Resonance fluctuations of a whispering gallery mode biosensor by
particles undergoing Brownian motion

D. Keng, S. R. McAnanama, I. Teraoka, and S. Arnold
Appl. Phys. Lett. 91 (2007, in press)

Nanoparticles suspended in the vicinity of a whispering gallery mode WGM biosensor are
detected from fluctuations in the driving light-guide transmission. These fluctuations are described
by Brownian particles perturbing the resonance wavelength in reaction to being polarized by the
WGMs evanescent field. Comparison between the autocorrelation of the measured fluctuations and
theory provides a first order approximation for the nanoparticle size and lays the basis for future
studies of interfacial dynamics. With this advance, the WGM biosensor goes beyond low-frequency
measurements of adsorption and desorption and into a world which has been dominated by
fluorescence correlation spectroscopy, but without labels.