Estimation of Purcell factor from mode-splitting spectra in an optical microcavity

We investigate scattering process in an ultrahigh-Q optical microcavity coupled to subwavelength scatterers by introducing splitting quality Q(sp), a dimensionless parameter defined as the ratio of the scatterer-induced mode splitting to the total loss of the coupled system. A simple relation is introduced to directly estimate the Purcell factor from single-shot measurement of transmission spectrum of scatterer-coupled cavity. Experiments with polystyrene (PS) and gold (Au) nanoparticles, erbium ions, and Influenza A virions show that Purcell-factor-enhanced preferential funneling of scattering into the cavity mode takes place regardless of the scatterer type. Experimentally determined highest Q(sp) for single PS and Au nanoparticles are 9.4 and 16.19 corresponding to Purcell factors with lower bounds of 353 and 1049, respectively. The highest observed Q(sp) was 31.2 for an ensemble of Au particles. These values are the highest Q(sp) and Purcell factors reported up to date.