Studies Of Amplified Spontaneous Emission And Single Mode Laser In Organic Semiconductors

Organic semiconductor has advantages of high photolumimescence(PL)quantum yield,large stimulated emission cross-section,significant stokes shift between absorption and emission spectra as well as easy fabrication and mechanical flexible.Therefore,it has been widely studied as an ideal candidate for laser gain material.Among them,?-conjugated polymer(PCP)is a newly-developed organic electro-optic material.The morphology plays an important role in its photophysical property.Therefore,to study morphology dependent amplified spontaneous emission(ASE)is very important.In addition,the laser resonator cavity has significant influence in laser properties.Some novel microcavities such as random laser and whipering gallery microcavity have attracted great attention.They are easy preparation and suitable for the organic material based on solution method.However,they are usually multimodal due to the lack of mode selection strategies.As a result,to achieve single mode operation in lasers such as random laser and whispering gallery microcavity laser is an important issue.For the above issue,four achievements have been made in this thesis:1.Optical measurement system set up.We have built up homemade setup for optical measurement including photoluminescence and photoinduced absorption and laser spectroscopy.2.Morphology dependent amplified spontaneous emission in ?-conjugated polymer was studied.We have presented ASE spectra of as-cast,annealed MEH-PPV film,1 mg/ml and 2.5 mg/ml MEH-PPV solution,respectively.The morphology as well as the fraction of aggregates in the film was adjusted by annealing the film at 420 K,large extent of aggregation was formed in the film.We found that the threshold in the annealed film is much higher than that in an as-cast one.However,the temperature had less effect on the ASE threshold than annealing.We conclude from the results that the morphology rather than long-lived photo-excitation is more essential for the threshold of ASE in MEHPPV film.In addition,in MEH-PPV solutions,the fraction of aggregates increased with the concentration.In dilute solution of 1 mg/ml,ASE spectra show both 0-0 peak and 0-1 peak in 1 mg/ml solution,but only 0-1 peak in 2.5 mg/ml one.When the temperature of solution increases slightly from 300K to 330K,the ASE threshold however increases dramatically in 1 mg/ml solution,but slightly in another one.Our results show that the morphology plays an important role in the ASE spectra in both film and solution.Therefore,to enhance the performance of the organic lasers of PCPs,the aggregation process,which is determined by chain-chain interaction,must be better understood and be controlled.3.Single mode random lasing in concentrated PCPs solutions was studied.Using thin stripe excitation of a 10 ns 532nm laser,we observed robust and bright single mode random laser(RL)emission in high concentrated solutions of MEH-PPV and PCPDTBT within the proper excitation intensity range.Our experimental results have demonstrated that refractive index fluctuation due to aggregation in concentrated solution results in spatially extended random lasing modes.The mode is amplified through traveling light along the interface between the solution and cuvette.The laser beam has good direction with far field divergence angle of 1.3°(parallel)and 3.7°(vertical).For using large volumes of gain materials,the conversion efficiency could be on the order of10%resulting in bright single-mode random laser output.Compared to other methods to effectively achieve single mode random laser,our work provided a simple method to achieve bright single-mode random lasing,which is obtained in pure polymer solution of ?-conjugated polymer solutions by stripe excitation.To the best of our knowledge,this is the first time that random lasing features have been observed in a pure polymer solution;moreover,a single mode RL from extended mode,which only existed in theory before,is demonstrated in the high concentrated solution of a?-conjugated polymer.4.Single mode whispering gallery mode(WGM)based on evanescent-wave-coupled gain was obtained.An optical fiber was put in organic dye solutions.When the refractive index of the solution is smaller than the fiber,the fiber can form a cylindrical microcavity,which can support WGM emission.In our experiment,when the fiber was surrounded by the rhodamine 6G dye molecules in an ethanol solution,the fiber was sticked to the cuvette wall by capillary.When the fiber was spot excitied by 532 nm pulsed laser,we can observe stable single mode WGM emission within a proper excitation en ergy range;however,if the fiber was separated from the cuvette wall,multimode WGM emission was achieved.When the single WGM was obtained,if the refractive index was gradually increased(the concentration was keeped same),the single mode can evolve to multimode.Our experiment has demonstrated that single mode WGM laser can be controlled,which is related to the depth of the evanescent wave leaking into the outside solution.