Dye-doped Liquid Crystal Random Laser Research Based On Waveguide Structure

There are many differences between random laser and traditional laser in the generation mechanism and luminescence properties.Random lasing does not require a resonant cavity and optical feedback depends on the multiple scattering of 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,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 random resonant cavity.In this paper,random lasing from dye-doped liquid crystals(DDLC)based on polydimethylsiloxane(PDMS)planer waveguide is investigated.And the effect of PDMS wrinkle on the threshold of random laser emitted from the dye-doped liquid crystals(DDLC)is studied.Random laser from dye liquid crystals based on PDMS planar waveguide is realized.Azo dye used in the experiment is Methyl Red(MR),which can undergo trans-cis isomerization after 532 nm of polarized light irradiation and then LC molecules are anchored on the surface of substrate and aligned with their long axes perpendicular to the polarization of the pump beam.After the orientation of liquid crystal molecules,the scattering efficiency is enhanced because of the refractive index difference at different thicknesses.The optical feedback is provided by both the limiting effect of waveguide and the multiple scattering of liquid crystal molecules.The threshold of random laser is 1.5 m J/cm~2.The laser characteristic of random laser is related to the size of pump region,it is proved that random laser can emit only when the pump region is larger than the critical gain volume.Finally,the effect of gain layer thickness on random laser threshold is studied.It can be seen that there is an optimal gain layer thickness of 50μm in the sample.Therefore,it indicates thas the PDMS planer waveguide is a promising platform for random lasing from dye-doped liquid crystals.We envision that this lab on chip systems can further affect the application of sensing and display imaging.In PDMS wrinkle structure,the threshold of the random laser can be affected by the roughness of the surface.Multiple scattering in the wrinkle structure is provided by both liquid crystal molecules and rough surfaces of the wrinkle.The pump light is amplified by the laser dye PM597,multiple scattered by liquid crystal molecules and wrinkle and finally limited in the wrinkle structure waveguide.The threshold of random laser based on PDMS wrinkle structure and planar waveguide structure is compared.The threshold of planar waveguide structure is 1.5m J/cm~2,while in wrinkle the threshold in 0.9m J/cm~2 and obviously reduced.The rough surface of the PDMS wrinkle can limit more photons by multiple scattering,thus improving the scattering efficiency and reducing the threshold.Finally,different wrinkle patterns can be produced by changing the tensile ratio and the surface oxygen plasma treatment time.With the increase of the roughness of the waveguide surface,the larger rough surface supports more effective multiple scattering,resulting in a higher magnification factor,so it is more favorable to the emitting of random laser.This random laser based on flexible substrates can be used in sensing and display imaging.