Second-harmonic generation from subwavelength metal heterodimers

Area of Science:

• Plasmonics
• Nonlinear Optics
• Nanophotonics

Background:

• Optical second-harmonic generation (SHG) is a key nonlinear optical process.
• Conventional understanding attributes SHG in nanoparticle dimers to individual particle surface and bulk contributions.
• Deep subwavelength nanoparticle dimers present unique physical regimes.

Purpose of the Study:

• To experimentally investigate SHG in deep subwavelength gold-silver heterodimers and compare it with homodimers.
• To challenge the conventional model by proposing an alternative mechanism for SHG enhancement in heterodimers.
• To develop a model explaining the observed SHG intensity and spectral characteristics.

Main Methods:

• Experimental fabrication and characterization of gold-silver heterodimers, silver-silver homodimers, and gold-gold homodimers.
• Measurement of optical second-harmonic generation (SHG) from these nanoparticle dimers.
• Theoretical modeling based on Coulomb interactions between lumped oscillating charges.

Main Results:

• Gold-silver heterodimers exhibit SHG intensity an order of magnitude greater than homodimers.
• Experimental findings contradict full-wave calculations based on conventional surface/bulk contributions.
• The proposed Coulomb interaction model successfully explains the enhanced SHG and spectral features of heterodimers.

Conclusions:

• The dominant mechanism for SHG in deep subwavelength gold-silver heterodimers is interparticle Coulomb interaction, not individual particle surface nonlinearity.
• This Coulombic nonlinearity provides a new pathway for enhancing nonlinear optical responses in plasmonic nanostructures.
• The findings necessitate a revision of the conventional understanding of SHG in closely spaced nanoparticles.