Photonic Time-Crystals

Photonic Time-Crystals (PTCs) are media whose electromagnetic properties are modulated periodically in time. When the modulation period is comparable to a single cycle of the wave propagating within them, and the modulation amplitude is at least 10-20% of the refractive index, the dispersion relation in PTCs exhibits momentum bands separated by significant momentum gaps. The momentum gaps are especially interesting – as they give rise to exponential amplification (and de-amplification) of the waves associated with those gaps, extracting energy from the modulation (or giving away energy to it).  This talk will review the basic concepts of PTCs and will focus on light-matter interactions, especially on light emission in PTCs. The last part of the talk will describe recent experimental progress on observing time-refraction within a single optical cycle, and on nonlinear phenomena in these time-varying media.