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Whispering-Gallery-Mode (WGM) microresonators have shown great promise for ultra-sensitive and label-free chemical
and biological sensing. The linewidth of a resonant mode determines the smallest resolvable changes in the WGM
spectrum, which, in turn, affects the detection limit. The fundamental limit is set by the linewidth of the resonant mode
due to material absorption induced photon loss. We report a real-time detection method with single nanoparticle
resolution that surpasses the detection limit of most passive micro/nano photonic resonant devices. This is achieved by
using an on-chip WGM microcavity laser as the sensing element, whose linewidth is much narrower than its passive
counterpart due to optical gain in the resonant lasing mode. In this microlaser based sensing platform, the first binding
nanoparticle induces splitting of the lasing line, and the subsequent particles alter the amount of splitting, which can be
monitored by measuring the beat frequency of the split modes. We demonstrate detection of polystyrene and gold
nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions. The built-in self-heterodyne
interferometric method achieved in the monolithic microlaser provides a self-referencing scheme with extraordinary
sensitivity, and paves the way for detection and spectroscopy of nano-scale objects using micro/nano lasers.
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