The ability to detect and size individual nanoparticles with high resolution
is crucial to understanding behaviours 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
30 nm in radius, using mode-splitting in a monolithic ultra-high-Q
whispering-gallery-mode (WGM) microtoroid resonator. Particle binding splits a
WGM into two spectrally shifted resonance modes, forming a self-referenced
detection scheme. This technique provides superior noise suppression and
enables extracting accurate size information in a single-shot measurement. Our
method requires neither labelling of the particles nor apriori information on
their presence in the medium, providing an effective platform to study
nanoparticles at single particle resolution.Comment: 23 pages, 8 figure