Miniaturized refractive index sensor based on bound states in the continuum combined with lateral confinement

Optical refractive index sensors with small footprints are essential components in compact biomedical and chemical analysis, owing to their unique advantages in size and real-time detection. In this work, we propose a miniaturized refractive index sensor based on bound states in the continuum (BICs) by utilizing compound gratings combined with two silver (Ag) mirrors on both sides. By confining light within the central grating region through reflection of the two Ag mirrors, we achieve a sensitivity of 362.07 nm/RIU and a figure of merit (FOM) of 1956.18 RIU⁻¹, while maintaining a small horizontal dimension of 20.96 μm. Additionally, BIC occurs when the grating height is tuned within the range of 771 nm–803 nm, resulting in a narrow linewidth that can be utilized to achieve an enhanced FOM of 8726.22 RIU⁻¹. Our research provides valuable insights for the development of high-performance on-chip sensors with compact footprints for future applications.