Nonlinear saturable absorption and defect-assisted Auger recombination of environmentally stable OHB-T

The search for environmentally stable nonlinear optical materials with ultra-high third-order nonlinear absorption response is attractive for the development of novel optoelectronic devices. To this end, the nonlinear absorptions and related carrier dynamics of ionic organic 2-(4-hydroxystyryl)-3-methylbenzothiazolium 4-methylbenzenesulfonate (OHB-T) are investigated in this work. Results indicate that stable OHB-T with a high initial weight-loss temperature of 314.3 oC and melting temperature of 265.3 oC exhibits outstanding third-order nonlinear saturable absorption properties in both liquid and solid phases at 520 nm with the figures of merit of 2.94−4.50×10-13 esu cm and saturation intensities of 2.80−3.60 GW cm-2. Transient absorption measurements reveal that the saturable absorption behavior originates from a rapid ground-state bleaching process caused by the Pauli blocking principle. Particularly, the subsequent relaxation processes of single-photon excited carriers are dominated by the defect-assisted Auger recombination involving a fast relaxation of 5.68−11.95 ps and a slow relaxation of 43.30−64.30 ps. These findings not only are helpful for understanding the third-order nonlinearities and carrier dynamics of organic benzothiazolium salts, but also disclose a promising candidate material for the design of next-generation high-performance optoelectronic devices in the future.

面向新型光电子器件（optoelectronic devices）的开发需求，探寻兼具环境稳定性与超高三阶非线性吸收（third-order nonlinear absorption）响应的非线性光学材料（nonlinear optical materials）始终是颇具吸引力的研究课题。为此，本工作针对离子型有机化合物2-(4-羟基苯乙烯基)-3-甲基苯并噻唑鎓对甲苯磺酸盐（OHB-T）的非线性吸收行为与相关载流子动力学展开了研究。结果表明，热稳定性优异的OHB-T初始失重温度达314.3 ℃、熔融温度为265.3 ℃，其在520 nm波长下的液相与固相体系中均表现出突出的三阶非线性可饱和吸收特性，品质因数（figures of merit）为2.94−4.50×10^-13 esu·cm，饱和光强为2.80−3.60 GW·cm^-2。瞬态吸收（transient absorption）测试显示，该可饱和吸收行为源于泡利阻塞原理（Pauli blocking principle）引发的快速基态漂白（ground-state bleaching）过程。尤为关键的是，单光子激发载流子的后续弛豫过程以缺陷辅助俄歇复合（Auger recombination）为主导，该过程包含两个弛豫阶段：快速弛豫时长为5.68−11.95 ps，慢速弛豫时长为43.30−64.30 ps。本研究结果不仅有助于深化对有机苯并噻唑鎓盐类材料的三阶非线性光学特性与载流子动力学的理解，同时也为下一代高性能光电子器件的设计提供了极具应用前景的候选材料。