Mapping of Whispering-Gallery-Modes in coupled disk microresonators

Here we show a different method of mapping WGM in disk microresonators using a small SNOM-tip for perturbing the modes evanescent field but tracking the transmission and reflection signals in the tapered fiber used for excitation. Surface roughness of our samples induces coupling of co- and counter-propagating waves forming a standing wave in the disk and leading to a spectral splitting of the WGMs. Placing the tip in the evanescent field of an anti-node of the standing wave while the laser is locked to a resonance increases the scattering and therefore the reflection signal.