Photon trapping in a time cavity flying at the speed of light

Abstract A refractive index step, moving at the speed of light, can serve as a trap for weak light waves. Here we shed light on highly unusual features of such trapping, making it fundamentally different from conventional “resting” cavities. First, such trapping can be described as time-tapping rather than space-trapping. Trapping in time provides superior quality Q, but this quality does not anymore impose very narrow spectral lines, breaking thereby the fundamental time-bandwidth limit inherent to most traps of any physical nature. Although the trapped states are almost entirely disconnected from the outer world, a nonreciprocal connection channel involving resonant radiation can be opened and tunably manipulated. Nonreciprocity breaks the balance between the energy currents directed into the trap and back, allowing to load the trap faster than it is possible for reciprocal systems. Exotic, up to now unobserved in optics states such as halo states, and phenomena as anomalously huge delays in transmission occur naturally in a time trap. Beatings between the trapped-in-time modes provide a novel tool for ultrafast shaping of both spectral and temporal waveforms for arbitrary weak wavepackets, even for single photons. Altogether, this promises a highly efficient platform for energy storage, pulse compression, waveform shaping and frequency conversion for ultrafast classical and quantum optics.