We investigate the quadratic nonlinear optical response from metallic gold nanoparticles homogeneously dispersed in a
medium or deposited on glass substrates. The nanoparticles are prepared by the wet chemistry method in solution used
afterwards. The diameter of the gold nanoparticle is 80 nm. In a homogeneous medium, hyper Rayleigh scattering,
effectively incoherent second harmonic generation is used to determine the origin of the response. It is shown that for 80
nm diameter gold nanoparticles, the overall response stems from the deviation of the shape of the nanoparticle from that
of a perfect sphere and from retardation effects with a similar weight. The latter retardation effects occur because the
diameter of the nanoparticle is no longer vanishing before the wavelength of the incoming electromagnetic field. For
deposited nanoparticles, the sample is illuminated through the transparent glass slide and the light at the harmonic
frequency, produced through the second harmonic generation phenomenon, is observed in the retro-reflection. From the
collected SHG images, it can be unambiguously concluded that the origin of the nonlinearity in 80 nm diameter gold
nanoparticles stems from the substrate influence normal to the interface. It can also be concluded that the gold
nanoparticles can be used to map out the electromagnetic field in the focal volume.
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