Cross-polarization coupling of whispering-gallery modes due to the spin-orbit interaction of light

摘要

Light can couple between TE and TM whispering-gallery modes (WGMs) of a microresonator; the effect is easilyobservable when those modes are frequency-degenerate, and can result in coupled-mode induced transparency (CMIT).Fitting experimental observations of CMIT with a numerical model in which the cross-polarization coupling strength is afree parameter shows that the coupling strength is typically 10~(-8) – 10~(-7) per round trip. It is shown here that polarizationrotation of this magnitude can result from optical spin-orbit interaction through the asymmetry of a WGM. Using theeikonal approximation to describe a WGM, along with asymmetry of the microresonator about its equator, the maximumpossible polarization rotation per round trip can be calculated. Then accounting for spatial overlap and phase mismatchof the coresonant WGMs, using coupled-mode theory, gives coupling strengths in agreement with experiment.