organic SA materials have been identified and investigated in the past decades. [4][5][6][7][8][9] Nevertheless, most inorganic SA materials, such as graphene (G), [6] black phosphorus (BP), [7] transition metal sulfides, [4,8] and topological insulators, [9] still suffer from some inherent limitations. For example, the saturation modulation depth of a monolayer G is only 0.5−1.8% in the wavelength range of 800−1500 nm, unsuitable for stable mode-locking. [10] The oxidation in ambient air of BP has been an impediment to its commercialization and industrialization. [11] The bulk transition metal sulfides are unable to generate ultra-short laser pulses, due to their long intraband electron relaxation times, at the scale of picosecond. [10,12] The extremely low saturation intensities of topological insulators at the scale of W cm −2 hinder their practical applications in continuous-wave modelocking. [10,13] Particularly, these aforementioned inorganic SA materials generally lack a controllable, reproducible, high-yield, and low-cost preparation process. [10] In contrast, organic SA materials have shown the advantages of good flexibility, reproducible preparation processes, diverse molecular design possibilities, and fast NLO responses. [14][15][16] However, the nonlinear SA performances of most organic SA materials are generally lower than those of inorganic SA materials, limiting practical applications of the former in the optoelectronic devices. [16] Moreover, the transient carrier dynamics of most organic SA materials, important for Third-order nonlinear optical (NLO) absorption properties and ultra-fast carrier dynamics of organic 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) at the wavelength of 520 nm are investigated by femtosecond open-aperture Z-scan technique and transient absorption spectroscopy, respectively. Z-scan measurements indicate that DAST solution exhibits ultra-high nonlinear saturable absorption (SA) responses with a figure of merit of 4.57 × 10 −13 esu cm and a saturation strength of 3.20 GW cm −2 , far superior to those of the most known 2D SA materials under similar excitations. Transient absorption results reveal that the outstanding SA performances of the solution-phase DAST are rooted in ultra-fast ground-state bleaching based on the Pauli blocking effect. The subsequent excited state carrier relaxation processes are dominated by rapid intraband carrier cooling and defect-assisted interband Auger recombination. Moreover, high-quality DAST−polyvinyl alcohol composite films prepared by solution casting exhibit similar ultra-high SA responses and reliable long-term stability without any degradation of their SA responses even after exposure to air moisture for 100 days. These findings establish an experimental and theoretical foundation for the development of high-performance ultra-fast optoelectronic devices based on organic NLO materials in the future.
