RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY

Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.