Quasi-bound state in the continuum supported by a compound grating waveguide structure for high-figure-of-merit refractive-index sensing

Herein, we realize quasi-bound states in the continuum (quasi-BICs) in a compound grating waveguide structure with a complex lattice. The first layer of the structure is a grating layer composed of two dielectric ridges with different heights and the same width, while the second layer of the structure is a dielectric waveguide layer supporting the guided mode. As the height difference in two dielectric ridges changes from a non-zero value to zero, the lattice of the structure turns from a complex lattice with the period $Λ$ to a simple lattice with the half of the previous period $Λ′=Λ/2$⁠. Therefore, the previous excitable odd-order resonant guided modes cannot be excited and become BICs. Assisted by the ultra-high Q factor of the quasi-BIC, we achieve high-performance refractive-index sensing at near-infrared wavelengths. Under the proper design, the optimum sensitivity and figure of merit (FoM) reach 676.8 nm/RIU and 9543 RIU⁻¹, respectively. Compared with the reported refractive-index sensors based on surface plasmon polaritons, localized surface plasmon resonances, and Bloch surface waves, the optimum sensitivity achieved in this work reaches the same order of magnitude, while the optimum FoM achieved in this work is one or two orders of magnitude higher. Our work reveals that quasi-BICs would become a powerful competitive mechanism to achieve high-performance refractive-index sensing.