MODE SPECTROSCOPY OF COUNTER-PROPAGATING LIGHT IN MICRORESONATORS
Whispering gallery mode resonators are circular structures that have the ability to guide light around their walls in optical mode areas of the order of μm2. In combination to their high Quality factors (~ 108), they have a low threshold power for third order nonlinear effects taking place.
A strong beam changes the refractive index of the medium as experienced both by itself (self-phase modulation), but also twice as much by other overlapping beams (cross-phase modulation) circulating the resonator. Even though self- and cross-phase modulations are well known third order effects, they have yet to be observed as direct resonance shift in microresonators; They are often negligible compared to thermal resonance frequency shifts, caused by the strong beam warming up the resonator enough for it to expand, leading to a change in optical path and hence a resonance frequency shift. Like all third order nonlinear effects, self- and cross-phase modulations are dependent on the intensity of the beam.