  We report on very high enhancement of thin layer's absorption through
band-engineering of a photonic crystal structure. We realized amorphous silicon
(aSi) photonic crystals, where slow light modes improve absorption efficiency.
We show through simulation that an increase of the absorption by a factor of
1.5 is expected for a film of aSi. The proposal is then validated by an
experimental demonstration, showing an important increase of the absorption of
a layer of aSi over a spectral range of 0.32-0.76 microns.
