  We consider interaction of a femtosecond light pulse with a one-dimensional
photonic crystal with relaxing cubic nonlinearity in the regime of
self-trapping. By use of numerical simulations, it is shown that, under certain
conditions, the spectra of reflected and transmitted light possess the
properties of narrow-band (quasi-monochromatic) or wide-band (continuum-like)
radiation. It is remarkable that these spectral features appear due to a
significant frequency shift and occur inside a photonic band gap of the
structure under investigation.
