Image of the Electron Energy-Loss Function in Light Emitted from Tunnel Junctions

Tunnel junctions of the type Al-$I-M$ ($M\equiv \mathrm{A}\mathrm{u},\mathrm{C}\mathrm{u},\mathrm{A}\mathrm{g}$) prepared on ${\mathrm{CaF}}_2$-roughened substrates emit broadband visible light. It is found that the light intensity variation with wave-length in the range 350 to 700 nm images the electron energy-loss function, $\mathrm{Im}(-\frac{1}{\epsilon })$, of each metal $M$ as calculated from its optical constants. It is concluded that surface plasmons are damped above the interband transition in Au at 2.5 V and in Cu at 2.25 V. The findings are in harmony with data from surface-enhanced Raman spectroscopy.