Characteristics of polarizable particles in a fluid are detected using an optical cavity comprising opposed optical reflectors containing the fluid. A particle is introduced through the fluid into the optical cavity. The particle may be transiently in the cavity or optically trapped. The optical cavity containing the particle is illuminated with light that excites resonance of an optical mode of the optical cavity that is affected by the particle. A measurement of a parameter of the excited resonance is derived, for example while tuning through the resonance. Repeated measurements may be used to derive a measure of a characteristic of the particle that is dependent on the motion of the particle in the optical cavity.