Optical detection of nanoparticles by mode splitting in whispering-gallery-mode microcavities

Optical microcavities with high quality factors (Q factor) and small mode volumes have shown their potentials in various sensing applications. Here we experimentally demonstrate the real-time detection of single nanoparticles down to 30 nm in radius, using an ultra-high-Q microtoroid on a silicon chip. Mode splitting phenomenon of WGMs caused by their strong interactions with a single nanoparticle is utilized as the sensing signal. Frequency and linewidth information of the split modes is used to accurately derive the size of the particle detected. Theoretical calculations and finite element simulations are in good agreement with the experimental results. The mode splitting technique provides a self-reference scheme that is more immune to noise than the techniques based on the detection of changes of a single mode.