Study On Mode Behaviors Of Liquid-Crystal Random Lasers And Liquid-Crystal Nonlinearity Dependent Random Lasing

The random laser (RL) with its optical feedback provided by multiply scattering in disorder system rather than mirror cavities, is a very young and unique kind of laser. Besides its great value in the fundamental research of science, this micro-sized RL possesses a vast application prospect in industrial fields. Numerous disordered amplifying media have been demonstrated to exhibit RL action, among which the dye-doped nematic liquid crystal (DDLC) has received considerable interest due to its unique, flexibly controllable optical performance. In this article, by using DDLC as disordered amplifying media, random lasing in different confining geometries was experimentally investigated, and the existence of a boundary-free, coherent lasing was explored. Then, we characterized the mode behavior and its dynamic evolution of RLs in DDLC films, and the influence of the nanosecond pump pulse induced nonlinearly optical effects in nematic liquid crystals on the random lasing emission.Using a DDLC cuvette as the sample, the pump threshold, lasing mode and angular intensity distribution of random lasing emission were characterized, and controllable RLs with random mode distribution were realized. The results demonstrated that the window of the cuvette was not indispensable to the lasing process, and a boundary-free and coherent RL in DDLC was realized. Then, the influence of the boundary reflection on the mode, polarization and directionality of the liquid-crystal RL was investigated by further fabricating DDLC samples with planar and concave boundary structures which have a feature dimension commensurate with the pump size. It is found that the cavity formed by the boundary reflection can compel the mode distribution of the RL to develop from randomly spacing to nearly constant spacing which resembles that of the cavity resonance modes. The transition or competition between the resonance modes and the random modes depended on the boundary structure, pump area, and so on. Besides, remarkable wavelength shift of RL emission induced by reabsorption of the emission light was observed by changing the focused position of the pump pulse.The mode of RL emission in DDLC films was studied systematically by altering the thickness or the dye doping concentration of the special disordered amplifying medium. The overall wavelength positions of RL peaks do not change with the pump position on the sample and the thickness of the DDLC film, but depend strongly on the dye doping concentration. According to the details of the random lasing spectra, Fabry-Perot cavity-like emission mode behavior is observed, and the relationship between the average wavelength spacing and the thickness of the DDLC film satisfies the resonance condition of the cavity formed by the boundary reflection.Based on the nonlinearly optical effects in DDLCs induced by the nanosecond pump pulse, the strongly pump angle dependent RL output intensity versus input pump energy were characterized. The threshold intensity, slope efficiency, and maximal output intensity of the RL depend heavily on the pump angle with a nonmonotonical variation of relationship. According to the obtained special dependence of the random lasing emission on the pump energy and the pump angle, the underlying mechanism for the control of the RL output by the pump pulse induced nonlinearly optical effects was considered. The theoretical results fit the experimental data quite well.