Figure 1

(a) Spectrum of the light emitted from a Au-Au junction at $V=1.3\mathrm{V}$ and $I=5\mu \mathrm{A}$ (solid line) and detector response (dashed line). (b) Series of 64 light emission spectra recorded at $V$ from 0.7 to 2.0 V at 20 mV intervals. Light intensity is represented by false colors on a logarithmic scale. A horizontal line indicates the spectrum shown in (a). To avoid saturation of the CCD camera, spectra have been acquired in two steps below and above 1.46 V with different exposure times. Expected cutoffs for one- ($h\nu <eV$) and two-electron ($eV<h\nu <2\mathrm{eV}$) processes are indicated by dashed lines.Reuse & Permissions

Figure 2

(a) Spectra of a Au-Au junction measured—with the same tip and at the same area—at different tunneling conditions: $V=2.15\mathrm{V}$, $I=50\mathrm{nA}$ (black line); $V=1.32\mathrm{V}$, $I=5\mu \mathrm{A}$ (gray line, scaled by a factor of 300). (b) Spectra recorded at 41 tip-sample distances which correspond to $2.4\mu \mathrm{A}<I<52.4\mu \mathrm{A}$, with $V=1.33\mathrm{V}$ and an exposure time of 80 ms per spectrum. The logarithm of the light intensity is represented by false color. (c) Total photon intensity vs current $I$ evaluated from (a), for $h\nu <eV$ (triangles) and $h\nu >eV$ (dots). The inset shows the quantum efficiency (photons per electron) of light with $h\nu >eV$.Reuse & Permissions

Figure 3

(a) Sketch of the hot-hole mechanism leading to the $2e$-light emission. The detailed shape of the barrier, which varies with tip-sample separation, is unknown. (b) Calculated spectra of $2e$ light for $I=10\mu \mathrm{A}$ and $V=1.2\mathrm{V}$.Reuse & Permissions

Figure 4

(a) Junction conductance in units of $G_0$ vs tip displacement towards the sample, from tunneling to contact ($V=1.35\mathrm{V}$). The upper inset shows a STM image of the single adatom remaining on the surface after the contact measurement. (b) Light emission spectra acquired during the tip approach in (a). (c) $1e$- and (d) $2e$-light intensities extracted from (b) by integrating the relevant parts of the spectra.Reuse & Permissions