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.