Non-hermitian physics and exceptional points in high-quality optical microresonators

High-quality optical microresonators that can significantly enhance light-matter interaction have found broad applications in many fields including optical communications, microlasers, optical sensing, optomechanics, frequency combs and cavity quantum electrodynamics. Recently, they have become an excellent platform to study non-Hermitian physics and exceptional points by judiciously tailoring modal coupling and gain-loss distribution. Typical configurations include two or more coupled optical resonators, hybrid resonator-scatterer systems, and a single resonator with complex index modulation. In this chapter, we summarize the recent progress of non-Hermitian optical microresonators especially parity-time symmetric microcavities with balanced gain and loss and their applications in light transport, laser mode management, optical sensing, and optomechanics. These studies not only unveil the intriguing features associated with non-Hermitian physics and exceptional points but also provide new degrees of freedom to design optical systems with novel functionalities.