Probing dynamics of time-varying media: Beyond abrupt temporal interfaces

TL;DR

This paper explores the effects of ultrafast time-varying media on electromagnetic waves, deriving new models for wave flux ratios and intensity saturation, emphasizing the importance of precise temporal control in experiments.

Contribution

It introduces a switching parameter to model ultrafast media changes and provides a unified framework for understanding wave behavior in such media, supported by experimental data.

Findings

Derived a generalized flux ratio expression for plane waves.

Identified intensity saturation in temporal transition radiation.

Showed dependence of maximum intensity and transition time on electron speed.

Abstract

This work investigates the effects of time-varying media, where optical properties change over time, on electromagnetic wave propagation, focusing on plane waves and free-electron evanescent waves. We introduce a switching parameter, $\tau$, to model ultrafast switching in the femtosecond to nanosecond range. For plane-wave incidence at angular frequency $\omega_0$, we derive a generalized expression for the backward-to-forward flux ratio as a function of $\omega_0$ and $\tau$, aligning with recent experimental data and providing a unified interpretation framework. For free-electron incidence, we observe intensity saturation in temporal transition radiation at $I_{\textrm{max}}$ for $\tau \leq \tau_{\textrm{0}}$, with both $I_{\textrm{max}}$ and $\tau_{\textrm{0}}$ depending on electron speed. These results highlight the importance of precise $\tau$ control in experiments to probe time-varying media effectively.