Random lasing and weak localization of light in dye-doped Nematic liquid crystals

The first observation of random laser action in a partiallyordered, optically anisotropic nematic liquid crystal with long-rangedielectric tensor fluctuations is reported. Above a given pump power thefluorescence curve collapses and the typical narrowing and explosion effectleads to discrete sharp peaks. The unexpected surviving of interferenceeffects in recurrent multiple scattering provide the required optical feedbackfor lasing in nematics. Coherent backscattering of light waves inorientationally ordered nematic liquid crystals manifests a weak localizationof light which strongly supports diffusive laser action in presence of gainmedium. Intensity fluctuations of the speckle–like emission pattern indicatethe typical spatio-temporal randomness of diffusive laser emission. Acomparison of the laser action is reported for systems with different orderdegree: fully disordered semiconductor powders, self-ordered cholestericsand partially ordered nematic liquid crystals.