Microsphere-Integrated Photonic Crystal Sensors for Spatially-Resolved Refractive Index Measurement

Refractometers based on interferometric methods require sophisticated systems and stable environment. Non-interferometric refractometers can offer advantages in terms of reducing device costs and simplifying device configurations. In this Letter, we proposed a microsphere-integrated photonic crystal (PC) refractometer with a high spatial resolution. The refractometer forms magnified patterns of the PC substrate when immersed in liquid. By invoking the two-dimensional windowed Fourier transform to analyze the patterns, the refractive index of liquid can be determined. The validity of the proposed method is verified by measuring both standard uniform liquids and the dynamic process of oil-water merging, suggesting it is a simple and effective tool for measuring RI with spatially resolved micrometer-scale resolution.