  The time-dependence of the Bragg diffraction by one-dimensional photonic
crystals and its influence on the short pulse reflection are studied in the
framework of the coupled- wave theory. The indicial response of the photonic
crystal is calculated and it appears that it presents a time-delay effect with
a transient time conditioned by the extinction length. A numerical simulation
is presented for a Bragg mirror in the x-ray domain and a pulse envelope
modelled by a sine-squared shape. The potential consequences of the time-delay
effect in time-dependent optics of short-pulses are emphasized.
