Subject: conservation in reversed doppler effects in photonic crystals?

I have read the journal article titled, Reversed Doppler Effect in Photonic
Crystals written by Evan J. Reedm Marin Soljacic, and John D. Joannopoulos
(Physical Review letters, Volume 91, Number 13, 26 September 2003), in which
they describe a new method of changing the frequency of light by using a
shockwave in a photonic crystal.  Basically, they use a dielectric with a high
strain dependence, and create a crystal with a band gap above the frequency of
the incident light.  As the shockwave propagates through the crystal the
dielectric constant is changed the the band gap is shifted into the frequency
range.  This creates a moving reflector in the crystal, capable of "doppler"
shifting the frequency to a lower frequency.  My question is, depending on the
choice of dielectric material, this down shift in frequency can be accomplished
with the shockwave moving in either direction, how is conservation of energy
maintained?  Is the energy from the frequency shifted light transfered to the
shockwave?  How is energy conserved in both shockwave directions?

Re: conservation in reversed doppler effects in photonic crystals?