Improved refractive-index sensing performance in medium contrast gratings by asymmetry engineering

Silicon nitride (Si3N4) subwavelength medium contrast gratings (MCGs) directly integrated with CMOS photodetectors are a promising option for on-chip label-free biosensing. The narrow spectral features required for sensing are often realized in Si3N4 nanostructures by weakly corrugated gratings which limit design flexibility. We numerically investigate the optical properties of asymmetry-engineered MCG gratings and predict the formation of ultra-sharp spectral features via the excitation of quasi-bound states in continuum (QBIC) resonances. Systematic investigation of the design parameter space shows that sharp spectral features are obtained for a wide range of parameters without requiring ultrathin grating profiles. Transmissionmode refractive index sensing simulations for bulk and surface sensing, considering both wavelength-shift and intensity-shift modalities, indicate performance gains using these structures.