  Nonlinear optical phenomena play a critical role in understanding microscopic
light-matter interactions and have far-reaching applications across various
fields, such as biosensing, quantum information, optical switching, and
all-optical data processing. Most of these applications require materials with
high third-order absorptive and refractive optical nonlinearities. However,
most materials show weak nonlinear optical responses due to their perturbative
nature and often need to be improved for practical applications. Here, we
demonstrate that the charge donor-acceptor hybrid of VSe2-reduced graphene
oxide (rGO) hybrid exhibits enhanced ultrafast third-order absorptive and
refractive nonlinearities compared to the pristine systems, at least by one
order of magnitude. Through density functional theory and Bader charge
analysis, we elucidate the strong electronic coupling in the VSe2-rGO hybrid,
involving the transfer of electrons from VSe2 to rGO. Steady-state and
time-resolved photoluminescence (PL) measurements confirm the electronic
coupling and charge transfer. Furthermore, we fabricate an ultrafast optical
limiter device with better performance parameters, such as an onset threshold
of 2.5 mJ cm-2 and differential transmittance of 0.42.
