Subcycle Mid-Infrared Electric-Field-Driven Scanning Tunneling Microscopy with a Time Resolution Higher Than 30 fs

Abstract

Electric-field-driven scanning tunneling microscopy (STM) that uses carrier envelope phase-controlled pulses in the THz region (THz-STM) has been attracting considerable attention because of its capability of probing ultrafast dynamics in materials. However, since the electric field is in the THz region, its time resolution of about 1 ps limits the range of measurable targets. Therefore, to pave the way for studying the local dynamics of non-equilibrium states such as elementary excitations and phase transitions in condensed materials, it is desirable to develop an STM system with a higher time resolution. Here, we report a mid-infrared (MIR) electric-field-driven STM system enabling an atomic-scale pump-probe method to be used over a wide range of time with a time resolution higher than 30 fs. We demonstrate the high potential of the new MIR-STM system by visualizing the photo-induced ultrafast non-equilibrium dynamics in MoTe2. We succeeded in measuring ultrafast carrier dynamics in the time range of 0 to over 1 ps, which were well explained by the change in band structure associated with the carrier dynamics. In addition to time-resolved signal measurement, atomically resolved MIR-STM imaging was also realized.