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.