  Optical resonant microcavities with ultra high quality factors are widely
used for biosensing. Until now, the primary method of detection has been based
upon tracking the resonant wavelength shift as a function of biodetection
events. One of the sources of noise in all resonant-wavelength shift
measurements is the noise due to intensity fluctuations of the laser source. An
alternative approach is to track the change in the quality factor of the
optical cavity by using phase shift cavity ring down spectroscopy, a technique
which is insensitive to the intensity fluctuations of the laser source. Here,
using biotinylated microtoroid resonant cavities, we show simultaneous
measurement of the quality factor and the wavelength shift by using phase shift
cavity ring down spectroscopy. These measurements were performed for
disassociation phase of biotin-streptavidin reaction. We found that the
disassociation curves are in good agreement with the previously published
results. Hence, we demonstrate not only the application of phase shift cavity
ring down spectroscopy to microcavities in the liquid phase but also
simultaneous measurement of the quality factor and the wavelength shift for the
microcavity biosensors in the application of kinetics measurements.
