Before the cycle is over: Subcycle time refraction at optical frequencies

Large and abrupt variations in the electromagnetic properties of materials lead to dramatic effects: Even a single steplike 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 time 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 nonresonant 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 subcycle 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 exciting phenomena expected from light-matter interactions in time-varying media.