Terahertz controllable double-resonance plasmon metasurface sensor based on quasi-continuous domain bound state

Abstract

A terahertz adjustable and controllable double-resonance plasmon metasurface sensor based on a quasi-continuous domain bound state belongs to the field of optical sensors and biomolecule detection, and comprises a low-loss high-transparency flexible material polyimide as a substrate, periodic resonance units are distributed on the substrate in an array mode, and the periodic resonance units are arranged on the substrate. The periodic resonance unit is formed by arranging five metal resonators, specifically, the periodic resonance unit is formed by combining three elliptical resonators with the same long axis length and another two elliptical resonators with slightly shorter and different long axes, slits are formed among the five resonators, and the periodic resonance unit presents a double-resonance characteristic in a transmission spectrum; the BIC has a high Q value and great surface electric field enhancement capability, and effective sensing of dynamic frequency modulation of the BIC in a broadband can be realized by adjusting resonator parameters. According to the invention, the design is simple, high-sensitivity double-harmonic-peak sensing can be realized through a high Q value and electric field enhancement, and the sensor has important application prospects in the field of terahertz biosensing detection.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical sensors and biomolecule detection, and particularly relates to a terahertz adjustable double-resonance plasmonic metasurface sensor based on quasi-bound continuous state. BACKGROUND

[0002] Terahertz (THz) spectroscopy has become a powerful tool for studying a variety of materials, especially for organic and biological molecules that have characteristic responses to terahertz radiation. The frequency range of terahertz waves is 0.1 to 10 THz, which is exactly in the middle of the transition from macroscopic electronics to microscopic photonics, with the advantages of low photon energy, strong penetration of non-polar substances, and fingerprint spectrum characteristics. It is worth mentioning that the molecular vibrations of most biological molecules can be observed in the terahertz region, so terahertz has a wide application in the field of biological sensing and identification. However, due to the weak interaction of most substances in nature with terahertz waves, the sensing sensitivity and accuracy are low, which has been a major difficulty in many related applications in the past. The emergence of metamaterials effectively solves this problem. Metamaterials are a kind of artificially designed composite materials, generally composed of periodic arrangement of subwavelength structure units. Because most of them are two-dimensional structures, they are also called terahertz metasurfaces. Metasurface sensors can produce a variety of high-intensity local resonances in the terahertz band through the design and arrangement of unit structures, such as multipole ring resonances, Fano resonances, continuous domain bound states, and electromagnetic induced transparency, thereby achieving a breakthrough in transmission capabilities.

[0003] Among them, the principle of continuous domain bound state breaks through the traditional wave bound mechanism, and the frequency exists in the continuous spectrum but has an infinite high Q factor, which can effectively enhance the interaction between light and matter. However, BIC cannot be monitored and obtained in an experimental system.

[0004] Based on this, the present application proposes a terahertz adjustable double-resonance plasmonic metasurface sensor based on quasi-bound continuous state (QBIC) by breaking the in-plane symmetry of the structure, which has a high Q value and a great surface electric field enhancement capability. At the same time, dynamic frequency modulation BIC can be realized to achieve double-resonance peak sensing by adjusting the parameters, which has important application prospects in terahertz metasurface sensing and detection. SUMMARY

[0005] The present application aims to solve the above-mentioned problems in the prior art, and provides a terahertz adjustable double-resonance plasmonic metasurface sensor based on quasi-bound continuous state, which has a strong near-field enhancement effect and can realize high-Q factor resonance through continuous domain bound state. Moreover, the present metasurface sensor can realize double-resonance peak sensing by adjusting parameters, and thus can realize high-sensitivity sensing and detection in the terahertz band.

[0006] To achieve the above object, the present application adopts the following technical solutions:

[0007] A terahertz adjustable double-resonance plasmonic metasurface sensor based on quasi-continuous domain bound state, comprising a transparent flexible substrate, wherein a periodic resonant unit array is arranged on the flexible substrate, and the periodic resonant unit comprises at least five metal elliptical resonators arranged at intervals.

[0008] The at least five metal elliptical resonators arranged at intervals are configured to support quasi-continuous domain bound state resonance by introducing structural asymmetry, so that the sensor presents at least two independent resonance peaks in the transmission spectrum in the terahertz wave band.

