Whispering gallery modes of microspheres in the presence of a changing surrounding medium: A new ray-tracing analysis and sensor experiment

A simple plane wave, ray-tracing approach was used to derive approximate equations for the dielectric microsphere whispering gallery mode (WGM) resonant wavenumber and quality factor, as dependent on the surrounding medium's refractive index. These equations are then used to determine the feasibility of a micro-optical sensor for species concentration. Results indicate that the WGMs are not sensitive enough to refractive index changes in the case of gas media. However, they can be sufficiently sensitive for measurements in liquids. Experiments were carried out to validate the analysis and to provide an assessment of this sensor concept.