  Nonlinear optical absorption of light by materials are weak due to its
perturbative nature, although a strong nonlinear response is of crucial
importance to applications in optical limiting and switching. Here we
demonstrate experimentally and theoretically an extremely efficient scheme of
excited state absorption by charge transfer between donor and acceptor
materials as the new method to enhance the nonlinear absorption by orders of
magnitude. With this idea, we have demonstrated strong excited state absorption
(ESA) in reduced graphene oxide that otherwise shows increased transparency at
high fluence and enhancement of ESA by one orders of magnitude in graphene
oxide by attaching gold nanoparticles (AuNP) in the tandem configuration that
acts as an efficient charge transfer pair when excited at the plasmonic
wavelength. To explain the unprecedented enhancement, we have developed a
five-level rate equation model based on the charge transfer between the two
materials and numerically simulated the results. To understand the correlation
of interfacial charge-transfer with the concentration and type of the
functional ligands attached to the graphene oxide sheet, we have investigated
the AuNP-graphene oxide interface with various possible ligand configurations
from first-principles calculations. By using the strong ESA of our hybrid
materials, we have fabricated liquid cell-based high-performance optical
limiters with important device parameters better than that of the benchmark
optical limiters.
