Optical interactions with the scanning tunneling microscope

Abstract

This work investigates optical interactions with scanning tunneling microscopy with the intention of better understanding how light affects the output of the STM for different samples as well as how light can be emitted from the STM, possibly providing additional information about the surfaces examined. In Chapter 1, a variety of topographical, electronic and even magnetic surface maps, which were produced by STM, are presented to demonstrate the capabilities of state-of-the-art probe microscopy techniques. These images represent the results from a variety of materials investigations. The second chapter examines the results of direct illumination of the tunnel junction. Seven effects are treated, each section describing measurements conducted in the laboratory as well as the theory and a review of work by other researchers. First, heating effects, induced by illumination, which give rise to thermal expansion and thermovoltages are studied. Enhanced conductivity, as well as induced photovoltage as a result of carrier excitations are then explored. A description of the theory of photoemission, and photo- and thermal-assisted tunneling is presented along with an examination of how electromagnetic fields incident on the tunnel junction can be rectified by the nonlinear I - V curve of the junction. Chapter 3 describes photon emission from the tunnel junction, the majority of this chapter dealing with our results on photon emission from Au surfaces in air which include spatial maps of the photon intensity. Surface modification of the Au films by the tip in the form of pitting, etching, and surface damage is illustrated and attributed with enabling photon emission to be attainable in air, where contamination effects were shown to quench the emission process. A survey of other experiments involving photon emission is also presented. Finally, the appendix discusses how optically exciting surface plasmons in metal films can be detected with an STM. Experiments conducted by other researchers, have demonstrated that kinetic energy energy from surface plasmons can be efficiently coupled into the tip resulting in heating and subsequent thermal expansion. Other investigations have revealed that the AC fields of the surface plasmons can be directly coupled into the junction, resulting in the rectification.