Cavity gain enhanced slow light in a hybrid photonic molecule system

We propose a hybrid photonic molecule system, which includes a two-level system coupled to two optical cavities and the two cavities interact with each other by a phase-dependent photon-photon interaction. The absorption spectra of the two-level system manifest one or two transparent windows (zero absorption deeps) by the dark-mode effect or by breaking the dark-mode effect, which is accompanied by the rapid dispersion leading to the fast or slow light propagation effect. Combining the phased-dependent photon-photon coupling with the interactions between the two-level system and two optical cavities, the dark-mode effect is controllable due to the quantum interference effect, which together determine the process form fast to slow light effect. Moreover, we consider one optical cavity is loss and the other one can be loss, neutral, or gain. The manipulation and periodic switching of group index can be achieved by tuning the modulation phase, and the fast- and slow-light effects are particularly pronounced in the scenario of one optical cavity is active (gain), compared to those are loss or neutral. This study lays the foundation for the application of photon-mediated optical information storage and processing.