On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator

The ability to detect and size individual nanoparticles with high resolution is crucial to understanding the behaviour of single particles and effectively using their strong size-dependent properties to develop innovative products. We report real-time, in situ detection and sizing of single nanoparticles, down to 30nm in radius, using mode splitting in a monolithic ultrahigh-quality-factor (Q) whispering-gallery-mode microresonator. Particle binding splits a whispering-gallery mode into two spectrally shifted resonance modes, forming a self-referenced detection scheme. This technique provides superior noise suppression and enables the extraction of accurate particle size information with a single-shot measurement in a microscale device. Our method requires neither labelling of the particles nor a priori information on their presence in the medium, providing an effective platform to study nanoparticles at single-particle resolution.