Keywords:microcavity, nano particle, sensor
Hollow WGRs, such as the microbubble, have shown to have an improved sensitivity due to the large evanescent field penetrating into the liquid core of the resonator. A microbubble WGR, is essentially a thin glass shell supported on a hollow stem that allows fluids to be passed through the core of the resonator. WGMs are supported in the wall of the hollow resonator and create an evanescent field along the inner and outer wall surfaces. For thin walls, the mode can enter into the quasi-droplet regime and it strongly interacts with the material in the core. Minute changes to the environment result in a change of the WGM spectrum so the strongly interacting quasi-droplet modes make hollow WGRs uniquely suited for sensing applications.
Here, we demonstrate, for the first time, single nanoparticle detection using truly quasi-droplet WGRs and show that they can outperform solid WGRs in terms of dispersive and dissipative sensitivity. We also observe a number of interesting effects, such as regenerative self-modulation driven by the particle motion arising from strong scattering at the waveguide/cavity coupling junction.