Cavity-enhanced spectroscopy on silica microsphere resonators

Microcavities are a powerful tool for chemical detection and sensing, but also to study chemical processes at interfaces. In most experiments the cavity mode spectrum is used to infer the chemical composition of the resonator medium and its immediate environment. For example, frequency shifts of cavity modes can be related to either cavity length changes or refractive index changes. Photon lifetime measurements, on the other hand, allow in principle for an independent measurement of the optical loss experienced by a cavity mode, but time-resolved cavity ring-down measurements are difficult due to the small dimensions of the cavity and the consequent short photon lifetimes (ring-down times). This chapter describes how phase-shift cavity ring-down spectroscopy can be adapted to extract the optical loss of a whispering gallery mode in a microresonator. By combining different phase-shift measurements of the total optical loss one can furthermore separate the contributions from intracavity loss due to absorption and scattering from the contributions of optical loss due to coupling to a light delivery waveguide. The experimental focus is on the use of silica sphere microresonators. The frequency of the high-Q whispering gallery modes in these microspheres is strongly dependent on the size of the sphere whereas the intracavity loss is influenced by surface absorption and scattering. A sub-monolayer of ethylene diamine on a 300 ?m sphere has the effect of, simultaneously, changing the resonance frequencies and the ring-down times of the whispering gallery modes. The absolute surface coverage can be extracted from the resonance frequency, and can be combined with the measurement of intracavity loss to determine the absolute absorption cross section of ethylene diamine at sub monolayer coverage. © 2014 Springer-Verlag Berlin Heidelberg.