On the free-space Gaussian beam coupling to droplet optical resonators

In this paper, light coupling into droplet optical resonators by means of a free-space Gaussian beam (GB) is investigated through numerical simulations and experiments. This method is introduced as an alternative to previously reported methods based on coupling through tapered fibers or prisms. Though applicable to solid-state optical resonators, this method is investigated here in the context of optofluidics for preserving the integrity of the droplet shape and for facilitating the steps of alignment and light coupling. The glycerol droplet under study is supported by a super-hydrophobic surface, which consists of Teflon-coated nanostructured silicon, to provide the advantage of keeping the droplet at a specific location, while maintaining a nearly spherical shape. The effectiveness of this method is tested with millimeter-sized droplets through measurements of their spectral responses. Quality factors Q in excess of 6 x 10(3) have been recorded. An analytical model for the external quality factor associated with this coupling technique has been derived, and the effect of the coupling parameters is demonstrated, allowing discussion about the scaling effects.