First hyperpolarizability ($\beta )$ of bare and polymer protected copper nanoparticles

We have prepared bare as well as polyvinyl pyrrollidone (PVP) capped Cu nanoparticles (NPs) of $<$10 nm size by laser ablation and measured their first hyperpolarizabilities ($\beta $ values) using the hyper-Rayleigh scattering technique in solution. The $\beta $ values for the bare and capped NPs are 414 ($\pm $19) x 10$^{-30 }$and 808 ($\pm $12) x 10$^{-30 }$esu/atom$^{1/2}$, respectively. The bare NPs are stable in isopropanol for weeks but are short-lived compared to the capped particles. Our results of capped NPs having a $\beta $ value twice as high compared to the bare NPs of the same size show that surface capping is necessary for enhancing $\beta $ in noble metal NPs. In addition to the bulk and surface quadrupolar contributions which exist in bare NPs, dipolar contribution to $\beta $ becomes important for the capped NPs due to the destruction of centro-symmetry at the surface, leading to a significant increment in $\beta $. Experiments with smaller size NPs show that $\beta $ goes down with size. The bulk quadrupolar polarization which decreases with particle size, perhaps, rationalizes the size dependence of $\beta $.