i-RTG Lectures

This lecture will give an introduction into the theoretical background and technical aspects of ultrafast scanning tunneling microscopy (USTM). I will introduce different theoretical approaches to classify light-matter interaction in tunnel junctions: (i) Possible criteria for adiabatic (quasi-static) tunneling and their applicability to STM and (ii) the criterion for detecting the photon nature of light as quantum steps in the current-voltage curve of a tunnel junction. I will further discuss possible processes that can produce an ultrafast current in STM, the physical information they can provide about the nonequilibrium state of the sample, and the challenges that can arise in their unambiguous identification. In the second part of the lecture, I will introduce important technical aspects of USTM operation. This includes a discussion of suitable femtosecond laser systems, understanding of the antenna response of the STM tip (plasmonic and non-plasmonic) in the THz and VIS spectral range, how to deal with light-induced heating, and the importance of tight focusing and the integration of high-NA optics into low-temperature STM setups. The lecture will provide the background knowledge for a comprehensive understanding of modern USTM experiments and their value for contemporary condensed matter research.