Investigation On Organic Solid-state Lasers Based On Self-assembled Micro-/nanocrystals

Organic semiconductor materials have been ideal alternative for semiconductor lasers due to their large absorption and emission cross sections,inherent four-level systems,abundant molecular structures,and solution processability.In particular,organic micro-/nanocrystals self-assembled through weak non-bonding interactions(such as ?-? stacking,hydrogen bonding,halogen bonding,and van der Waals forces)between small organic molecules have been widely applied in solid-state lasers owing to the simultaneous function as both gain medium and resonant cavities.However,due to the lack of stable four-level energy system and high-quality self-assembled resonators,the development of organic micro-nanocrystalline lasers is still hampered.Therefore,our researches focus on these two key issues and start from the optimization of the four-level system and the preparation and regulation of the resonant cavity towards high-performance organic solid-state lasers.The specific research content is as follows:(1)We introduced excited state intramolecular proton transfer(ESIPT)-active small organic molecule DMHC and prepared organic single-crystal microwires by the solution exchange method.The dynamic intramolecular proton transfer process within?2.5 ps establishes a effective four-level system consisting of two sets of So and S1 energy levels from enol form and keto form.Consequently,the 775 nm near-infrared(NIR)lasing with a low threshold of 5.3 ?J cm-2 was realized in the single micro wire with the length of 40 ?m.This work clearly demonstrated the mechanism of the ESIPT laser,and achieved organic micro-/nanocrystalline laser above 760 nm for the first time,which will promote the development of the integration of the coherent NIR light sources at micro-/nanoscale.(2)Further,to meet the large-scale application,we prepared single-crystal nanowire arrays of the above-mentioned organic small molecule DMHC by a capillary-bridge confined assembly method.These organic one-dimensional single-crystal arrays with tunable size and precise position can effectively reduce dispersion and waveguide loss due to their smooth surface with Rq=0.3 nm,yielding high-Q(?2340)Fabry-Perot(FP)resonator with efficient optical feedback.Consequently,we realized room temperature single-mode NIR laser emission with a low threshold(9.9 ?cm-2)based on the single nanowire with L=10 ?m.The mode of laser can be modulated by changing the length of the nanowires.These highly ordered organic one-dimensional single-crystal arrays will contribute to the application of integrated organic NIR micro-/nano lasers.(3)Through the "co-crystal" approach,we modulated the yellow-emitting polyhedron(Whispering-gallery mode,WGM resonator)of 1,4-bis(4-cyanostyryl)benzene(p-BCB)to the sky-blue-emitting microwires(Fabry-Perot,FP resonator)of 1,4-bis p-BCB:1,4-diiodotetrafluorobenzene(p-BCB:DIFB),both of which are prepared by solution self-assembly method at room temperature.With the formation of co-crystals,the radiation decay rate(kr)of these organic micro-/nanocrystals can be adjusted from 0.04 to 0.12 ns-1 due to the absence of excimer.The increase of the radiation decay rate can make sure more electrons participate in stimulated emission,which can help increase the optical gain.Therefore,the "co-crystal" approach can simultaneously modulate the morphology and optical performance of these self-assembled resonators,which will advance the rational design and synthesis of organic solid-state laser materials.