Low threshold lasing in cholesteric liquid crystals

Cholesteric liquid crystals, due to their periodic structure and large bire fringence, are 1-d photonic band-gap materials. Circularly polarized light of the same handedness as the cholesteric structure cannot propagate in the reflection band. We have studied the effects of this band-gap structure on the fluorescence spectrum of dyes dissolved in the cholesteric liquid crys tal, as well as on the fluorescence spectrum of the liquid crystal itself. We have found that emission is enhanced at the band edges, and, above a cer tain pump threshold, lasing occurs both in dye-doped and in pure liquid cry stals. Extremely low lasing thresholds and high efficiencies were observed in dye-doped systems. The results suggest an active energy transfer mechani sm between the LC host and the fluorescent dye molecules.