Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

TL;DR

This paper introduces an externally-triggerable optical pump-probe scanning tunneling microscopy system with tens-picosecond temporal resolution, enabling real-space imaging of carrier dynamics at the nanoscale in advanced materials.

Contribution

It presents a novel OPP-STM setup with improved temporal resolution and stable laser illumination, overcoming previous limitations of nanosecond resolution and enabling detailed carrier dynamics studies.

Findings

Achieved tens-picosecond temporal resolution in OPP-STM.

Demonstrated carrier decay time of ~170 ps on GaAs(110).

Revealed local carrier dynamics at nanoscale features.

Abstract

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of $\sim170$ ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.