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 are 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.