Whispering Gallery Mode Resonators as Opto-Mechanical Probes to Nanoparticle-Microcavity Interaction and Charge

Whispering Gallery Mode (WGM) resonators demonstrate nearfield interactions that sensitively shift resonance frequencies with interaction lengths less than 200 nm. This range is ideal for observing the effect that double layers have on the motion of protein, virus, and artificial nanoparticles. In exploring the measurement possibilities we were immediately drawn to the opportunity for determining physical parameters that have not revealed themselves through microcavity experiments in the past. The first example of this general interest is the measurement of charge. Charge significantly influences the binding of virus and other nano-particles to microcavity bio-sensors, although surprisingly there have been no reports of the determination of either cavity charge density sigma(w) or nanoparticle charge q(p) using these sensors. In this paper we review our recent progress in experimentally demonstrating an approach for the determination of both. We use an opto-mechanical Whispering Gallery Mode (WGM) Carousel trap to extract the electrostatic interaction energy versus separation s between the cavity surface and an orbiting nanoparticle of know charge, from WGM frequency fluctuations induced by the particle. Next we fit this interaction energy to wallcolloid theory (first Debye-Huckel, and then Gouy-Chapman theory) and determine sw. This charge density on a just-prepared silica microsphere is far less than literature values. With this microcavity charge density in hand, a larger particle having unknown charge and orbiting the same microcavity has its charge measured from its associated electrostatic interaction energy. This charge is in agreement with independent measurements.