Density of States in Photonic Band Gap Liquid Crystals

Liquid crystals with modulated ground states are periodic dielectric structures, hence they are self-assembled photonic band gap materials. Fluorescent emission is suppressed in the stop band, but is enhanced at the band edges, and, above a pump threshold, distributed feedback lasing occurs. We interpret experimental observations of fluorescence alteration and lasing in terms of the density of states, and give a detailed description for helical cholesteric liquid crystals. Lasing occurs due to the singularity in the density of states at the band edges.