Hyper-Rayleigh scattering from gold nanoparticles: effect of size and shape

Area of Science:

• Nonlinear optics
• Materials science
• Nanotechnology

Background:

• Gold nanoparticles (GNPs) exhibit unique optical properties influenced by their size and shape.
• Hyper-Rayleigh scattering (HRS) is a nonlinear optical technique sensitive to molecular symmetry and electronic structure.

Purpose of the Study:

• To investigate the influence of different GNP shapes on their hyper-Rayleigh scattering (HRS) properties.
• To determine the first hyperpolarizability (β) for various GNP morphologies.
• To explore the potential of specific GNP shapes for sensing applications.

Main Methods:

• Synthesis of GNPs with five distinct shapes: quasi-spherical, rod, tetrapod, flower, and star.
• Excitation using an 800 nm, 150 fs pulsed laser.
• Measurement of HRS signals and calculation of hyperpolarizability (β) values relative to a reference (∼10 nm spherical GNPs).
• Analysis of polar plots to determine the nature of the nonlinear optical response (dipolar vs. quadrupolar).

Main Results:

• Star and flower shaped GNPs showed the highest hyperpolarizability enhancements (∼130 and ∼52 times, respectively).
• Rod and tetrapod shaped GNPs exhibited modest enhancements (∼7 times), comparable to larger quasi-spherical GNPs.
• Normalized hyperpolarizability values confirmed the superior performance of flower and star shaped GNPs.
• Polar plots indicated a predominantly dipolar nonlinear response, attributed to sharp tips on the GNPs, even for larger particles.

Conclusions:

• GNP shape significantly impacts nonlinear optical properties, with sharp-tipped morphologies yielding the highest hyperpolarizability.
• The observed dipolar response in larger GNPs is likely due to the presence of sharp tips.
• GNPs with sharp tips are promising candidates for nonlinear optical sensing applications.