  Whispering gallery mode (WGM) optical resonators utilizing resonance shift
(RS) and mode splitting (MS) techniques have emerged as highly sensitive
platforms for label-free detection of nano-scale objects. RS method has been
demonstrated in various resonators in air and liquid. MS in microsphere
resonators has not been achieved in aqueous environment up to date, despite its
demonstration in microtoroid resonators. Here, we demonstrate scatterer-induced
MS of WGMs in microsphere resonators in water. We determine the size range of
particles that induces MS in a microsphere in water as a function of resonator
mode volume and quality factor. The results are confirmed by the experimental
observations.
