Enhanced sensing performance of a resonance-trap assisted quasi-bound states in the continuum sensor at oblique incidence

Asymmetric nanostructures that support quasi-bound states in the continuum (quasi-BIC) are known to exhibit an extremely sharp and sensitive optical resonance, making them attractive platforms for optical sensing applications. In this work, we numerically propose an asymmetric grating based on quasi-BIC for surface refractive index sensing with enhanced sensitivity and figure of merit. The gratings are realized by three asymmetric types: ridgeasymmetry, groove-asymmetry, and dual-asymmetry. Numerical simulations of the proposed structure were conducted by using the rigorous coupled wave approach and finite-difference time-domain method. At normal incidence, the three asymmetric gratings support a single quasi-BIC at 910.3 nm, a single quasi-BIC at 1067 nm, and double quasi-BICs at 910.4 nm and 1067.1 nm, respectively. The single quasi-BIC at 910.3 nm, achieved through ridge-asymmetry, and the quasi-BIC at 910.4 nm, achieved through dual-asymmetry, exhibit almost the same electric field distribution and high sensitivity of about 444 nm/RIU. At oblique incidence, another quasiBIC can be induced in ridge-asymmetry or groove-asymmetry types, which has the same resonance wavelength as the quasi-BIC in dual-asymmetry type. As the incident angle increases, the sensitivity of the quasi-BICs at longer wavelengths consistently increases in three types. Remarkably, as the angle approaches approximately 28 degrees , these quasi-BICs collapse into resonance-trapped BICs, leading to an extremely sharp resonance. Therefore, it is possible to achieve high resolution at this angle while maintaining high sensitivity simultaneously. Our study represents the first attempt to enhance the sensing performance of BIC sensors by using oblique incidence, which has significant implications for the design and application of such sensors.