On the electrodynamics in time-dependent linear media
  In this work we study the classical electrodynamics in homogeneous conducting and nonconducting time-dependent linear media in the absence of charge sources. Surprisingly, we find that the time dependence of the permittivity gives rise to an additional term in the Ampere-Maxwell equation and an asymmetry between the electric and magnetic field wave equations. As special cases we consider a linear and an exponential growth of the permittivity, as well as a sinusoidal time-dependent permittivity. 
I. A. Pedrosa, A. Yu. Petrov and Alexandre Rosas
physics.class-ph cond-mat.other