This book focuses on the optical characterization of wide band gap semiconductor micro-nano structures and advanced optoelectronic devices including GaN-based laser diodes, GaN-based micro-LEDs, ZnO-based micro-lasers, Ga2O3-based deep UV photodetectors, etc., written by leading scholars in the field from the perspective of applied research and development of advanced optoelectronic devices.This book consists of 12 chapters, mainly presenting the authors’ own new findings, new theoretical models and experimental results, which can benefit researchers, engineers and postgraduate students in the field of semiconductor optoelectronics.