Abstract

Whispering-Gallery-Modes (WGM) in all types of microresonators show a very strong dependence on the surrounding medium. The refractive index distribution, especially in the area of the evanescent tail of a WGM influences its scattering and resonance frequency. Due to the narrow bandwidth of the resonances even a very small perturbation of the refractive index leads to significant changes of the resonant frequency as well as the amount of scattered light of the system. This property can be used to manipulate the spectral features as well as to map the spatial distribution of WGM in microresonators with only one nano-scale scatterer using a scanning near-field optical microscope (SNOM). Early approaches used the SNOM in so-called collection mode where the SNOM-tip, acting as a scatterer, at the same time collects a small amount of the scattered light [1,2]. Another approach uses the SNOM in scattering mode, where the tip provides only scattering and the scattered light is collected with a multimode fiber [3]. The latter has the advantage of very high resolution (approx. 50 nm) depending on the size of the tip, but needs an extremely sensitive detection scheme due to the small scattering signal.

© 2011 Optical Society of America