| Random laser(RL)is a special kind of laser radiation with multiple scattering as optical feedback and it can be prepared to a size of several tens of micrometers or less.It not only has important basic research significance,but also shows a very broad application prospect.In this paper,lateral random lasing(LRL)was realized in dye-doped nematic liquid crystals(NLCs).First,the dependence of threshold,facula distribution and polarization of LRL on the NLC-cell substrate and its surface treatment were characterized.Then the electrical and thermal behaviors of LRL in dye-doped NLCs was investigated in detail.Finally,the effect of the electrically controlled boundary formed by two regions with different nematic director orientations and its position on the LRL was studied.Dye-doped NLC cells with a thickness of several micrometers were prepared,and the absorption,spontaneous fluorescence and substrate-surface-treatment dependent director orientation distribution of the samples were characterized.Then a stable liquidcrystal LRL was realized by controlling the width of the pump pulse.The parallel arrangement of nematic director had different order parameters before and after surface treatment,which makes the absorption,spontaneous fluorescence emission and light scattering of samples with different polarization dependences,and then it affected the pumping threshold energy,polarization and facula distribution of LRL.By applying an alternating voltage across the parallel aligned dye-doped NLC film,it will reorient the nematic director from parallel toward vertical to the substrate,thereby driving the transition dipole of the dye molecules doped in the liquid crystal similarly.The electrically controlled change of the orientation of the transition dipole led to electrically tuning of absorption of pump pulse and spontaneous fluorescence,thus the electrically induced reorientation of the nematic director was able to tune the strength and dominant polarization component of LRL.The effect of temperature on LRL was investigated by heating the NLC sample.Increasing the temperature below 50 °C caused a gradually decrease of the orientation order,and thus the threshold energy required for LRL increased slightly.When the temperature rises from the namatic phase to the isotropic phase,the absorption of the linearly polarized pump pulse along the nematic director and the pumping efficiency of spontaneous fluorescence are significantly reduced in the sample,and the light scattering feedback also becomes weak.LRL could still be realized at these high temperatures under a much larger pumping energy.The most striking thing is that near the phase transition temperature ?52 °C of dye-doped NLCs,the special balance between gain and loss made LRL show an unusual facula distribution,polarization and mode behavior.Edge-etching sub-millimeter-sized electrodes of different widths are used to control the orientation of the nematic liquid crystal director by an applied voltage,resulting in an electrically controlled rotation of liquid crystal director in the non-etching area to influence the generation of LRL.The length of the etched area and the applied voltage across the NLC film in non-etching area can control the absorption of the pump pulse,the pump efficiency of spontaneous fluorescence and the strength of optical feedback,so that LRL shows etching-size dependent,electrically tuned output intensity,and owns a different variation of mode behavior with the applied voltage compared to that for the sample without edge-etching electrodes. |
