responses in the THz range. The refractive index of biomolecules usually varies between 1.4 and 2.0. To achieve the sensing effect of detecting the refractive index of the analyte and changing the refractive index of the analyte, the frequency shifts of the three QBICs are as Figure 15 (analyte thickness h = 20 μm) and Figure 16 (analyte thickness h = 2 μm) shown, and different QBICs can be selected to detect the refractive index of the analyte in different environments. Among them, the sensitivity of the sensor is defined as:

[0086]

[0087] Among them, δf represents the change in the resonance frequency. In addition, δn is the change in the refractive index of the analyte.

[0088] For sensing applications, the Q factor is also an important factor affecting the sensing performance. The FoM is used to evaluate the performance of the ultrasensitive sensor, and the FoM is defined as:

[0089]

[0090] Among them, S is the sensitivity of the accidental QBIC, FWHM is the full width at half maximum, and f(n) is the resonance frequency of the accidental QBIC when the refractive index of the analyte is n. Since the sensitivity of the accidental QBIC is generally higher than that of the SP-QBIC, the FoM of the accidental QBIC with an analyte thickness of 20 μm is as Figure 17As shown in the figure, the FoM shows an exponential upward trend when the refractive index of the analyte changes from 1 to 2. This shows that the interaction between the analyte and the electric field increases with the increase of the refractive index of the analyte. Therefore, due to its ultrahigh sensitivity and FoM, the proposed metasurface structure is suitable for ultrasensitive sensing applications at THz frequencies.

[0091] In order to explore the dependence between the QBIC of the degraded all-dielectric metasurface and the THz incident wave, the metasurface structural parameters (g1=g2=5.2μm, d=2μm) were selected. By incident y-polarized light at different angles, the functional relationship between QBIC and the incident wave angle is shown in the figure below. Figure 18 As shown. The positions of QBIClow, QBIChigh, and occasional QBIC have been marked. As the incident angle increases slowly, QBIClow slightly redshifts and the bandwidth slightly increases, while QBIChigh increases in bandwidth, but the resonance frequency gradually blueshifts. Since SP-QBIC has very strict requirements on symmetry, when the symmetry of the structure is broken and the incident light is asymmetric, the coupling strength between the radiation channel and the bound state is enhanced, and the resonance bandwidth gradually increases, forcing the Q factor to decrease. SP-QBIC is very dependent on the incident angle of the THz wave. Relatively speaking, the change of occasional QBIC is not obvious, and it has no dependence on the incident angle of the THz wave.

[0092] Sensors should have high sensitivity, wide response frequency range, and reliable performance. However, analytes with uncertain sensing ranges are a huge challenge for sensors. Structural parameters are usually changed to select the sensing range and manufacture sensors. However, in actual production and application, re-fixing parameters increases costs and integration difficulties, severely limiting their applications. Therefore, active tuning of resonance is of great practical value.

[0093] In order to realize the active tuning function of QBIC, based on the design inspiration of the all-dielectric metasurface, this structural shape is used to adjust the thickness t2 of the top layer of the all-dielectric metasurface structure to 0.5μm, replace the silicon material with the Dirac semi-metallic metamaterial, and add a bias voltage to the structure. The initial Fermi level Ef = 100meV, and at the same time, to increase the coupling strength between the resonant arms, g0 is adjusted to 2μm, as shown in Figure 19 It can be proved that the dark state BIC emerges near 6.5THz based on the Dirac semimetal metasurface, and the shape of the metasurface remains unchanged, and the coupling mode remains basically unchanged. This is still an occasional BIC controlled by the parameters g1 and g2 at the same time, as shown in Figure 20 As shown in the circle. Since the sensitivity of the incidental QBIC based on the all-dielectric metasurface sensor is higher than that of the SP-QBIC, the tuning function of the QBIC with d = 0 μm is selected for analysis without destroying the structural symmetry. The structural parameters of the metasurface are selected as g1 = g2 = 1 μm. Figure 21Shows the transmission spectrum of the metasurface as a function of the Fermi level in the absence of an analyte. By changing the bias voltage Vg, as the Fermi level increases, the QBIC resonance gradually blueshifts, and the resonance amplitude remains basically unchanged, ensuring that the signal response can be detected in a noisy environment. When an analyte (n = 1.6, h = 20 μm) is added above the metasurface, the sensor based on the Dirac semimetal metasurface still has a good tuning effect, as Figure 22 shown, where the curve marked by the circle is the reference transmission spectrum when no analyte is added and Ef = 300 meV. After adding the analyte, the QBIC shows a redshift. As the Fermi level decreases, the analyte with the same refractive index achieves a larger frequency shift. To quantitatively analyze the dynamic change of the frequency after adding the analyte, the frequency shift change curve of the QBIC with respect to the Fermi level compared to the reference transmission spectrum (the curve marked by the circle) is shown in Figure 23 the triangular line. As the Fermi energy decreases from 0.3 eV to 0.1 eV, the corresponding frequency shift changes from 48 GHz to 132 GHz. Therefore, by changing the Fermi level (changing by 0.2 eV), a compensation of 84 GHz can be obtained. At the same time, the frequency shift changes of the QBIC before and after adding the analyte at each corresponding Fermi level are analyzed, as shown in Figure 23 the star line. At Ef = 0.1 eV, the frequency shift is the largest, reaching 76 GHz. As the Fermi level increases, the frequency shift of the QBIC before and after adding the analyte gradually decreases. The sensitivity of the sensor corresponding to each Fermi level is different. In practical applications, different Fermi levels can be selected to switch the target sensitivity. It is worth mentioning that the sensitivity of the sensor based on the Dirac semimetal metasurface is about 117 GHz / RIU at Ef = 0.1 eV.

