A joint paper with colleagues from Graz and Cardiff has just appeared in Laser & Photonics Reviews. In this work, we develop a resonant-state theory for finite photonic systems that are periodically modulated in time. While these photonic time crystals are often discussed in terms of bulk bandgaps, realistic implementations are finite and open. We show that their dynamics is governed by resonant states (quasinormal modes), revealing how parametric amplification emerges as a resonant process. Congratulations to Adrià, Sergei, David, Egor, Thomas and everyone involved!
