Liquid refractive index measurements using cavity in silicon via near-infrared interferometry

Refractive index, a key parameter of the optical properties of materials, is sensitive to the purity and concentration of liquids, making it a promising measure for monitoring various physical, chemical or biological processes. The refractive index of liquids is commonly measured using methods such as refractometry or interferometry, which are accurate but require relatively large sample volume and are limited in real-time measurement. Microfluidic devices have been proposed to handle small sample volume and provide real-time analysis, but they often require waveguide integration, which complicates the overall device design and fabrication. Here, we present a novel platform that integrates a self-assembled cavity in silicon with near-infrared (NIR) interferometry to measure the refractive indices of small volume liquids without the use of waveguides. Once a liquid is injected into the cavity beneath a thin silicon membrane through parallel circular fluidic ports, scanning is performed perpendicular to the silicon membrane and the cavity and acquires the sample-specific interferogram. After measuring five different pure liquids, the interface between immiscible mineral oil and DI water injected sequentially into the cavity is also detected with a refractive index contour. In addition, the NaCl concentration in DI water is successfully obtained with the refractive index measurement. Our refractive index measurements are accurate within 0.5% of the reported values.