Ultrafast tilting of the dispersion of a photonic crystal and on-the-fly adiabatic frequency compression of light pulses

We demonstrate the ultrafast tilting of the dispersion curve of a photonic crystal waveguide following the absorption of a femtosecond pump pulse. By shaping the pump cross section with a shadow mask, different waveguide eigenmodes have different spatial overlaps with the perturbation, leading to a local flattening of the dispersion by up to 11%. Such partial mode perturbation can be used to adiabatically compress the spectrum of a light pulse in the waveguide.