A monolithic optical sensor based on whispering-gallery modes in polystyrene microspheres

We experimentally demonstrate a monolithic sensor based on sharp whispering-gallery optical resonances of polystyrene microspheres in water attached onto an optical coupler via a solid separation layer index matched to water. A Q-factor of 0.9×106 is measured on a microsphere of 44 μm diameter with a 650 nm thick separation layer. As a proof of principle of the sensing capabilities, we measure the response to refractive index changes and infer a shot-noise limited precision of 1×10−8 refractive index units