Magnetic Plasmon-Enhanced Second-Harmonic Generation on Colloidal Gold Nanocups

Area of Science:

• Plasmonics
• Nanophotonics
• Materials Science

Background:

• Three-dimensional (3D) nanostructures exhibit unique optical responses due to magnetic plasmons, crucial for optical nanoresonators and nanoantennas.
• Controlling nanostructure geometry is key to tuning plasmonic properties and enhancing nonlinear optical effects like second-harmonic generation (SHG).

Purpose of the Study:

• To synthesize colloidal gold (Au) and gold-silver (AuAg) nanocups with controlled asymmetric geometry.
• To investigate the relationship between nanocup geometry, magnetic plasmon resonance, and second-harmonic generation (SHG) enhancement.
• To explore the synergistic effects of combining magnetic and electric plasmon resonances in heteronanocups for nonlinear optics.

Main Methods:

• Synthesis of colloidal Au and AuAg nanocups with tunable geometric parameters (opening size, normalized depth h/b).
• Experimental characterization of optical responses, including magnetic plasmon resonance and second-harmonic generation (SHG).
• Fabrication of AuAg heteronanocups via silver overgrowth on Au nanocups.

Main Results:

• The most efficient SHG in bare Au nanocups was observed at a normalized depth (h/b) of approximately 0.78-0.79.
• Maximum magnetic field enhancement occurred at h/b ≈ 0.65, contributing to maximal SHG alongside the lightning-rod effect.
• AuAg heteronanocups demonstrated a 21.8-fold enhancement in SHG intensity compared to bare Au nanocups by synergizing magnetic and electric plasmon resonances.

Conclusions:

• Optimized magnetic plasmon resonance and the lightning-rod effect are crucial for maximizing SHG in Au nanocups.
• AuAg heteronanocups offer a powerful strategy for enhancing nonlinear optical responses by combining dual plasmon resonances.
• These findings provide a design pathway for advanced nonlinear optical nanoantennas with potential applications in nanophotonics and biological spectroscopy.