|MadSci Network: Physics|
James, The article you refer can be referenced for others interested in the subject as PRL, 91, 133901 (2003). The article deals with an analytical approach and computational simulation of a temporary compression and relaxation in non-linear (photonic) materials. One effect of this compression and relaxation is a change in the dielectric constant of the material(and thus its index of refraction). The investigators have utilized a shockwave compression to establish a sharp, rapidly moving boundary between the compressed material and the relaxated material. The net effect is to lower the optical bandgap of the compressed material to lower energy. It is well established that at the bandgap edge, the reflectivity of a material increases sharply (this is really the heart of the matter and involves surface and bulk exciton energetics which I will not deal with here). So, by shifting the bandgap edge to lower energy photons with energies within that compressed bandgap can be tranmsitted through the relaxed portion of the material until they meet the sharp, rapidly moving boundary edge with the compressed material. At that point, they undergo bandgap reflection. But at what frequency? The investigators access that since the band gap acts at a lower energy than the relaxed material, it will reflect photons associated with this lower energy. They term this a "reverse Doppler effect", in which they attribute an object-type character to their shock front. Whether a shock front is an "object" could be debated, but their arguments for the physical phenomena seem sound. As for your argument that this process is non-symmetric and mean a loss of energy conservation, I find little support. That is, if you are implying that an oppositely moving shockwave will also "downshift" (sic) the photon energy. I don't think you appreciate that not only is the bandgap narrowing on compression, its position with respect to the vacuum level is also changing. I would suspect without thorough investigation that the oppositely moving shockwave will reflect photons with higher energy than are incident, and that the process is symmetric. Yours, ---* Dr. Ken Beck
Try the links in the MadSci Library for more information on Physics.