A sensing device based on whispering-gallery mode splitting in response to changes in refractive index between coupled microresonators

Coupled microspherical whispering gallery modes are highly sensitive to environmental and structural changes in the region of contact between two microcavities. Placing a sample between two microspheres changes the refractive index of their contact region, which can be useful for sensing applications. Here, we propose a solid-liquid two-phase resonator structure (“bi-sphere”) based on a pair of coupled microspheres with a water droplet in the gap between them, and investigate its spectral characteristics by monitoring the transmission spectral signal during water evaporation. In addition, the feasibility of the proposed sensor device is demonstrated by a refractive index sensing experiment where the resultant mode splitting is linearly related to the refractive index in the range of 1.400–1.450. The proposed bi-sphere device could have important applications in the detection of substances.