All-dielectric metasurface two-parameter sensor based on quasi-bound states in continuum

<sec>Refractive index sensors based on metal metasurfaces are commonly limited by their low quality factors due to significant Ohmic losses in the metal material. In contrast, sensors based on all-dielectric metasurfaces can overcome this disadvantage. Currently, all-dielectric metesurface sensors based on symmetry-protected bound states in the continuum (BIC) have aroused intense research interest due to their ability to achieve ultrahigh quality factors. Such a metasurface sensor is mainly based on single BIC and single form of symmetry breaking. There are few studies on metasurface sensors of multiple BICs and multiple forms of symmetry breaking. In additon, the refractive-index sensors commonly neglect the influence of temperature fluctuation and thus suffer the crosstalk between the refractive index and temperature of the environment.</sec><sec>In this work, an all-dielectric metasurface composed of a periodic array of asymmetric semicircular-cylinder pairs is designed and two quasi-bound states in the continuum (QBIC) with high quality factors are obtained. By choosing three different forms of symmetry breaking (two in-plane and one out-of-plane), two robust QBIC modes can be generated in the selected near-infrared frequency band, and their resonance wavelengths, quality factors and polarization dependences exhibit different characteristics. Full-wave simulation results show that by measuring the resonance wavelengths of the two QBICs (denoted by QBIC1 and QBIC2), two-parameter sensing of refractive index and temperature can be achieved, which then solves the problem of crosstalk between the refractive index and temperature of the environment in refractive-index sensing. The dependence of quality factor on asymmetric parameters follows an inverse quadratic relation for the two QBICs. By adjusting the asymmetric parameters, the quality factor can be theoretially increased to any value, so that the sensing performance can be improved and adjusted. For refractive-index sensing, the QBIC1 can achieve a sensitivity of 194.7 nm/RIU and a highest figure of merit (FOM) of 8197 (corresponding to a quality factor of 45829); the QBIC2 can achieve a sensitivity of 170 nm/RIU, and a highest FOM of 4970 (corresponding to a quality factor of 28097). For temperature sensing, the QBIC1 can achieve a sensitivity of 7.77 pm/℃, and the QBIC2 can achieve a sensitivity of 24 pm/℃.</sec>