[littlestymaar]

Stupid question, where does the energy comes from?

With reflection, I see how you can harvest energy through red-shift (the reflected photon ends up with a lower frequency after it bounces due to Doppler effect, which means it lost energy and gave it to the sail as kinetic energy), but here I don't really understand were the energy comes from.

[kurthr]

It's momentum transfer from deflecting (diffracting) the light through an angle. Similarly, any photon would be slightly red-shifted after hitting the surface whether it goes through or reflects off.