Interference between selected dipoles and octupoles in the optical second-harmonic generation from spherical gold...

Area of Science:

• Plasmonics and Nanophotonics
• Nonlinear Optics

Background:

• Gold nanoparticles exhibit unique optical properties due to surface plasmon resonance.
• Second-harmonic generation (SHG) is a key nonlinear optical process sensitive to material structure and symmetry.

Purpose of the Study:

• To investigate optical second-harmonic generation from gold nanoparticles.
• To identify and characterize the contribution of octupolar responses to the SHG emission pattern.
• To compare experimental findings with theoretical simulations.

Main Methods:

• Experimental measurements of SHG from gold nanoparticles with varying sizes (up to 100 nm).
• Theoretical analysis using finite element method (FEM) simulations.
• Analytical calculations based on nonlinear Mie scattering theory.

Main Results:

• The contribution of octupoles to the second-harmonic emission pattern was observed for the first time, particularly with harmonic polarization in the scattering plane.
• Experimental results showed excellent agreement with FEM simulations that included only the normal surface term in the nonlinear polarization source.
• Nonlinear Mie scattering theory calculations demonstrated constructive and destructive interference between dipolar and octupolar responses under specific polarization configurations.

Conclusions:

• Octupolar responses play a significant role in the second-harmonic generation from gold nanoparticles.
• The combination of experimental and theoretical approaches provides a comprehensive understanding of SHG mechanisms in plasmonic nanoparticles.
• This study advances the understanding of nonlinear light-matter interactions at the nanoscale.