A review on optical characterization of refractive index in photonic related devices and applications

As one of the most important optical properties of a material, refractive index (RI) and its spatial distribution play important roles in managing the performances of photonic structures and devices. The capability to accurately and reliably characterize RI can be crucial for precise control of specifications of photonic devices, and is required in diverse scenarios, ranging from material inspections, processing controls and device stage characterizations. In this review, we discuss a variety of optical characterization techniques for RI profiling and measurements, leveraging optical interference contrast effects, phase-shifting effects, as well as spectroscopic responses in reflectometric and ellipsometric manners. In addition, we give a quick account of recent progress on these techniques empowered by advanced data treatments.