Light-matter interactions in photonic temporal crystals

Recently, the emerging field of time-varying photonics has called for a broader theoretical framework to include the effects of temporal variations on light-matter interactions. A notable development in this context is the photonic temporal crystal, whose optical properties are modulated in a time-periodic manner. A distinctive feature of these crystals is their non-Hermitian band structures, which lead to momentum gaps with non-Hermitian degeneracies, or exceptional points, at their edges. In this talk, I will discuss how the spontaneous emission rate is net positive at the momentum gap frequency. This observation highlights the importance of the non-orthogonality of photonic Floquet eigenstates and brings the Petermann factor into focus in the context of photonic temporal crystals.