Cavity-enhanced evanescent-wave chemical sensing using microresonators

Abstract

Scope and Method of Study: The purpose of this research was to examine fused silica microresonators into which light was evanescently coupled into whispering-gallery modes (WGMs) using a tapered optical fiber. The high quality factor (Q/) of these microcavities makes a sharp dip in a frequency scan when the resonance criterion is met. The depth and width of the dip are determined by the coupling coefficient between the coupler and the microsphere, and the intrinsic loss of the cavity. The use of these modes as sensors was examined.

Findings and Conclusions: Introducing an absorber in the evanescent field of the microresonator's WGM changes the round-trip loss of the cavity and hence a change in dip depth, width, and frequency of a WGM is observed. Absorbers studied in the dissertation include methane, methyl chloride, ethylene, indocyanine green dye in methanol at 800 nm, SDA2072 in methanol at 1650 nm, and ruthenium complex in water. Less than nM concentrations of dye in solution can be easily detected. Trace gas amounts in atmosphere could also be detected.