Design of subwavelength grating-based bimodal waveguide interferometric sensor in visible range for enhancing diagnostics sensitivity

The demand for point-of-care biosensors capable of rapid, accurate, and specific detection has grown significantly, particularly in the wake of the COVID-19 pandemic. Silicon photonics-based biosensors, based on evanescent wave sensing, have positioned as competitive candidates to meet these needs due to their high index contrast, compact size, high sensitivity, and specificity, CMOS compatibility, and low cost. Among various configurations, the bimodal waveguide (BiMW) interferometer stands out for its high sensitivity and compact footprint. Meanwhile, subwavelength grating (SWG) waveguides offer enhanced sensitivity by providing a larger active sensing area compared to conventional waveguides. In this work, we propose integrating SWG waveguides with BiMW interferometers, using silicon nitride (Si3N4) as the guiding material in the visible wavelength range to improve sensing performance through enhanced sensitivity and compactness. Si3N4 offers advantages such as low scattering losses and chemical stability in biological environments, while operation in visible range increases compatibility with biological assays, making this approach novel. We present a comprehensive device design, optimization, and analysis of the SWG-based BiMW interferometer for refractive index sensing applications. Three - dimensional (3D) full vectorial simulations were conducted using the finite element method (FEM) in COMSOL Multiphysics for the device design and optimizations. The proposed device demonstrates notable improvements in sensing performance, with an intrinsic bulk sensitivity of 0.1479 RIU/RIU, intrinsic surface sensitivity of 2.41×10⁻⁴ RIU/nm, and a theoretical phase sensitivity of 1318 rad/RIU/mm, all achieved within a compact footprint.