Photonic Time-Crystals

Photonic Time-Crystals (PTCs) are materials in which the refractive index varies considerably (on order one) periodically on the timescale of a single optical cycle. These media exhibit unusual properties: they conserve momentum but not energy, and display momentum bands separated by gaps within which waves can be amplified exponentially, extracting energy from the modulation. These features are especially exciting in the context of light-matter interactions, leading to new laser sources, new schemes of generating entangled photon states, and new ideas in nonlinear optics.  I will describe the concepts involved and highlight recent experimental and theoretical progress on PTCs and light-matter interactions therein.