GaN Optical Refractometer With Hemispherical Resin Lens

Refractive index (RI) sensing is critical for diverse applications, yet conventional optical approaches face limitations, including complex instrumentation and alignment requirements. In this work, a compact optical refractometer integrates a GaN-based chip with a resin lens. The device, fabricated via wafer-scale epitaxy and microfabrication, incorporates on-chip light-emitting diode (LED) and photodiode (PD). A cost-effective molding technique enables the integration of a hemispherical resin lens, eliminating external components and enhancing optical coupling. Device characterization confirms robust electro-optical performance, with lens integration significantly enhancing optical coupling. Variations in the ambient RI modulate the total internal reflection (TIR) at the lens interface, altering the light received by the on-chip detector. The sensor demonstrates a sensitivity of $69.36~\mu $ A/RIU across a broad RI range (1.3331–1.5405) and a resolution of $7.2\times 10^{-{7}}$ RIU. It exhibits rapid response (0.118 s) and recovery (0.140 s), along with excellent repeatability over 100 cycles. Real-time monitoring of glucose dilution dynamics and quantitative detection of honey adulteration validate its practicality. This compact, high-performance platform offers a promising solution for scalable RI sensing in biomedical, environmental, and food integrity applications.