Study On Polarization Of Dye-doped Liquid Crystal Random Laser

Compared to traditional lasers,random lasing does not require a resonant cavity,and optical feedback mainly depends on the multiple scattering of random medium in random medium.The multiple scattering get the photons travel a longer path in the media,and result in a narrower spectra and stronger emission intensity when the system is in low disorder degree,in which case the incoherent random laser occurs.And the coherent random may take place with distinguished sharp laser spikes on the spectra when the system is in high disorder degree,in which case the photons have a certain probability to return to the original location after multiple scattering,forming a resonant cavity of conventional laser,in which the light field get feedback.Currently based on semi-classical theory of coherent random laser,researchers have discussed theoretical methods for calculating random laser modes.Random laser has the advantages of low manufacturing cost,small size,and simple structure.Random laser have broad application prospects in many fields such as display,polarization imaging technology,ambient light sources,and sensing.The liquid crystal based random laser is a kind of random laser using liquid crystal as scattering medium.In this paper,photoalignment method was used to align liquid crystal molecules and the effect of photoalignment on the characteristics of the random laser was studied,such as the threshold and polarization;the effects of shear stress on the intensity and polarization of the random laser emitted from the dye-doped polymer-dispersed liquid crystals(DDPDLC)were studied.Photoalignment method was used on liquid crystal based random laser and the effect of photoalignment on the the random laser was studied.Firstly,we compared the characteristics of random lasers emitted from orientated and non-oriented samples.The observation of the 50 ?m non-oriented DDPDLC without azo dye shows that the random laser threshold is 1.1m W/Pulse,which is lower than that from oriented sample(1.3m W/pulse).This is mainly due to the competitive absorption of(Methyl red)MR and PM597 in the oriented sample during the pumping process.The degree of polarization of the random laser emitted from the non-orientated sample is 0.1,which is significantly lower than the random laser polarization emitted by the oriented sample.This is because the liquid crystal molecules in the non-oriented sample are randomly distributed and the absorption of PM597 is anisotropic.Then,by studying the properties of random laser exited from different thickness DDPDLCs with orientation,it was found that as the sample thickness increases from 12.5 ?m to 50?m,the threshold decreases from 2 m W/pulse to 1.3 m W/pulse;as the sample thickness increases from 50 ?m to 100 ?m,the threshold increases from 1.3m W/pulse to 3.1m W/pulse.This is because the pump efficiency becomes saturated as the thickness increase continually,but the loss increases.As the sample thickness increases from 12.5 ?m to 50 ?m,the degree of polarization increases from 0.76 to0.98;as the sample thickness increases from 50 ?m to 100 ?m,the degree of polarization decreases from 0.98 to 0.75.This is mainly due to the fact that as the thickness increases,the overlap area of the pumping penetration depth and the orientation depth gradually decreases,so that the influence of the alignment layer becomes smaller and smaller.Photoalignment makes the random laser emitted from DDPDLC have a high degree of polarization,which can provide an important reference for random laser applications in display,polarization imaging,sensors and other fields.Moreover,the successful realization of the photoalignment method in the field of random lasers has made it possible to realize multi-domain orientation of random lasers,which has great significance for the realization of vector-random lasers.In DDPDLC,the threshold and polarization degree of the random laser can be adjusted by applying shear force.With the emission intensity weakens,the threshold of random laser from DDPDLC increases from 2.0 m J/pulse to 4.0 m J/pulse,and the degree of polarization increases from 0.1 to 0.78 obviously when the shear distance increases from 0mm to 4mm.The liquid crystal droplets are gradually oriented in the shear direction because of the alignment direction of polymer chain and anisotropy of droplet shape.Scattering in DDPDLC is provided by the difference in refractive index between the polymer and the liquid crystal molecules.Defining the shear direction as parallel direction.As the liquid crystal droplets are gradually oriented in parallel direction,the scattering intensity in perpendicular direction gradually decreases and that in parallel direction gradually increases.The controllable intensity and polarization degree of random laser based on shear force have great application value in the fields of light sensing and liquid crystal display.