Imaging Confined Electrons with Plasmonic Light

Variations of the spectra of plasmonic light emitted from the junction of a scanning tunneling microscope have been observed for different lateral positions of the scanning tunneling microscope tip on a Au(111) surface. Subnanometer spatial variations of the light emission intensity over a triangular island and in the vicinity of surface step edges have been recorded at different photon energies. They reveal surface standing wave patterns characteristic for two-dimensional confined electrons.

Figure 1

(a) Spectra (vertically offset for clarity) of STM-induced light emission from Au(111) obtained at two locations of a triangular island [see image in (b)]. The inset shows the difference between the spectra. (b) $11\times 11{\mathrm{nm}}^2$ constant-current STM image of a triangular island on Au(111). (c) Energy-resolved ($1.65<h\nu <1.7\mathrm{eV}$) map of the detected photon intensity (“photon map”) acquired simultaneously with the STM image in (b). Data acquired with a sample bias $V=1.73\mathrm{V}$ and a tunneling current $I=300\mathrm{nA}$.

Figure 2

(a) Series of light emission spectra recorded during a topographic scan perpendicular to a step edge ($V=-1.73\mathrm{V}$). (b) Dependency of the photon intensity plotted for each photon wavelength as a function of the distance from a linear step edge. The top (bottom) panel has been acquired with $V=+1.73$ ($-1.73$) V. The right abscissa indicates the approximate energy of the surface state electrons involved in the light emission according to the model presented in Fig. 3.

Figure 3

Models of the influence of the Au(111) surface state, which is schematically represented by a parabola, on the energy of the emitted photons, for (a) positive and (b) negative sample bias $V$.

Figure 4

(a) Isochromatic light intensity (dots) versus distance from a linear step edge fitted by $1-J_0(2\kappa x)$ (solid line). [Photon wavelength 725 nm, $V=-1.73\mathrm{V}$, dashed line in Fig. 2b]. (b) Dispersion data. Circles: Photon energy $h\nu$ versus wave vector $\kappa$ extracted from the oscillatory pattern in Fig. 2b. Horizontal bars reflect the uncertainty of distance measurements in STM images. Vertical bars indicate the photon energy range used. Dashed line: Dispersion ($E$ versus parallel momentum $k$) of Au(111) surface state as determined by photoelectron spectroscopy (PES) [37]. Solid line: Photon energy $h\nu$ versus averaged wave vector $\overline{k}$ calculated using Eq. (3).