Efficient random laser effect in a new dye-nematic liquid crystalline composite

Random lasing effects in fully disordered systems with organic and inorganic nature have been the subject of extensive studies since the beginning of the past decade. In multiple scattering materials the optical diffusion may induce a phase transition in the photon transport behavior. Beyond a critical scattering level, the system makes a transition into a strongly localized state and light transmission is therefore inhibited. This effect can be used as a photon trapping mechanism to obtain laser action in the presence of a gain medium. In this paper we present the first experimental evidence of random laser action in a dye doped nematic liquid crystal (ZLI-2659) with negative dielectric anisotropy. This material exhibits long-range dielectric tensor fluctuations, favoring the multiple scattering. When exceeding a certain threshold pump power value, the fluorescence curve in the system collapses and distinct sharp peaks emerge above the residual spontaneous emission spectrum. The laser emission has several interesting features - linewidth of the laser peaks is extremely narrow banded (approximately 0.5nm FWHM), very low threshold (30 µJ/mm 2 ), high efficiency (~ 20%). The unforeseen surviving of constructive interference in repeated multiple scattering events of the dye emitted photons provide the necessary optical feedback for lasing in our nematic liquid crystalline system. We analyze the main physical characteristics of these relatively novel phenomena, by also including here the case of various confinement geometries for the active medium. These scientific aspects overlook features of great interest characteristic of laser physics and material science.