New delay-time modulation scheme for optical pump–probe scanning tunneling microscopy (OPP-STM) with minimized light-intensity modulation

Abstract

Optical pump–probe scanning tunneling microscopy (OPP-STM) with square-wave delay-time modulation is capable of resolving nanoscale ultrafast dynamics because it does not require illumination intensity modulation that disturbs precise STM measurement. It has been overlooked, however, that when the delay time between the pump and probe pulses is modulated by periodically changing the emission timing of either the pump pulse or the probe pulse, with the other remaining the same, the illumination intensity is slightly but finitely modulated with the same periodicity as a side effect. We propose a new delay-time modulation scheme that periodically changes the emission timing of both pulses, which can greatly reduce the side effect and remove the induced fake signal in OPP-STM measurements.