Laser action in dye doped liquid crystals: from periodic structures to random media

The birefringence and natural ability to form periodic structures make cholesteric liquidcrystalline (CLC — chiral nematics) materials particularly attractive as 1D photonicband gap systems. If a CLC is doped with dye fluorescent molecules, in such a way thatthe maximum peak of fluorescence matches one of the edges of the selective stop band,laser action is expected at that spectral position. By confining the helical super-structureof chiral liquid crystals in polymeric micro-cavity channels, a tunable microcavity laserarray was achieved. In multiple scattering systems, the propagation of the light wavesis quite different, as optical scattering may induce a phase transition in the photontransport behavior. Beyond a critical scattering level, the system makes a transitioninto a strongly localized state and light transmission is inhibited. This effect can be usedas a photon trapping mechanism to obtain laser action in the presence of a gain medium.Random lasing modes come from interference effects which survive in disordered systemsand open a particular chapter in the study of the interplay between localization andamplification. Here, experiments performed on systems having different order degreeand confinement are presented and possible technological implications are discussed.