Cavity enhanced droplet spectroscopy: Principles, perspectives and prospects

Micron-sized liquid droplets exhibit unique optical properties, including lower threshold energies for non-linear optical processes than are observed in the bulk liquid phase. Whispering gallery modes (WGMs) and cavity quantum electrodynamics lead to cavity enhancement of fluorescence and Raman scattering at wavelengths commensurate with WGMs. The principles of cavity enhanced droplet spectroscopy will be reviewed, along with the major developments in the field over two decades. The prospect of applying cavity enhanced techniques in the simultaneous determination of droplet size and composition is discussed. A systematic experimental examination of the cavity enhanced Raman spectroscopy of water droplets is presented. In particular, the influences of laser power and illumination geometry, ionic strength and the presence of a second Raman active scatterer, nitrate, on the shape of the water Raman band are explored. Prospects for applying cavity enhanced techniques to studies of aerosol dynamics are also discussed.