Transit-time of electromagnetic Gaussian packets traversing nonlinear periodic layered systems

We investigate the transmission of Gaussian packets of light in periodic binary multilayers with the two constituent materials having dielectric constant dependent on the electric field intensity. The transmission spectra for the linear response and for the second and third harmonics were calculated within the undepleted approximation. For the transit-time calculation we use the stationary phase method. The wave packets were constructed with the mid-frequency lying either at the lower gap edge, at the mid-gap or at the higher gap edge of the first photonic band gap of the linear contribution. In some cases the transmitted packets (of both the linear and harmonic responses) preserve the form, but not the mid-frequency values; in other cases the transmitted packets unfold producing peaks that leave the superlattice at different times.