Modulating time refraction via collisional plasma-formed photonic time interfaces

Temporal interfaces created by abrupt refractive-index changes enable frequency conversion and underpin photonic time crystals, but bidirectional control in a single physical system remains difficult. Here we show that collisional plasma provides pressure-controlled bidirectional microwave frequency modulation. We develop a self-consistent framework coupling finite-difference time-domain electrodynamics with particle-in-cell Monte Carlo collision kinetics, avoiding a prescribed Drude permittivity for the transient plasma response. The model predicts that high gas pressure produces a redshift dominated by collisional momentum relaxation, whereas low pressure produces a blueshift driven by rapid ionization and electron-density dispersion. Experiments using a waveguide-loaded gas discharge system reproduce the pressure-dependent shift directions predicted by the simulations. These results identify gas pressure as a control parameter for time refraction in plasma and provide a microscopic framework for designing dynamic temporal photonic systems.