Direct Observation of the Optical Plasma Resonance of Ag by Photon-Assisted Tunneling

Photon-assisted tunneling is proposed for the investigation of optical plasma resonances; this is demonstrated by the direct observation of the Ag resonance in appropriate Ag-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al structures. A spectral scan of the ratio of the signals photoinduced by the $p$ and $s$ polarizations shows a large enhancement corresponding to the plasma resonance of Ag.