Shock waves propagating in photonic crystals can be used to lower or raise the frequency of light passing through them. It seems (to me) to be some kind of Doppler effect at work here. The electrons in the atoms of the crystal absorb the light, and as they move because of the propagating shock wave, they emit the photon, which then experiences a Doppler shift. Can anyone confirm this hypothesis, or is there another explanation for this? Would a similar, if less noticeable, but equally less destructive effect, be visible (watch the pun ) if sound waves were used in photonic crystals propagating light?