Optical detection of surface plasmons on tunnel-junction structures

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Abstract
The surface-plasmon-polariton mode at the Ag-air interface of typical Al-alumina-Ag tunnel junctions and of novel Ag-Al-alumina-Ag tunnel junctions is excited in an attenuated-total-reflection arrangement. It is found that the photon-SPP coupling efficiency is determined principally by the amount of Al in the base electrode. The use of an extremely thin ( approximately 5 nm) Al base electrode or a base electrode ( approximately 20 nm thick) composed largely of Ag can lead to coupling efficiencies of up to 100%. These results indicate that the efficiency of light emission from prism-coupled tunnel junctions could possibly be increased by an order of magnitude over that previously observed. In addition, the fitting of theoretical curves to the experimental spectra affords a most useful characterisation, in terms of values for the dielectric function and thickness, of the thin films comprising the tunnel-junction structures. In particular the thickness of the alumina tunnel barrier in Al-alumina-Ag junctions is found to be approximately 2.2 nm, in good agreement with thickness values demanded by electron tunnelling studies.