  Ultrasensitive optical detection of nanometer-scaled particles is highly
desirable for applications in early-stage diagnosis of human diseases,
environmental monitoring, and homeland security, but remains extremely
difficult due to ultralow polarizabilities of small-sized, low-index particles.
Optical whispering-gallery-mode microcavities, which can enhance significantly
the light-matter interaction, have emerged as promising platforms for
label-free detection of nanoscale objects. Different from the conventional
whispering-gallery-mode sensing relying on the reactive (i.e., dispersive)
interaction, here we propose and demonstrate to detect single lossy
nanoparticles using the dissipative interaction in a high-$Q$ toroidal
microcavity. In the experiment, detection of single gold nanorods in an aqueous
environment is realized by monitoring simultaneously the linewidth change and
shift of the cavity mode. The experimental result falls within the theoretical
prediction. Remarkably, the reactive and dissipative sensing methods are
evaluated by setting the probe wavelength on and off the surface plasmon
resonance to tune the absorption of nanorods, which demonstrates clearly the
great potential of the dissipative sensing method to detect lossy
nanoparticles. Future applications could also combine the dissipative and
reactive sensing methods, which may provide better characterizations of
nanoparticles.
