Ultrafast carrier dynamics and transient nonlinear absorption in metallic Co9S8

Metallic cobalt sulfide (Co9S8), featuring high intrinsic carrier density and delocalized electronic states, remains insufficiently understood in terms of its ultrafast photoinduced dynamics and nonlinear optical response. Using femtosecond transient absorption (TA) spectroscopy combined with reflection Z-scan measurements, we investigate ultrafast nonlinear dynamics in epitaxial Co9S8 thin films. TA analysis reveals the coexistence and competition among ground-state bleaching, reverse saturable absorption (RSA), and excited-state absorption (ESA). Increasing excitation fluence promotes the accumulation of excited-state carriers, thereby enhancing ESA and weakening the relative contribution of bleaching. At the same time, the elevated carrier density accelerates carrier relaxation through many-body interactions. Reflection Z-scan measurements further confirm the RSA-type nonlinear absorption and yield a peak effective nonlinear absorption coefficient of α2eff ≈ 3.14 × 103 cm GW−1. These results elucidate the interplay between ultrafast carrier dynamics and nonlinear absorption, extending the understanding of photophysics in metallic chalcogenides.