[0009] The periodic resonant unit comprises three first elliptical resonators with a long axis length y1, one second elliptical resonator with a long axis length y2, and one third elliptical resonator with a long axis length y3, and the second elliptical resonator and the third elliptical resonator are arranged at intervals by the three first elliptical resonators, wherein y1>y2, and y2≠y3.

[0010] The long axis length y1 ranges from 75 to 85 μm; the long axis length y2 ranges from 65 to 75 μm; and the long axis length y3 ranges from 40 to 65 μm.

[0011] In the present application, the short axis width x of the elliptical resonator is 8-12 μm.

[0012] In the present application, the gap distance between adjacent elliptical resonators is 10-16 μm.

[0013] The lateral period px of the periodic resonant unit array is 160-180 μm, and the longitudinal period py is 180-200 μm.

[0014] The flexible substrate is a polyimide substrate with a thickness h of 40-60 μm.

[0015] The sensitivity of the sensor based on the resonance peak of the quasi-continuous domain bound state is not less than 200 GHz / RIU.

[0016] A terahertz sensing method using the sensor, comprising: placing a to-be-measured object on the surface of the sensor, incidenting a terahertz wave to the sensor, and acquiring a transmission or reflection spectrum; determining the dielectric property or concentration of the to-be-measured object based on the frequency shift of at least one resonance peak in the spectrum.

[0017] A method of tuning the resonance frequency of the sensor, comprising: changing the long axis size of at least one elliptical resonator in the periodic resonance unit to adjust the asymmetry factor delta, wherein delta is the difference between the long axis lengths of different groups of elliptical resonators; by changing the asymmetry factor delta, continuous tuning of the quasi-continuous domain bound state resonance frequency is achieved.

[0018] The technical scheme of the present application has the following beneficial effects relative to the prior art:

[0019] 1. The present application places an array of periodic resonance units on a polyimide substrate, and forms a slit between the elliptical resonators. When a terahertz wave is incident, a high-Q factor resonance is formed by the continuous domain bound state due to the asymmetry, and the electric and magnetic fields are greatly enhanced, effectively enhancing the interaction between the substance and the terahertz wave.

[0020] 2. The Q factor of the present application is high, and when there is no substance to be detected, the maximum quality factor can reach 40 or more. By introducing an asymmetry factor to excite double resonance peaks, the resonance sensing sensitivity dominated by the dipole can reach 177 GHz / RIU; and the resonance sensitivity induced by the continuous domain bound state can reach 223 GHz / RIU, which is much higher than the traditional structure in the terahertz wave band, greatly improving the sensitivity of the sensor.

[0021] 3. The BIC resonance frequency of the present application can be flexibly regulated according to the sensing requirements, and the specific mode is to realize dynamic frequency modulation by adjusting the long axis length of the elliptical resonator. The resonances in the dipole mode and the continuous domain bound state mode do not interfere with each other, realizing double resonance peak sensing.

[0022] 4. The periodic unit structure of the present application is designed flexibly, and can be composed of high-conductivity elliptical, circular, square, etc. gold resonators and low-refractive-index substrates to achieve extremely high near-field enhancement.

[0023] 5. The present application can be used for terahertz sensing and detection of various biomolecules and drugs, such as glucose, ibuprofen, and roxithromycin, etc., which fills the gap of sensing of such samples in the terahertz frequency band.

[0024] 6. The unit structure of the present application is simple and only consists of five elliptical shapes, which is convenient for processing and manufacturing. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A structure schematic diagram of an elliptical metal double-resonance metasurface sensor based on quasi-continuous domain bound states is provided for the embodiments of the present application.

[0026] Figure 2 A simulated transmission spectrum when the typical size (y3=50μm) is provided for the embodiments of the present application.

[0027] Figure 3 (a) is an electric field distribution diagram of a dipole resonance mode provided by an embodiment of the present application, Figure 3 (b) is an electric field distribution diagram of a quasi-continuous domain bound state mode provided by an embodiment of the present application.

[0028] Figure 4 The simulation transmission spectrum of the six typical sizes (y3 = 40, 45, 50, 55, 60, 65 μm) provided by an embodiment of the present application.

