Whispering gallery mode optical resonators
Over the last few decades, optical resonators allowed enormous progresses in a number of different fields of fundamental and applied science. At small length scales, from microns to millimeters, dielectric micro-sphere and microtoroids showed impressive Q-factors which were exploited for bio-chemical sensing, lasers and nonlinear optics experiments. Silica microresonators were realized and whispering-gallery modes were excited by a fiber-taper applying cavity ring-down spectroscopy techniques for chemical detection in the near-infrared spectra region. We recently explored the idea of liquid optical microcavities, i.e. a self-sustained drop of liquid where light can be trapped and resonate with very low loss, demonstrating, for the first time, the feasibility of droplets as passive sensors. In a recent work, we showed evidence that simple oil droplets, suspended by a vertical wire thanks to surface tension, can be used in the same way as solid microspheres. The droplet itself serves as the sensor and the sample at the same time, where the internal optical field is directly used to probe dissolved analytes or nanoparticles. Free-space light excitation of whispering-gallery modes and laser frequency locking on resonant modes is achieved, recording Q-factors ranging from 10<sup>5</sup> to 10<sup>7</sup> in the near-infrared and visible spectral regions. In acqueous solutions, at certain visible wavelengths, the Q-factor exceeds 10<sup>8</sup>. Nanostructures and nanoparticles can be readily combined with such resonators thereby opening the way to further extension of this scheme to spectroscopic techniques and ultra-sensitive refractive-index measurements. Appealing applications for spectroscopy, bio-sensing, material characterization and non-linear optics are envisaged. In particular, it is possible observing the coupling of radiation with vibration modes of the droplet microcavity in order to realize all-liquid mechanical microoscillators.