The critical phenomena in a one-dimensional periodic stratified medium with time-varying dielectric permittivity functions are theoretically investigated for both TM and TE waves by solving the time-dependent electric displace ment equation and the magnetic field equation, respectively. It is found that there exists a critical value of the varying amplitude when the dielectric permittivity function is changing with time. When the varying amplitude is below or at the critical value, the periodic structure can reflect an incident plane wave into a periodic femtosecond pulse train with respect to time. On the other hand, when the varying amplitude is much above the critical value, the reflectivity is descending more rapidly with time, which means that, at one moment, the incident light is transmitted thoroughly and the periodic structure is becoming transparent. The effect of the layer number on the reflectivity is also presented. Especially at the critical value, the pulse train generated is periodic with almost 100% peaks and zero troughs for both TM and TE waves when the layer number is up to 11. These interesting conclusions could find potential realistic applications in the future.
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