  We demonstrate, by theory and experiment, the ultrafast tilting of the
dispersion curve of a photonic-crystal waveguide following the absorption of a
femtosecond pump pulse. By shaping the pump-beam cross section with a
nanometric shadow mask, different waveguide eigenmodes acquire different
spatial overlap with the perturbing pump, leading to a local flattening of the
dispersion by up to 11 %. We find that such partial mode perturbation can be
used to adiabatically compress the spectrum of a light pulse traveling through
the waveguide.

Optics