Random Lasing Boosted By Active Scatterers

Random laser(RL)has attracted extensive attention due to its special characteristics like disorder,multi-direction and miniaturization of devices compared with traditional lasers.In this paper a new active scatterer formed by molecular aggregation was firstly used in the field of RL and the synthesis as well as characteristic of the molecule has been carefully demonstrated.Besides,the random lasing of the active scatterers dissolved in solution as well as doped in polymer has been carefully researched.Below are the details:1.Nano-scale aggregates of hybrid molecules,which is named N,N'-di[3-(isobutyl polyhedral oligomeric silsesquioxanes)propyl]perylene diimide(DPP)and is composed of perylene diimide(PDI)as gain group and polyhedral oligomeric silsesquioxanes(POSS)as scattering group at a mole ratio of 1:2,are formed in CS2 solution within concentration ranges from 3×10-5M to 10-3M.The aggregation of DPP in the solution is characterized by UV-Vis and fluorescence spectroscopy.Under pumping of 532nm pulse laser,coherent random laser(RL)is produced from the aggregates in CS2 solution with threshold of 17.4?J and Q value of 2440 at DPP concentration of 10-4M.Enhanced near-zone scattering by aggregation is thought to boost coherent RL.This finding not only extends RL system to the extremely weak scattering regime but also provides a new avenue to nano-scale materials for lasers on the basis of molecular designing.2.The effect of solvents on random lasing in active scatterers(DPP aggregates)solution system is carefully researched.The random lasing is detected in such solutions with a series of solvents ordered by refractive index(RI),from which it is found that the solution system with larger RI solvent is easier to boost RL for a suitable "?l=ls-lg"(ls>lg),while a smaller RI solvent,one situation is a small RI difference between scatterers and surroundings makes an extremely large ls,therefore,the extremely large ?l in these samples makes no observation of random lasing;the other is due to the increased lg characterized by UV-Vis spectroscopy,though the ls remains nearly unchanged compared with that in larger RI solvent system for the large RI fluctuations between scatterers and surroundings,the smaller ?l also results in no detection of random lasing as the laser pumping energy in our experiments couldn't compensate the gain loss.Results from this work help us to quantitatively judge necessary condition of "?l=ls-lg" to RL in solutions and are meaningful to design suitable solution system to boost RL.3.A series of polystyrene(PS)doped with different concentrations of DPP(10-3M-10-6M)was synthesized in capillary tubes with different inner diameters from 100?m to 700?m under moderate reaction temperature.The random lasing detection results of these samples show that,when the inner diameter of capillary tube is confirmed,for example,300?m,while changing the concentration of doped DPP,the results are similar to that in DPP solution system,that is,coherent RL is boosted with the concentration of DPP is larger than 10-4M,below that,normal fluorescent emission was detected.When the concentration of DPP is confirmed in 10-3M while changing the inner diameter of capillary tube,the results show coherent RL is detected when the diameter is no more than 300?m,otherwise incoherent RL is observed.Besides,to sample doped with 10-3M DPP in capillary tube with 300?m inner diameter,incoherent RL instead of coherent RL is boosted when carefully removed the capillary tube.Such experimental results directly show that the formation of coherent RL in this system is assisted by the whispering gallery modes(WGM)of capillary tube.4.The relationship between random lasing and dynamic distribution of active scatterers(DPP)in a solution system is firstly demonstrated via viscosity tuning.At low viscosity solution via adding less than 20wt%polystyrene(PS)into the 10-3M DPP dissolved in CS2,coherent random lasing is detected.However,in high viscosity system with more than 20wt%PS added into the DPP solution,only incoherent random lasing is observed.Further research found that the Brownian motion confined by the viscosity had an effect on the dynamic distribution of scatterers,which results in stable dynamic distribution of scatterers in low viscosity system,then stable scattering loops are easily formed,thus coherent random lasing is boosted.However,in high viscosity system,the slow moving scatterers couldn't form stable loops,therefore,only incoherent random lasing is detected.5.A series of one-armed cholesterol-linked azobenzene molecules named CholXAzo with different spacers were synthesized,in which Chol6Azo was found to have induced blue phases(BPs)with a concentration of 4.0 wt%.Under irradiation of a 385 nm UV light with a density of 15.0 mW·cm-2,photo-responsive behavior of the 4.0 wt%Chol6Azo doped sample named B3 shows a sensitive temperature dependence,which means that at 38.0?,a phase transition from BPs to isotropic phase is induced.However,at 33.0?,this phase transition does not take place.Results from the research show that the optically binary phase transition behavior of B3 is sensitive to the isomerization degree of Chol6Azo,which is closely related to the stability of the BP structure and there is a critical isomerization degree of 13.7%for the phase transition of the B3 liquid crystals.Further POM observation shows that the liquid crystal samples doped with different concentrations of Chol6Azo have an increasing transition temperature for photo-induced phase transition from BP to isotropic phase along with the increasing concentration of Chol6Azo,which are found to have the same changing tendency with phase transition temperature from isotropic phase to BPs and a phase diagram is made to map the optically binary behavior of Chol6Azo doped blue phase liquid crystals.At last,a simple pattern with BP and isotropic phase arranged at interval was made with this optically binary liquid crystalline blue phase under a suitable photomask.