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 large, 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, extracting energy from the modulation. This talk will review the EM concepts of PTCs, and will focus on light-matter interactions and especially on light emission and nonlinear frequency conversion in PTCs. The last part of the talk will describe recent experimental progress on observing time-refraction within a single optical cycle.

Acknowledgements:  European Research Council (ERC-Consolidator) under grant agreement No. 101045135 (Beyond Anderson).