Random lasing in dye doped nematic liquid crystals: the role of confinement geometry

The first experimental evidence of random laser action in a partially ordered, dye doped nematic liquid crystal with long-range dielectric tensor fluctuations is reported. Above a given pump power the fluorescence curve collapses and discrete sharp peaks emerge above the residual spontaneous emission spectrum. The spectral linewidth of these emission peaks is narrow banded, typically around 0.5nm. The unexpected surviving of interference effects in recurrent multiple scattering of the emitted photons provide the required optical feedback for lasing in nematic liquid crystalline materials. Light waves coherent backscattering in orientationally ordered nematics manifests a weak localization, strongly supporting the diffusive laser action phenomenon in the presence of a gain medium. Unlike distributed feedback mirror-less laser, this system can be considered as a cavity-less microlaser where the disorder unexpectedly plays the most important role, behaving as randomly distributed feedback laser. The far field spatial distribution of the emission intensity shows a huge number of bright tiny spots spatially overlapped and the intensity of each pulse strongly fluctuates in time and space. Here, we report the main characteristics of this novel systems for various confinement geometries and under different conditions. A brief presentation of boundary-less systems such as free standing and freely suspended dye doped nematic films and droplets is also introduced, revealing unique emission features because of the complete absence of confining borders.