Tunneling Limits of Plasmonic Enhancement Probed in the Near-Field by Third Harmonic Generation

Ghazal Hajisalem Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia, Canada Mohammadreza Nezami Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia, Canada Reuven Gordon Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia, Canada

Plasmonic focusing achieved by narrow gaps faces a fundamental limit from quantum tunneling that shorts out the sub-nm gap. Using third harmonic generation, we probe the onset of the quantum tunneling regime, which is observed by a sharp reduction in the local field intensity. Unlike past works that use far-field scattering spectra to try to access the local response, this measurement nonlinearly sensitive to the near-field intensity as the third power in the tunneling gap. The experimental results agree quantitatively with a quantum corrected model implemented with finite-difference time-domain calculations. The self-assembled monolayer actually increases the gap size at which the tunneling regime arises, which will be critical for many applications of plasmonics, including nonlinear optics and surface enhanced Raman spectroscopy.

rgordon@uvic.ca