We study Raman scattering of single nanoparticles coupled to a high-$Q$ whispering-gallery microresonator. It is found that cavity resonances greatly enhance the Raman signal, and the enhancement factor is as high as ${10}^8$. Unlike the noncavity case, the signal power exhibits a nonmonotonic dependence on particle size, and it reaches the maximum when the Rayleigh scattering loss and the cavity intrinsic loss are comparable. We further analyze how the Raman signal intensity is influenced by different parameters including cavity quality factors and taper-cavity coupling strength. The detection limit of observing single-nanoparticle Raman signal is discussed finally. As a potential application, this mechanism may provide an alternative way to detect specific biological targets without the need of precovered biorecognitions.

• Received 27 November 2014

DOI:https://doi.org/10.1103/PhysRevA.91.043836