Abstract
Gold nanoparticles are widely used in different fields. They are currently under extensive investigation with regard to their linear and nonlinear optical properties. So far, nanoparticles with centrosymmetrical shapes, such as nanospheres or nanorods, have received the main attention. In this work, the properties of gold nanoprisms exhibiting a highly noncentrosymmetrical threefold symmetry are investigated with hyper-Rayleigh scattering (HRS). Aqueous solutions of gold nanospheres and nanoprisms with different sizes were synthesized. The first hyperpolarizability magnitudes of both the nanospheres and nanoprisms were determined separately with the assumption of a surface origin of the nonlinearity. Using polarization-resolved HRS, retardation is shown to be largely underdeveloped for the nanoprisms as opposed to the nanospheres. The nanoprism shape noncentrosymmetry has therefore a leading role in the HRS response although it is also shown that surface defects induce deviations from the ideal threefold symmetry.
Highlights
Metallic nano-objects, such as gold, silver, or platinum nanoparticles with a size of the order of 100 nanometers or smaller, have been gaining growing interest over the last few years driven by the potential applications envisaged
Aqueous suspensions of gold nanoprisms with an average edge length ranging from 26 to 87 nm were successfully synthesized combining various synthesis protocols known in the literature and investigated with polarization-resolved hyper-Rayleigh scattering (HRS)
The determined first hyperpolarizabilities are rather low as compared to the values reported in the literature for other plasmonic nanoparticle shapes, such as rods or spheres. This feature emphasizes the sensitivity of this parameter to the nature of the nanoparticle surface and must be taken into account when selecting nanostructures with an efficient HRS response
Summary
Metallic nano-objects, such as gold, silver, or platinum nanoparticles with a size of the order of 100 nanometers or smaller, have been gaining growing interest over the last few years driven by the potential applications envisaged. In order to provide a deeper understanding of the experimental data and to unravel the origin of the HRS signals from nanoprisms, theoretical simulations were performed using a discrete nonpolarizable nonlinear dipole model.[38] A D3h symmetry for equilateral triangles with a defined edge length was assumed, see Figure 3. To that of molecules with the same symmetry like crystal violet.[36] The polar plot of small edge length nanoprisms corresponds well to the experimentally obtained graph for sample #26, see Figure 2, i.e., showing a very low level of retardation. Introducing three unbalanced nonlinear dipoles disposed at the three tips of the nanoprisms provides lower ζV values, in agreement with the experimental observations This breaking of the perfect threefold symmetry is seen through the depolarization ratio measured, which is weaker than the theoretical one
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