Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy

Abstract

Time-resolved tunneling current measurement in the subpicosecond range was realized by ultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pair method. A low-temperature-grown GaN x As 1−x ͑x = 0.36% ͒ sample exhibited two ultrafast transient processes in the time-resolved tunnel current signal, whose lifetimes were determined to be 0.653± 0.025 and 55. Smaller and faster are the key words in the progress of current nanoscience and technology. Thus, for further advances, a method of exploring the ultrafast transient dynamics of the local quantum functions in organized small structures is eagerly desired. Ultrashort optical pulse technology in the near-infrared to ultraviolet region has allowed us to observe transient phenomena in the femtosecond range, the optical-monocycle region, 1,2 which, however, has a drawback of a relatively low spatial resolution due to electromagnetic wavelength. On the other hand, scanning tunneling microscopy (STM), although its time resolution is limited by circuit bandwidth ͑ϳ100 kHz͒, enables us to observe spatial dynamics at the atomic level in real space. 3 Therefore, the integration of ultrashort optical technology with STM has been one of the most exciting goals since their invention. [4][5][6][7] Pioneering works were performed by Hamers et al., 4-7 which have attracted the extensive interest of researchers in various fields. However, there remain critical problems which have prevented the achievement of the laser-combined STM measurement, such as the displacement current due to the stray capacitance of the tunneling gap and photoelectrons produced by multiple photoabsorption. [4][5][6][7] In such cases, since a large area is included in the processes, the superior space resolution of STM cannot be utilized. In particular, the thermal expansion of the STM tip by photoillumination causes much large noise in the tunneling current, making the measurement difficult.Here, we show the results of the time-resolved tunneling current measurement in the subpicosecond range, which can advance the development of future research in terms of ultimate temporal and spatial resolutions.A schematic of the measurement system is shown in Fig. 1. We adopted the recently developed shaken-pulse-pairexcited STM (SPPX-STM) method, which realizes highly sensitive measurement free from the thermal expansion effect of the tip and sample. 8 The tunneling junction is directly illuminated by a sequence of laser pulse pairs and average tunneling current, I t ͑t d ͒, is measured as a function of the delay time between the two pulses, t d . To decrease broadband noise, the delay time of the two pulses t d is modulated with a small amplitude ⌬t d at frequency , and the tunneling current is detected by a lock-in amplifier. Since the tunneling current I t responds to the modulation as In the pulse-pair-excited STM measurement, the first laser pulse in each pulse pair acts as the pump pulse to excite and modulate the electronic structure of the sample surface, which might cause d...

Real-space imaging of transient carrier dynamics by nanoscale pump–probe microscopy

“…However, the spatial and temporal resolutions of the former method were limited to the 1 µm scale and laser-pulse repetition rate, respectively, and the PG-STM probes dI/dV or the quantity mediating the signal rather than the transient effect itself [18]. With the shaken-pulse-pair-excited STM (SPPX-STM) [19,20], which was designed to detect a weak tunneling current of the transient signal under optical excitation, the time-resolved tunneling current in the subpicosecond range was successfully probed; however, its temporal range was very narrow and it was still difficult to reliably measure a very weak time-resolved STM signal in a short period of time, preventing the microscopy technique from revealing transient carrier dynamics in nanostructures consisting of composite materials with a wide variety of lifetimes. The long measurement time interferes the imaging of dynamics.…”

Laser-combined scanning tunnelling microscopy for probing ultrafast transient dynamics

“…This idea has been realized recently and named SPPX-STM [26][27][28][29][30][31][32]. In SPPX-STM, the optical intensity is nearly unchanged at the modulation frequency, and hence, the thermal interference is markedly suppressed.…”

“…In principle, a newly developed microscopic technique, shaken-pulse-pair-excited STM (SPPX-STM) [26][27][28][29][30][31][32] In this article, we briefly review attempts to improve resolutions by combining STM with ultrafast laser technology and describe the features and critical issues. Furthermore, we show the development of SPPX-STM and its principle and application.…”

Laser-Combined STM and Related Techniques for the Analysis of Nanoparticles/Clusters

“…Therefore, if the tunneling process directly produced by optical excitation can be measured, high temporal and spatial sensitivity can be simultaneously achieved with the atomic-scale resolution of STM [32][33][34][35][36][37][38][39][40][41]. A promising setup for achieving this is pulse-pair-excited STM (PPX-STM), in which, in analogy with pump-probe experiments, a sequence of paired laser pulses with a certain delay time t d excites the sample surface beneath the STM tip, and the tunneling current I is measured as a function of t d .…”