Coherent Umklapp Scattering of Light from Disordered Photonic Crystals
  A theoretical study of the coherent light scattering from disordered photonic crystal is presented. In addition to the conventional enhancement of the reflected light intensity into the backscattering direction, the so called coherent backscattering (CBS), the periodic modulation of the dielectric function in photonic crystals gives rise to a qualitatively new effect: enhancement of the reflected light intensity in directions different from the backscattering direction. These additional coherent scattering processes, dubbed here {\em umklapp scattering} (CUS), result in peaks, which are most pronounced when the incident light beam enters the sample at an angle close to the the Bragg angle. Assuming that the dielectric function modulation is weak, we study the shape of the CUS peaks for different relative lengths of the modulation-induced Bragg attenuation compared to disorder-induced mean free path. We show that when the Bragg length increases, then the CBS peak assumes its conventional shape, whereas the CUS peak rapidly diminishes in amplitude. We also study the suppression of the CUS peak upon the departure of the incident beam from Bragg resonance: we found that the diminishing of the CUS intensity is accompanied by substantial broadening. In addition, the peak becomes asymmetric. 
A. Y. Sivachenko, M. E. Raikh, Z. V. Vardeny (University of Utah)