LIGHT-EMISSION FROM A SCANNING TUNNELING MICROSCOPE - FULLY RETARDED CALCULATION

The light-emission rate from a scanning tunneling microscope (STM) sca nning a noble-metal surface is calculated taking retardation effects i nto account. As in our previous, nonretarded theory [P. Johansson, R. Monreal, and P. Apell, Phys. Rev. B 42, 9210 (1990)], the STM tip is m odeled by a sphere, and the dielectric properties of tip and sample ar e described by experimentally measured dielectric functions. The calcu lations are based on exact diffraction theory through the vector equiv alent of the Kirchoff integral. The present results are qualitatively similar to those of the nonretarded calculations. The light-emission s pectra have pronounced resonance peaks due to the formation of a tip-i nduced plasmon mode localized to the cavity between the tip and the sa mple. At a quantitative level, the effects of retardation are rather s mall as long as the sample material is Au or Cu, and the tip consists of W or Lr. However, for Ag samples, in which the resistive losses are smaller, the inclusion of retardation effects in the calculation lead s to larger changes: the resonance energy decreases by 0.2-0.3 eV, and the resonance broadens. These changes improve the agreement with expe riment. For a Ag sample and an Ir tip: the quantum efficiency is appro ximate to 10(-4) emitted photons in the visible frequency range per tu nneling electron. A study of the energy dissipation into the tip and s ample shows that in total about 1% of the electrons undergo inelastic processes while tunneling. [S0163-1829(98)02040-2].