  We measured the angle- and polarization-resolved reflection and
photoluminescence spectra of ZnO inverse opals. Significant enhancement of
spontaneous emission is observed. The enhanced emission not only has good
directionality but also can be linearly polarized. A detailed theoretical
analysis and numerical simulation reveal that such enhancement is caused by the
abnormal dispersion of a high-order photonic band. The frozen mode at a
stationary inflection point of a dispersion curve can strongly modify the
intensity, directionality and polarization of spontaneous emission.
