Time-domain observation of strong coupling between counter-propagating ultra-high Q whispering gallery modes

An ultra-high Q whispering gallery mode (WGM) cavity is attractive because the light-matter interaction is enhanced inside it. In terms of science and engineering, an interesting use of a WGM cavity is as a coupled system. When two cavity modes are strongly coupled, they are split in the frequency domain and photons are transferred cyclically between the two modes in the time domain. Recently, the time-domain observation and control of the coupling states were reported with photonic crystal nanocavities, and this technology is essential for developing a quantum node and a quantum network. However, such experiments have not yet been achieved with ultra-high Q modes despite the potential benefit to be gained from the use of ultra-high Q cavities. In this study, we observed strong coupling between ultra-high Q modes in the time domain for the first time. We employed two counter-propagating modes that coupled with each other via surface scattering in a silica toroid microcavity. We employed two tapered fibers (add-drop configuration), one for excitation and the other for observing the energy oscillation between two cavities, which is a necessary technique for directly observing energy in a cavity. The results revealed clear oscillatory behavior, which was induced by the strong coupling. In addition, the oscillation period in the time domain precisely matched that inferred from the mode splitting in the frequency domain, and the measured results showed excellent agreement with those calculated with the developed numerical model.