Probing the Quantum Tunneling Limit of Plasmonic Enhancement by Third Harmonic Generation

• Published in: Nano letters 2014-11, Vol.14 (11), p.6651-6654
• Main Authors: Hajisalem, Ghazal, Nezami, Mohammedreza S, Gordon, Reuven
• Format: Article
• Language: English
• Subjects: Collective excitations (including excitons, polarons, plasmons and other charge-density excitations); Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Exact sciences and technology; Focusing; Harmonic generations; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Methods of nanofabrication; Nanostructure; Nonlinear optics; Physics; Plasmonics; Quantum tunnelling; Self-assembled monolayers; Self-assembly; Spectra; Surface and interface electron states; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl503324g

Metal nanostructures provide extreme focusing of optical energy that is limited fundamentally by quantum tunneling. We directly probe the onset of the quantum tunneling regime observed by a sharp reduction in the local field intensity in subnanometer self-assembled monolayer gaps using third harmonic generation. Unlike past works that have inferred local limits from far-field spectra, this nonlinear measurement is sensitive to the near-field intensity as the third power. We calculate the local field intensity using a quantum corrected model and find good quantitative agreement with the measured third harmonic. The onset of the quantum regime occurs for double the gap size of past studies because of the reduced barrier height of the self-assembled monolayer, which will be critical for many applications of plasmonics, including nonlinear optics and surface enhanced Raman spectroscopy.