[0029] Figure 5 The relationship diagram of the super surface Q factor and the asymmetry factor δ obtained by experiments provided by an embodiment of the present application.

[0030] Figure 6 The simulation transmission spectrum diagram of the refractive index n of the to-be-measured object ranging from 1 to 2 placed in the terahertz sensor provided by an embodiment of the present application;

[0031] Figure 7 The relationship diagram of the thickness of the to-be-measured object and the resonance frequency shift provided by an embodiment of the present application. DETAILED DESCRIPTION

[0032] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects more clear, specific embodiments will be described in detail below.

[0033] In the present application, the terms "first", "second", "third" and the like are only used to distinguish similar objects, and do not have to be used to describe a specific order or sequence, nor can they be understood as indicating or implying relative importance. In the description, the directions or positions indicated by "up", "down", "left", "right", "front" and "back" are based on the directions or positions shown in the drawings, and are only for the convenience of describing the present application, and cannot be understood as indicating or implying that the devices must have a specific direction, be constructed and operated in a specific direction, and therefore cannot be understood as limiting the scope of protection of the present application. For ordinary skilled persons in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0034] Referring to Figure 1 , the present application provides an elliptical metal double-resonance super surface sensor based on a quasi-continuous domain bound state, which comprises a low-loss high-transparency flexible polyimide substrate and a plurality of periodic resonance units arranged in an array on the substrate. The dielectric constant of the polyimide substrate is 3.5, and the thickness is 50 μm. The material of the periodic resonance unit is gold, and the conductivity is 4.56 x 10 7The periodic resonant unit includes five elliptical resonators, and a slit is formed between every two adjacent resonators. When a terahertz wave is incident, the continuous domain bound state is triggered due to the introduction of asymmetry, thereby forming a resonance with a high Q factor, realizing double-resonance-peak sensing of the dipole mode and quasi-continuous domain bound state mode, and generating great near-field enhancement, effectively enhancing the interaction between light and matter, and realizing high-sensitivity detection.

[0035] In the embodiment, the lateral period px of the periodic resonant unit array is 160 μm, the longitudinal period py is 200 μm, and the thickness of the flexible substrate is 50 μm. The major axis length y1 of the elliptical resonator is 80 μm, y2 is 65 μm, and y3 is 50 μm. The minor axis width x of the resonator is 8 μm. The slit distance gap between adjacent elliptical resonators is 10 μm.

[0036] The transmission spectrum obtained by simulation analysis is shown in Figure 2 The transmission characteristics between 0.4 and 1.2 THz are given, and dipole resonance f1 and quasi-continuous domain bound state resonance f2 are generated at 0.77 THz and 0.96 THz, respectively, showing double-band resonance with a high quality factor, which can realize high-sensitivity double-resonance-peak sensing.

[0037] Further, Figure 3 (a) in FIG. 1 shows the electric field distribution diagram of the super surface at the dipole mode resonance f1 = 0.77 THz. It can be seen that the resonance is mainly formed by the coupling of the left three elliptical resonators, and the local electric field is also mainly concentrated in the left three elliptical regions. Figure 3 (b) in FIG. 1 shows the electric field distribution diagram of the super surface at the quasi-continuous domain bound state mode resonance f2 = 0.96 THz. Compared with the dipole mode resonance, the local electric field is mainly concentrated in the right three elliptical resonators.

[0038] On the other hand, by changing the elliptical resonator length y3 in the periodic resonant unit, the change of the quasi-continuous domain bound state transmission spectrum frequency is simulated. As shown in Figure 4 It can be seen that the proposed structure shows geometric dependence, that is, the resonance frequency can be adjusted by adjusting y3. By changing y3, the quasi-continuous domain bound state can be observed, and as y3 increases, the resonance frequency has a clear tendency to shift to low frequency, and this resonance does not interfere with the dipole resonance of 0.75 THz on the left.

