Can Second-Order Nonlinearity in Metal–Dielectric Metamaterials Pave the Way toward Elusive Photonic Time Crystals?

Abstract While a tremendous amount of theoretical work has been dedicated to time-varying photonics, practical implementation in the optical range has relied almost exclusively on transparent conductive oxides, which remain severely constrained by being slow and highly lossy. To bypass these limitations, I propose an alternative platform utilizing ultrafast second-order nonlinearities within an epsilon-near-zero semiconductor-silver metamaterial. Due to the relatively low loss in silver and polarization-selective pumping, ultralow pump absorption prevents thermal degradation, while absorption of the probe is sufficiently low to simultaneously permit the multicycle interaction lengths necessary for signal detection. I find that with pump powers on a scale approaching 1 TW/cm2, one can open a wide momentum bandgap of tens of percent and achieve small yet observable net parametric gain in time, offering a robust pathway to realizing photonic time crystals in the optical range.