Abstract
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.
Highlights
A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices
The sizes of the molecular-like Au nanocrystals (AuNCs) are less than 2 nm (Figure 1b), and the high-resolution transmission electron microscopy (TEM) (HRTEM) image in the inset clearly shows that the atomic distance of the AuNC is,0.24 nm
The molecular-like AuNCs are generated from plasmonic AuNCs by thermal etching at a temperature of 190uC, which produces a mass of small AuNCs arranged around large structures and dispersed in aqueous suspensions (Figure S1)
Summary
A saturable absorber is a nonlinear functional material widely used in laser and photonic nanodevices. Metal nanocrystals can be used as either saturable absorbers or optical limiters depending on their sizes, shapes, and plasmon resonance wavelengths and strengths[6,8,12,13]. The plasmonic metal nanostructures (especially Au and Ag nanocrystals) with large local fields enhance the one-photon SA by increasing the absorption cross section of the ground state, which enhances the two-photon RSA by increasing the excited state absorption. Small Au nanocrystals (AuNCs) in the quantum size regime (with typical sizes of less than 2 nm) have an increased dielectric constant in the imaginary portion, which results in completely suppressed plasmon resonance and a molecular-like exciton absorption band edge. A critical size exists for the smallest two-photon absorption, which indicates that the SARRSA nonlinear processes could be suppressed in molecular-like AuNCs with an appropriate size[8,46]
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