Nonlinear Sensing with Whispering-Gallery Mode Microcavities: From Label-Free Detection to Spectral Fingerprinting.

Optical microresonators have attracted intense interests in highly sensitive molecular detection and optical precision measurement in the past decades. In particular, the combination of a high quality factor with a small mode volume significantly enhances the nonlinear light-matter interaction in whispering-gallery mode (WGM) microresonators, which greatly boost nonlinear optical sensing applications. Nonlinear WGM microsensors not only allow for label-free detection of molecules with an ultrahigh sensitivity but also support new functionalities in sensing such as the specific spectral fingerprinting of molecules with frequency conversion involved. Here, we review the mechanisms, sensing modalities, and recent progresses of nonlinear optical sensors along with a brief outlook on the possible future research directions of this rapidly advancing field.