Modified transmission spectrum induced by two-mode interference in a single silica microsphere

We theoretically and experimentally study the resonant transmission spectrum of light in a fibre taper coupled with a single silica microsphere cavity system, where two whispering-gallery modes (WGMs) are simultaneously excited. By changing the taper position (correspondingly, tuning the resonant frequencies of the two WGMs and modulating their coupling conditions with the fibre taper), a sharp electromagnetically-induced- transparency-like window can be observed in the transmission spectrum. This line shape origins from the taper-mediated interference between two co-existing WGMs in a single microsphere. This measurement result agrees well with the theoretical analysis.