[0039] Specifically, the asymmetry factor δ = y3 – y2 is defined, and by changing the asymmetry factor δ, the transition from the continuous domain bound state to the quasi-continuous domain bound state is realized. Figure 5 The relationship between different asymmetry factors δ and Q factors is shown, and the Q factor is obtained by typical Fano formula fitting:

[0040]

[0041] In the formula, T is the transmittance, a1, a2, and b are real constant factors, j is an imaginary number, ω is the angular frequency of the incident light, and ω0 and γ are the resonant frequency and damping ratio, respectively. . Figure 5 The relationship between the Q-factor and the asymmetry parameter δ is shown, with the simulated Q-factor reaching a maximum of 44. The asymmetry factor δ ranges from 0 to 30 μm. When δ = 0 μm, the structure is in a bound state, ideally possessing an infinite Q-factor. As δ increases from 0 to 5 μm, a typical Fano-shaped resonance peak appears, with its linewidth gradually increasing. By adjusting the asymmetry factor δ, the asymmetry of the structure can be controlled, allowing for the adjustment of the resonance linewidth and Q-factor according to the requirements of practical applications.

[0042] Using the structure of this embodiment, Figure 6 This demonstrates the relationship between the refractive index change of the analyte and the transmission spectrum frequency. Specifically, analytes with different refractive indices were selected, and their terahertz transmittance was obtained. The sensor sensitivity was defined as S = Δf / Δn, where Δf represents the frequency shift of the resonance peak before and after the addition of the analyte, and Δn represents the change in the refractive index of the analyte. From the above formula, it can be calculated that the sensing sensitivity of the dipole-dominated resonance f1 in this example can reach 177 GHz / RIU, while the sensing sensitivity of the quasi-continuous domain bound state-induced resonance f2 is as high as 223 GHz / RIU. Both modes of resonance can achieve high-sensitivity sensing.

[0043] On the other hand, this invention analyzes the relationship between the resonant frequency shift of the terahertz sensor and the thickness of the object under test, such as... Figure 7 As shown, when the thickness of the test object is low, the frequency shift changes significantly. When it reaches 15 μm, the change begins to slow down. At this point, the thickness of the test object is gradually approaching the boundary of near-field enhancement, and the frequency shift gradually reaches saturation.

[0044] The above are merely specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A terahertz-tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states, characterized in that: It includes a transparent flexible substrate, on which a periodic resonant unit array is provided, the periodic resonant unit including at least five metal elliptical resonators arranged at intervals between each other; The at least five metal elliptical resonators arranged at intervals to each other are configured to support quasi-continuous bound-state resonance by introducing structural asymmetry, such that the sensor exhibits at least two independent resonance peaks in the transmission spectrum of the terahertz band.

2. The terahertz-tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The periodic resonant unit includes three first elliptical resonators with a major axis length of y1, one second elliptical resonator with a major axis length of y2, and one third elliptical resonator with a major axis length of y3. The second and third elliptical resonators are separated by the three first elliptical resonators, where y1>y2 and y2≠y3.

3. The terahertz-tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 2, characterized in that: The major axis length y1 ranges from 75 to 85 μm; the major axis length y2 ranges from 65 to 75 μm; and the major axis length y3 ranges from 40 to 65 μm.

4. The terahertz-tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The minor axis width x of the elliptical resonator is 8~12 μm.

5. The terahertz-tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The slit distance between adjacent elliptical resonators is 10~16μm.

6. The terahertz tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The periodic resonant unit array has a horizontal period px of 160~180μm and a vertical period py of 180~200μm.

7. The terahertz tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The flexible substrate is a polyimide substrate with a thickness h of 40~60μm.

8. The terahertz tunable dual-resonant plasmonic metasurface sensor based on quasi-continuous domain bound states as described in claim 1, characterized in that: The sensor has a sensitivity of no less than 200 GHz / RIU based on the resonant peak of the quasi-continuous bound state.

9. A terahertz sensing method using a sensor as described in any one of claims 1 to 8, characterized in that, include: The object to be tested is placed on the surface of the sensor, a terahertz wave is incident on the sensor, and the transmission or reflection spectrum is obtained; The dielectric properties or concentration of the analyte are determined based on the frequency shift of at least one resonant peak in the spectrum.

10. A method for tuning the resonant frequency of a sensor as described in any one of claims 1 to 8, characterized in that, include: The asymmetry factor δ is adjusted by changing the major axis dimension of at least one elliptical resonator in the periodic resonant unit, where δ is the difference in the major axis length of different sets of elliptical resonators. By changing the asymmetry factor δ, continuous tuning of the resonant frequency of the quasi-continuous domain bound state can be achieved.

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