Sub-cycle time-refraction at optical frequencies

Abstract: Large and abrupt variations in the electromagnetic properties of materials lead to dramatic effects: even a single step-like change in the refractive index induces striking phenomena, such as time-refraction and time-reflection. When the refractive index varies periodically in time, multiple time-refractions and -reflections interfere, giving rise to photonic time-crystals (PTCs). Importantly, PTCs display momentum bands separated by gaps in which the modes experience exponential amplification, drawing energy from the modulation in a non-resonant fashion. Ordinary nonlinear optics does not operate in this regime: the material response is either very weak or very slow. One of the immediate consequences is that time-reflection of light at optical frequencies has never been observed in experiments. Here, we experimentally realize an order-unity change in the refractive index occurring at sub-cycle rates, and explore the phenomena emerging from it. By varying the duration of the index change from extending over many cycles to being significantly below a single cycle, we observe that the frequency shift of the time-refraction is enhanced as the index variation occurs faster. Our experiment is the gateway for realizing sharp time-interfaces at optical frequencies, which are the key for experimenting with time-reflection, PTCs and new phenomena expected from light-matter interactions in time-varying media.