Temporal modulation of photonic media generally induces effective gain and loss and thus non-Hermitian wave dynamics. Here, we show that under a temporally $\mathcal{PT}$-symmetric modulation, such media can nevertheless exhibit Hermiticity and a temporal $\mathcal{PT}$ phase transition. We prove that, for a homogeneously modulated photonic medium with complex-valued modulation $\epsilon \left(t\right)$, the temporal $\mathcal{PT}$ symmetry $\epsilon (-t)={\epsilon }^{*}\left(t\right)$ is a necessary but insufficient condition for obtaining a real eigenvalue spectrum, giving rise to $\mathcal{PT}$-symmetry phase transition. Specifically, the $\mathcal{PT}$ phase transition critically depends on the contrast between the modulation depth of the real and imaginary parts of permittivity when they are sinusoidally modulated with a $\pi /2$ phase difference—a minimal modulation scheme satisfying the temporal $\mathcal{PT}$ symmetry. We generalize the discretized temporal-interface transfer matrix method to a continuous differential operator framework, confirming the phase transition condition via Magnus expansion and Petermann analysis. Full-wave simulations and analytical calculations jointly confirm the occurrence of $\mathcal{PT}$ transition by examining the scattering behavior of a propagating pulse in such a type of modulated medium. The findings provide a temporal $\mathcal{PT}$-symmetric paradigm for controlling Hermiticity and non-Hermiticity in spatiotemporal photonic systems.

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• Metamaterials
• Photonic crystals
• PT-symmetry