Large Cubic Nonlinearity of Silver Nanoparticles

The cubic nonlinearity of Ag nanoparticles (NPs) with average sizes of $\sim $ 112 nm and $\sim $4 nm were investigated using polarization-resolved degenerate four-wave mixing (DFWM) at the regions of non surface plasmon resonance (SPR). The absorption spectra and TEM pictures of Ag NPs with an average size of $\sim $112 nm indicated wide morphology of size and shape distributions, and those of Ag NPs with an average size of $\sim $4 nm provided narrow size distribution. The SPR absorption peaks of Ag NPs with average sizes of $\sim $ 112 nm and $\sim $4 nm were $\sim $420 nm with an inhomogeneous spectral shape and $\sim $424 nm with a homogeneous shape. The excitation source was a spatially Gaussian shaped, $\sim $6 ns pulsed laser with 10-Hz repetition rate operating at 1064- and 532-nm wavelengths for the DFWM spectroscopy. The concentration- and polarization-resolved DFWMs revealed that the hyperpolarizabilities of Ag NPs with average sizes of $\sim $ 112 nm and $\sim $4 nm were $\sim $1.05$\times $10$^{-20 }$esu and $\sim $4.19$\times $10$^{-21 }$esu, and $\sim $3.05$\times $10$^{-26 }$esu and $\sim $6.6$\times $10$^{-27 }$esu for parallel and orthogonal polarizations. The possible origins of large hyperpolarizability enhancement with the Ag NPs with average size of $\sim $112 nm are a dielectric resonance and edge effects of the existing nanorods and nanoplates in the NPs.