Low threshold random lasing in dye-doped and strongly disordered chiral liquid crystals

Random lasing was experimentally investigated in pyrromethene 597-doped strongly disordered chiral liquid crystals (CLCs) composed of the nematic liquid crystal SLC1717 and the chiral agent CB15. The concentration of the chiral agent tuned the bandgap, and disordered CLC microdomains were achieved by fast quenching of the mixture from the isotropic to the cholesteric phase. Random lasing and band edge lasing were observed synchronously, and their behavior changed with the spectral location of the bandgap. The emission band for band edge lasing shifted with the change of the bandgap, while the emission band for random lasing remained practically constant. The results show that the threshold for random lasing sharply decreases if the CLC selective reflection band overlaps with the fluorescence peak of the dye molecules and if the band edge coincides at the same time with the excitation wavelength.