How does loss act on electromagnetic wave propagation in time-varying plasmas?

Time-varying media have attracted considerable attention for their novel properties, such as enabling light amplification within momentum band gaps. Dispersive media with near-zero permittivity, like plasmas, are promising candidates for temporal modulation because of their flexible electromagnetic characteristics. Existing studies on photonic time crystals typically treat material loss as an independent degree of freedom. However, we find that the intrinsic connection between loss and time modulation has been widely ignored. With the example of time-varying plasmas, we demonstrate that the electron density modulation process exhibits irreversibility, implying the existence of an inherent energy dissipation induced by time modulation. In this work, we establish a self-consistent density-modulated plasma model incorporating appropriate collision frequencies, further demonstrating that this precludes the possibility of achieving energy gain in such practical systems. Nevertheless, this dissipative model allows for unique mode selectivity, generating non-propagating waves under time modulation, offering prospects for implementing filters and subwavelength resonators. This study builds a bridge between the dynamic variation of electromagnetic parameters and energy dissipation and offers a fresh perspective on understanding the role of losses in time-modulated media.