Whispering-gallery modes in dielectric microspheres for biosensing applications

A dielectric microsphere in a lower refractive index medium exhibits sharp optical resonances, called whispering gallery modes. These are sensitive probes of refractive index changes near the sphere surface and can be thus exploited for biosensing applications. Using evanescent field coupling via frustrated total internal reflection, we measured whispering gallery optical resonances of polystyrene microspheres of 30-45 mum diameter, held in aqueous buffer by optical tweezers, and found Q-factors of up to 4 times 10⁶. We also demonstrated a monolithic sensor based on polystyrene microspheres in water attached onto an optical coupler via a solid separation layer index matched to water. As a proof of principle of the sensing capabilities, we measured the response to refractive index changes and inferred a shot-noise limited precision of 1 times 10^-8 refractive index units.