[0094] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A tunable terahertz sensor dominated by multiple continuous domain bound states, which is composed of a periodic arrangement of multiple unit cell structures, and is characterized in that, The unit cell structure includes: a substrate and a pair of slotted U-shaped resonant arms disposed above the substrate. A plurality of unit cell structures are arranged periodically to form a metasurface structure. The slotted U-shaped resonant arms are made of Dirac semimetal material or Si, and the substrate material is SiO2; Among them, the slotted U-shaped resonant arm is composed of two U-shaped structures symmetrically arranged. The bottom of the U-shaped structure has a slit, and the bottom slits of the two U-shaped structures are adjacent to each other; The distances between the two slits are represented by g1 and g2 respectively, the distance between the two slotted U-shaped resonant arms is represented by g0, the width of the resonant arm is represented by w, the heights of the substrate and the resonant arm are represented by t1 and t2 respectively, and the distance by which the two slotted U-shaped resonant arms move relative to the substrate is represented by d. By adjusting the parameter values of g1, g2, and d, three quasi-BICs, namely QBICs, are obtained.

2. The tunable terahertz sensor dominated by multiple continuous domain bound states according to claim 1, characterized in that the slit The initial values of the distances are g1 = g2 = 1 μm, the length P of the unit cell in the x-axis direction and the y-axis direction is 40 μm, the height of the substrate is t1 = 15 μm, the initial value of the height of the resonant arm is t2 = 5 μm, the initial value of the distance between the two resonant arms is g0 = 4 μm, the width of the resonant arm is w = 4 μm, and the two slotted U-shaped resonant arms are located within a square with a side length a = 32 μm.

3. The tunable terahertz sensor dominated by multiple continuous domain bound states according to claim 1, characterized in that, In order to obtain sharp quasi-bound states in the continuum QBICs, the parameter values of g1, g2, and d are adjusted. When g1 = g2 = 5.2 μm and d = 2 μm, three QBICs appear simultaneously, namely one accidental QBIC and two symmetry-protected QBICs.

4. The tunable terahertz sensor dominated by multiple continuous domain bound states according to claim 1, wherein The analyte to be detected is placed above the metasurface structure, and the thickness h of the analyte ranges from 0 ≤ h ≤ 20 μm.

5. An adjustable terahertz sensor dominated by multiple continuous domain bound states according to claim 1, characterized in that, When the resonant active tuning function of QBIC needs to be realized, Dirac semimetal material is selected as the material of the slotted U-shaped resonant arm, and electrodes are added to the periphery of the metasurface structure and a bias voltage is applied from the outside to change the Fermi level of the Dirac semimetal. The range of the Fermi level is 0.1 eV to 0.3 eV. The structural parameters of the metasurface are selected as g1 = g2 = 1 μm, g0 = 2 μm, d = 0 μm, the height of the resonant arm t2 = 0.5 μm, and the initial Fermi level Ef = 100 meV.

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