Characterization And Optimization Of The Properties Of Random Laser In Dye PM650Doped Nematic Liquid Crystals

Random lasers (RLs) with very narrow line-width and obvious threshold aredisordered optical structures of stimulated emission in which light waves are bothmultiply scattered and amplified. Its optical feedback to produce random lasing isprovided by the multiply scattering rather than a resonant cavity as in conventionallasers. Owing to its unique mechanisms and potential applications in micro-sized opticsand bio-medicine, random lasers have received considerable interest in recent years. Inthis article, using the dye-doped nematic liquid crystal (DDLC) as an amplifyingdisordered media, we characterized properties of liquid crystal RL in both experimentand numerical simulation, including spectrum, threshold, polarization and so on. Thenwe substantially improved the performance of liquid crystal RL by incorporating anone-side-reflection structure into the RL system.Dye (PM650)-doped nematic-liquid-crystal (E7) films were fabricated and then theprocedures and the fabrication technique to obtain effective random-laser sample wereexplored as well. Using the fresh DDLC films as the sample, the spectra and thresholdwere characterized by both high-resolution and low-resolution fiber-optic spectrometers.The divergence angle of the liquid crystal RL and intensity ratio of front to back emitionwere also measured. It was found that the ratio of front to back emition of RL fromDDLC sample appears favorable linear, both of which have a divergence angle of about20mrad. Based on the linear relationship of front to back emition intensity, we testedthe polarization of RL and found that liquid crystal RL is linearly polarized lights, thedirection of which is near to the rubbing direction.Based on the ring-laser theory, we constructed a two-dimensional RL system withMonte Carlo Method. We simulated the stimulating-scattering-emitting progress ofrandom lasing and studied the factors that have influence on the threshold of RL,including the thickness of the sample, the diameter of the pump beam, the densities ofdye and scatter-particles. Then we experimentally studied the effect of thethreshold-energy property of liquid crystal RL on the thickness of the sample, thedensity of dye, the area of pumped region and the disposal method of the inner surface.Both experiment and simulation results show that the threshold of RL can be effectively cut-down by increasing the thickness of the sample and the density of dye, or expandingthe pump region.By introducing a one-side-reflection structure into DDLC sample, we investigatedits effect on the properties of the threshold and pump-efficiency of liquid crystal RL bymeans of experiment, theoretical analysis and simulation. Experimental results showthat the incorporated of the reflection structure reduces the threshold of RL to about20%and raises the pump-efficiency to about10times. Then we explained how theone-side-reflection structure decreases the threshold of RL with localized-regime theory.Finally we simulated the two-dimensional RL with one-mirror structure using MonteCarlo Method. Simulation results confirm that one-side-reflection system candramatically decreases the threshold of RL.