Whispering-gallery-mode sensors for biological and physical sensing

The term whispering gallery mode (WGM) was first introduced to describe the curvilinear propagation of sound waves under a cathedral dome. The physical concept has now been generalized to include light waves that are continuously reflected along the closed concave surface of an optical cavity such as a glass microsphere. The circular path of the internally reflected light results in constructive interference and optical resonance, a morphology-dependent resonance that is suitable for interferometric sensing. WGM resonators are miniature micro-interferometers that use the multiple-cavity passes of light for very sensitive measurements at the microscale and nanoscale, including single-molecule and ion measurements. This Primer introduces various WGM sensors based on glass microspheres, microtoroids, microcapillaries and silicon microrings. We describe the sensing mechanisms, including mode splitting and resonance shift, exceptional-point-enhanced sensing and optomechanical and optoplasmonic signal transductions. Applications and experimental results cover in vivo and single-molecule sensing, gyroscopes and microcavity quantum electrodynamics. We also discuss data analysis methods and the limitations of WGM techniques. Finally, we provide an outlook for molecule, in vivo and quantum sensing. Whispering-gallery-mode sensors use multiple-cavity passes of light for very sensitive measurements at the microscale and nanoscale. This Primer introduces various whispering-gallery-mode sensor setups, describes different sensing mechanisms and highlights in vivo and single-molecule sensing applications in biology and physics.