Probing the origin of highly-efficient third-harmonic generation in plasmonic nanogaps

Area of Science:

• Photonics and Nanotechnology
• Nonlinear Optics
• Plasmonics

Background:

• Plasmonic nanostructures enable subwavelength localization of electromagnetic energy, enhancing nonlinear optical processes.
• The precise origin of nonlinear enhancements in dielectric-metal nanogap systems remains unclear.

Purpose of the Study:

• To investigate the source of third harmonic generation (THG) in plasmonic nanogap cavities containing dielectric materials.
• To determine whether the dielectric spacer or the surrounding metal dominates the nonlinear response.

Main Methods:

• Fabrication of plasmonic nanogap cavities with various dielectric spacers.
• Experimental measurement and spectral analysis of third harmonic generation (THG) signals.
• Comparison of THG signals from dielectric-metal nanogaps with bare metal films.

Main Results:

• Third harmonic generation (THG) primarily originates from the dielectric spacer layer within the nanogap.
• Observed a maximum enhancement factor exceeding six orders of magnitude compared to a bare gold film.
• Achieved a nonlinear conversion efficiency of 8.78 × 10^-4%.

Conclusions:

• The dielectric material within the nanogap is the dominant contributor to nonlinear optical enhancement.
• This understanding is crucial for designing efficient on-chip nonlinear optical devices.
• Potential applications include ultrafast optical switching and entangled photon sources.