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

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 $$\sim 170$$ ∼ 170 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.

Ultrafast Dynamics Revealed with Time-Resolved Scanning Tunneling Microscopy: A Review

“…Moreover, the coherent manipulation of light–matter interactions can now occur with atomic accuracy, which may provide an alternate approach to switch the quantum state population of a qubit. Even though TR-STM is currently a sophisticated technique only available in a limited number of research laboratories, recent advances in technology have made TR-STM more accessible and user-friendly. ,,,,,− Because of its versatility, we highly anticipate its future as a widely used technique that is accessible to users in a diverse array of fields.…”

Time-resolved force microscopy using the delay-time modulation method

“…In this study, we used an optical system with the delay time modulation of two pulse lasers (532 nm, 45 ps pulse width, 500 kHz) that can be driven by an external voltage pulse generator. 20,21) The delay time modulation shown in Fig. 1(b) is a method by which the delay time between the pump light and pulse light periodically changes (in this case at 20 Hz) between the measurement time t d and the sufficiently separated time t d,ref , and the corresponding signal is detected by lock-in detection.…”

“…With the delay time modulation method, it is possible to not only eliminate the effects of thermal expansion but also extract only the force component that reflects the dynamics of the sample excited by the pump light. 1,4,[20][21][22] As shown in Fig. 1, a tuning-fork-type cantilever was used, eliminating the need for an optical system for probe vibration detection.…”

“…This trend is similar to the signal detection principle of time-resolved STM when optical absorption bleaching is present. 1,20,21) On the other hand, for the image dipole force shown in Fig. 4(b), when contemplating the force transient response in the high-speed regime, the force is only measured during light irradiation that is sufficiently shorter than the rising time of α s due to the carrier accumulation on the surface, as shown by the orange and red bars in Fig.…”

Optoelectronic Readout of Single Er Adatom’s Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9 K)

“…1−4 Significant progress in this domain has focused on diverse aspects, among which, mapping the optical electromagnetic field in the junction of a scanning tunneling microscope (STM), 5,6 Raman spectroscopy at the molecular scale, 7 or probing surface carrier dynamics with pump−probe excitation. 8,9 Conducting this kind of investigation remains challenging. One major hurdle lies in effectively coupling laser light to the STM junction, ensuring the STM operates normally (measuring tunnel current, bias voltage, and tip surface distance) while maintaining control over the electromagnetic field parameters beneath the STM tip apex, including polarization, intensity, and spatial distribution.…”