Dynamic tuning of the Q factor in a photonic crystal nanocavity through photonic transitions.

Photonic devices based on a high-Q cavity are commonly constrained by narrow operation bandwidth, slow charging, and releasing speed. Here, we analytically and numerically demonstrate that this limitation can be broken via dynamically tuning the Q factors of a single nanocavity and driving controllable photonic transitions between a low-Q cavity mode and an ultrahigh-Q mode by temporal refractive index modulations inside a multimode cavity. During this dynamic process, the bandwidth of signal light is also reversibly compressed. As a result, the fundamental link between the photon lifetime and the operation bandwidth is broken, and a delay-bandwidth product of ∼76 is achieved, which is far greater than that of the usual optical resonator systems. This approach may pave the way for dynamic control of on-chip all-optical information processing.