Ultrafast scanning tunneling microscopy: Principles and applications

The growing field of nanotechnology requires special tools capable of probing ultrafast surface dynamics on atomic scales in order to unveil the fundamental relationships between material structure and its properties. The invention of the scanning tunneling microscope (STM) revolutionized the field of surface science, enabling the first images of surface structure on an atomic length scale. However, investigations using STM techniques have been largely restricted to the study of static surface properties such as electronic structure and topography, revealing little information on the dynamics underlying the phenomena under study. On the other hand, ultrafast optical techniques routinely reveal material dynamics on timescales as short as 10 fs, but with a spatial resolution > 1 mum. This chapter describes several approaches for combining these two techniques to measure ultrafast dynamics on an atomic scale. We describe here approaches for ultrafast STM based on photoconductive gating ultrafast STM, time-resolved STM through tunnel-distance modulation, and ultrafast junction-mixing STM. While much progress has been made to develop these techniques and understand both their limitations and exactly what each technique measures, we still have much to learn before ultrafast STM becomes a standard method for characterizing the dynamics on the nanoscale.