Wavelength-varying third-order nonlinear optical response of Ag nanoparticles-Si quantum dots integrated plasmonic system

We present an integrated plasmonic nanocomposite showing a nonlinear optical response changing its sign with wavelength, depending on its position with respect to the surface plasmon resonance of the nanocomposite. This nanocomposite is a SiO2 matrix containing both, embedded quasi-spherical Ag nanoparticles and silicon quantum dots. The wavelengths used for the picosecond Z-scan study were 355 and 532 nm, which are localized at both sides of the surface plasmon resonance of the Ag nanoparticles (395 nm), and 1064 nm, which is localized well far away of it. The integrated plasmonic system shows a positive nonlinear refraction below the plasmon resonance, changing to a negative value for wavelengths above resonance. On the other hand, below resonance the nonlinear absorption is cancelled due to opposite responses from the individual nanosystems, while above resonance only saturable absorption is observed.