The Study Of Long-wave Infrared Refractive Index Sensors Based On All-dielectric Metasurface

Refractive index sensing,a technology of sensing environmental information by optical signals,has been widely used in food safety,biomedical and chemical industry,with advantages of no labeling and high timeliness.As artificially layered two-dimensional materials,metasurfaces have been extensively investigated in related applications for modulating the properties of electromagnetic waves.Among them,metasurface-based refractive index sensors are considered to be the first choice for on-chip integrated sensing due to thier rather small size and excellent light trapping capability.All-dielectric metasurfaces are usually able to inspire higher-Q resonant responses compared to metallic plasmonic metasurfaces due to their low-loss characteristics,thereby exhibiting higher potential performance in refractive index sensing applications.In recent years,the bound state in the Continuum（BIC）based on symmetry-induced radiation suppression has greatly promoted the research of high-performance refractive index sensing,by introducing defects or perturbations to couple it to the radiation mode to achieve high-Q resonance.However,such defects and perturbations make the metasurface sensitive to the polarization state of the incident plane wave,which will cause inconvenience to practical measurements.Fano resonance has also been widely used in refractive index sensing researches in recent years due to its sharp spectral lines and near-field enhancement effect,but most works have focused on the excitation of single Fano resonance,and less on high-Q multiple Fano resonances in all-dielectric metasurfaces.In addition,recent researches on refractive index sensors mainly focus on the near-infrared and terahertz bands.In this paper,based on all-dielectric metasurfaces,the refractive index sensing application of high-Q resonances supported by BIC theory and Fano resonance theory is investigated,respectively.And theoretically realize the design of polarization-independent refractive index sensor device and the design of high-Q adjustable dual Fano resonances for refractive index sensing in the long-wave infrared.The main contents are as follows:A high-performance dual-band polarization-insensitive long-wave infrared refractive index sensor based on a silicon elliptical tetramer metasurface is designed.By carefully arranging two sets of asymmetric silicon elliptical nanodisks to form a fourfold symmetric cluster,two quasi-bound states（QBIC）in the continuum with low radiation loss are excited,including an electric dipole QBIC（ED-QBIC）and a magnetic dipole QBIC（MD-QBIC）.The analysis of the electromagnetic field distributions reveal the physical mechanism of the dual-band response of the all-dielectric metasurface.Two sharp resonance peaks with FWHMs of 3.36 nm and 4.53 nm appear in the reflection spectrum,and Q values are 1916.80 and 1636.16,respectively.The refractive index sensitivity test shows that the sensitivities of the two resonance peaks can reach2756.67 nm/RIU and 2319.93 nm/RIU,respectively,corresponding to FOM values of820.44 RIU^-1 and 532.09 RIU^-1.The dependence of the metasurface to the incident plane wave polarization was also investigated,and the proposed sensor remained polarization insensitive whether irradiated by linearly polarized plane waves with polarization angles varying continuously from 0 toπ/2,or by LCP and RCP plane waves.It is theoretically demonstrated that this combination between asymmetric structures can achieve the polarization-insensitive property,providing a design strategy for polarization-independent refractive index sensing devices.An asymmetricΦ-type all-dielectric metaurface model composed of a cuboid and a circular ring is established to realize dual high-Q Fano resonance and used for long-wave infrared refractive index sensing.The offset distance between the center of the cuboid and the center of the ring is set as an asymmetric parameter,and the simulation yields three resonant responses in the transmission spectrum of this metasurface,including two Fano resonances with high Q factors caused by the introduction of the asymmetric parameter.The effect of the metasurface asymmetry on the Q factor of the Fano resonance is studied,and the results show that the Q factor tends to decrease with the increase of the asymmetry parameter.The field distribution analysis shows that the two Fano resonances are originated from different physical mechanisms,which confirms that the two Fano resonances arise from the coupling of two narrow-band discrete states with broad-band continuous background scattering states,respectively.Two Fano resonances can be independently or simultaneously modulated by changing the structural geometric parameters.In addition,the dependence of the Fano resonance responses on the polarization states is investigated by changing the polarization states of the incident plane wave,and the transmission spectra are basically the same when the incident plane wave are LCP and RCP.Finally,the sensing performance of the metasurface is simulated and analyzed by changing the refractive index of the surrounding medium.The sensitivities of the two Fano resonances are 1690 nm/RIU and 1185.2 nm/RIU,corresponding to the maximum FOM values of 1000 RIU^-1 and5387 RIU^-1,and the maximum Q values of 4263 and 35276,respectively.