Study On Dye-doped Liquid Crystal Random Laser Based On The Enhancement Of Silver Film

Different from traditional lasers,random lasers are generated by multiple scatter in a disordered gain medium.As a controllable optically anisotropic medium,liquid crystal can provide refractive index difference for irradiated light to enhance scattering,and its molecular alignment is easily controlled by the external field,which makes it easy to control the random lasing behavior.Basing on the photo-alignment of dye-doped nematic liquid crystal(DDNLC),this paper studies the regulation of the random laser(RL)emission characteristics of DDNLC by rough silver film and oriented film substrate.A mixed solution of azo dye methyl red and polyvinyl alcohol is made to form an alignment film,in which methyl red absorbs linearly polarized light and undergoes cis-trans isomerization.After being prepared into a liquid crystal cell,the liquid crystal molecules can be induced to align perpendicular to the polarization direction of the polarized light.The comparison of the emission spectra of DDNLC with or without an orientation layer proves that the orientation layer is the key for random lasing action,and the threshold of one-side oriented DDNLCRL is 3.6 m J/cm2.Different concentrations of organic phase Ag nanoparticles(NPs)were used to make rough silver film substrates,and the relationship between the random lasing characteristics of DDNLC with one-side coated Ag or two-side coated Ag and the concentration of Ag NPs was studied.The results indicate that with the increase of the Ag NPs concentration,the single and double-layer rough silver film reduces the random laser emission threshold of DDNLC first and then increases,reaching the lowest threshold when the Ag NPs concentration is 1.12×10^-9 mol/L,1.6 m J/cm² and 1.5 m J/cm²respectively.After measuring the reflectivity of the silver film substrate and converting the pump light power,it is found that the Ag NPs in the rough silver film at low concentrations mainly exhibit enhanced scattering and surface plasmon resonance to the DDNLC radiation,which makes the random laser threshold decrease and the intensity increases.With the increase of Ag NPs concentration,the rough silver film will cause the fluorescence quenching of laser dye PM597,reducing its gain efficiency.And the increase of the areal density of Ag NPs increases the reflection loss and reduces the scattering of radiated light gain,thereby increasing the threshold.Based on the enhancement of surface plasmons,this research provides a new idea for the realization of DDNLC random lasers with high pumping efficiency.It has broad application prospects in the fields of integrated photonic devices,sensing and measurement.The widely used liquid crystal orientation method is rubbing,but rubbing is likely to generate static electricity and dust to damage the sample and can not realize the multi-domain orientation of liquid crystal,but photo-alignment can well solve these problems.To further study the effect of the light-controlled alignment layer on the DDNLC random laser,a linearly polarized light source with an optical power of 4.5 m W was used to irradiate the alignment substrate,and the effect of alignment time on the alignment effect of the liquid crystal in DDNLC was studied.The experimental results prove that the DDNLC random laser emission threshold first increases and then decreases with the irradiation time.The minimum threshold 3.6 m J/cm² can be obtained at the orientation time of 40min.The polarization microscope(POM)of the sample proves that the liquid crystal is in the vertical orientation laser polarization direction and the orientation effect is excellent.Comparing the random laser emission spectra of DDNLC with two-side vertical orientation and parallel orientation,it is found that the similar ordered structure of liquid crystal molecules in the two samples determines their similar random laser emission thresholds,which are 2.2 m J/cm² and 2.0 m J/cm²,respectively.The equivalent cavity length(56.15?m and 58.58?m,respectively)obtained through the power Fourier analysis of the spectrum shows that the coherent feedback random lasing is mainly provided by the closed loop formed with multiple scattering inside the liquid crystal system.The two-side parallel oriented DDNLCRL's threshold area is 3 mm²,which further confirms this point.Different photo-alignment layers realize the multi-domain orientation of the liquid crystal and lower the threshold of the DDNLC random laser,which provides a new idea for the design of optical switches and polarization devices.