Nanoparticle sensing beyond evanescent field interaction with a quasi-droplet microcavity

Sensing with whispering gallery resonators (WGRs) is largely limited by the weak perturbation of the whispering gallery mode (WGM) via the evanescent field. A new sensing regime using quasi-droplet WGMs allows WGRs to move beyond the limitation of the evanescent field and push the detection sensitivity to new heights. We present experimental results on the detection of 100 nm and 500 nm polystyrene particles in aqueous solution using thin-walled, hollow WGRs supporting quasi-droplet modes. The detection sensitivity in terms of mode shift and broadening is measured, with mode shifts of 400 MHz observed for 100 nm particles. In terms of the number of linewidths, this is 276 times larger than similar experiments with microsphere WGRs, thus showing a significant increase in detection sensitivity beyond the capability of standard evanescent field sensing with WGRs.