Overcoming the pulse-width limitation of third-order optical nonlinearity for above bandgap excitation in nanoparticles

The instantaneous nonlinear optical (NLO) responses of materials are typically limited to subbandgap excitation involving virtual states. However, achieving fast responses for above-bandgap excitation is extremely challenging because of the involvement of real states and associated electron-phonon (e-ph) scattering processes. Here, we investigate the NLO response of ${\mathrm{NiSe}}_{2}$ nanoparticles (NPs) using femtosecond (120 fs) and nanosecond (7 ns) laser pulses. Our experiments, supported by first-principles calculations, reveal that ${\mathrm{NiSe}}_{2}$ exhibits reverse saturable absorption (RSA) for femtosecond excitation, enabled by near-instantaneous intraband thermalization within the pulse duration. In contrast, ${\mathrm{CoSe}}_{2}$ NPs display saturable absorption, likely due to incomplete thermalization. Notably, when the laser interaction time is extended to nanoseconds, RSA emerges in both materials, which concurs with our model. Strikingly, ${\mathrm{NiSe}}_{2}$ NPs exhibit an unprecedented enhancement in the NLO response for nanosecond excitation, exceeding four orders of magnitude compared to femtosecond excitation, highlighting the role of prolonged excited-state dynamics. Our results provide a frontier in ultrafast optoelectronic device applications for above-bandgap excitations.