Single virus detection from the reactive shift of a whispering-gallery mode
F. Vollmer, S. Arnold, and D. Keng
PNAS, 105, 20701-20704 (2008).

We report the label-free, real-time optical detection of Influenza A virus particles. Binding of single virions is observed from discrete changes in the resonance frequency/wavelength of a whispering-gallery mode excited in a microspherical cavity. We find that the magnitude of the discrete wavelength-shift signal can be sufficiently enhanced by reducing the microsphere size. A reactive sensing mechanism with inverse dependence on mode volume is confirmed in experiments with virus-sized polystyrene nanoparticles. By comparing the electromagnetic theory for this reactive effect with experiments, the size and mass (5.2 10−16 g) of a bound virion are determined directly from the optimal resonance shift.