Light emission from a scanning tunneling microscope: Fully retarded calculation

TL;DR

This paper presents a fully retarded calculation of light emission from an STM, revealing small but significant effects on resonance features, especially for silver samples, aligning theory more closely with experimental observations.

Contribution

It extends previous non-retarded models by including retardation effects, providing more accurate predictions of light emission spectra in STM experiments.

Findings

Retardation effects cause minor shifts in resonance energies for Au and Cu.

For Ag samples, retardation significantly broadens and shifts resonance peaks.

Approximately 1% of tunneling electrons undergo inelastic processes, contributing to light emission.

Abstract

The light emission rate from a scanning tunneling microscope (STM) scanning a noble metal surface is calculated taking retardation effects into account. As in our previous, non-retarded theory [Johansson, Monreal, and Apell, Phys. Rev. B 42, 9210 (1990)], the STM tip is modeled by a sphere, and the dielectric properties of tip and sample are described by experimentally measured dielectric functions. The calculations are based on exact diffraction theory through the vector equivalent of the Kirchoff integral. The present results are qualitatively similar to those of the non-retarded calculations. The light emission spectra have pronounced resonance peaks due to the formation of a tip-induced plasmon mode localized to the cavity between the tip and the sample. At a quantitative level, the effects of retardation are rather small as long as the sample material is Au or Cu, and the tip consists of W or Ir. However, for Ag samples, in which the resistive losses are smaller, the inclusion of retardation effects in the calculation leads to larger changes: the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These changes improve the agreement with experiment. For a Ag sample and an Ir tip, the quantum efficiency is $\approx$ 10$^{-4}$ emitted photons in the visible frequency range per tunneling electron. A study of the energy dissipation into the tip and sample shows that in total about 1 % of the electrons undergo inelastic processes while tunneling.