Propagation of Cylindrical Waves in Media of Time-Dependent Permittivity

We present the propagation of cylindrical waves in the media whose permittivity varies with time abruptly or continuously. By the method of variable separation, we derive the general expression of electric field of TM-polarized waves in two-dimensional space with excitation of any point at any time. With this expression, the solution for a spatially and temporally distributed source can be obtained theoretically. The focusing of reflected waves in the cross section is shown when the media undergoes a sudden or continuous change. The wave propagation in time-invariant media can be considered as a special case of the media under exponential variance.