Black Gold Plasmon Response of a Raspberry Shell Grown on Lithium Niobate Nonlinear Nanoparticles

Lithium niobate nanoparticles (LN NPs) coated with a raspberry-like gold (Au) shell are synthesized herein. The strong plasmon resonance coupling between Au NPs in raspberry nanostructures is expected to modify the surface and cavity optical fields and thus significantly alter the second harmonic (SH) signal generated by the noncentrosymmetric crystalline core of LN. Plasmonic properties of the core-shell NPs are first modeled using the discrete-dipole approximation method. The influence of the size and filling fraction of Au NPs on the extinction cross section is correlated with the experimental responses. Comparison at 800 nm of the first hyperpolarizability of individual Au NPs, bare LN cores, and hybrid nanostructures demonstrates no enhancement. It reveals instead an octupolar response in the polarization-resolved data, consistent with a dominant surface contribution of the raspberry-like Au shell. The wavelength-dependent SH emission of the hybrid nanostructures is finally determined in the 700-1300 nm excitation range, and a continuous increase that reaches a factor up to 20 is only evidenced at the longest excitation wavelength of 1300 nm.