Largely Enhanced Saturable Absorption of a Complex of Plasmonic and Molecular-Like Au Nanocrystals

A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices. Metallic nanostructures have prominent saturable absorption (SA) at the plasmon resonance frequency owing to largely enhanced ground state absorption. However, the SA of plasmonic metal nanostructures is hampered by excited-state absorption processes at very high excitation power, which usually leads to a changeover from SA to reversed SA (SA→RSA). Here, we demonstrate tunable nonlinear absorption behaviours of a nanocomplex of plasmonic and molecular-like Au nanocrystals. The SA→RSA process is efficiently suppressed, and the stepwise SA→SA process is fulfilled owing to energy transfer in the nanocomplex. Our observations offer a strategy for preparation of the saturable absorber complex and have prospective applications in liquid lasers as well as one-photon nonlinear nanodevices.