Waveguide-based coupling of coated micro-spherical resonators

Coating of high-Q whispering gallery mode micro-resonators is typically performed in order to add the functionalities of the coating material to the unique properties of this type of resonators. Silica microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film is deposited. In order to effectively exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded. We applied this technique to the case of a SU-8 layer deposited on a silica microsphere. Main tests were performed around the wavelength of 770 nm because of potential application in biochemical sensing requiring low light absorption in aqueous environment. We show that by using integrated waveguides made with SU-8 polymer (rather than silica fiber tapers) we can fulfill the proper phase matching conditions thus exciting the fundamental WGM mainly confined in the coating. A further proof of the validity of the approach is obtained assessing the free spectral range of the excited modes which depends on the refractive index of the material in which the mode is confined.