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

摘要

Random lasing effects in fully disordered systems with organic and inorganic naturehave been the subject of extensive studies since the beginning of the past decade. In multiplescattering materials the optical diffusion may induce a phase transition in the photon transportbehavior. Beyond a critical scattering level, the system makes a transition into a strongly localizedstate and light transmission is therefore inhibited. This effect can be used as a photon trappingmechanism to obtain laser action in the presence of a gain medium. In this paper we present the firstexperimental evidence of random laser action in a dye doped nematic liquid crystal (ZLI-2659) withnegative dielectric anisotropy. This material exhibits long-range dielectric tensor fluctuations,favoring the multiple scattering. When exceeding a certain threshold pump power value, thefluorescence curve in the system collapses and distinct sharp peaks emerge above the residualspontaneous emission spectrum. The laser emission has several interesting features - linewidth of thelaser peaks is extremely narrow banded (approximately 0.5nm FWHM), very low threshold(30 μJ/mm2), high efficiency (20%). The unforeseen surviving of constructive interference inrepeated multiple scattering events of the dye emitted photons provide the necessary optical feedbackfor lasing in our nematic liquid crystalline system. We analyze the main physical characteristics ofthese relatively novel phenomena, by also including here the case of various confinement geometriesfor the active medium. These scientific aspects overlook features of great interest characteristic oflaser physics and material science.