Scanning Probe Microscopy

Next-Generation Time-Resolved
Scanning Probe Microscopy
Ushering in a New Era of Nanoscale Exploration with Enhanced Usability
Katsuya Iwaya1, Hiroyuki Mogi2, Shoji Yoshida2, Yusuke Arashida2, Osamu Takeuchi2, Hidemi Shigekawa2

Understanding the nanoscale carrier
dynamics induced by light excitationis the key to unlocking futuristic
devices and innovative functionalities inadvanced materials. Optical pump-probe
scanning tunneling microscopy (OPP-STM)
has opened a window to these phenomena.
However, mastering the combination ofultrafast pulsed lasers with STM requires
high expertise and effort. We have shattered this barrier and developed a compact OPP-STM system accessible to all.
This system precisely controls laser pulse
timing electrically and enables stable laser
irradiation on sample surfaces. Furthermore, by applying this technique to atomic force microscopy (AFM), we have captured time-resolved force signals with an
exceptionally high signal-to-noise ratio.
Originating from the dipole-dipole interactions, these signals provide insights into
the carrier dynamics on sample surfaces,
which are activated by photo-illumination.
These technologies are promising as powerful tools for exploring a wide range ofphotoinduced phenomena in conductive
and insulating materials.

34 • Imaging & Microscopy 1/2024

Introduction
The need for exceptional spatial and temporal resolutions is at the heart of deciphering
the intricate carrier dynamics in nanoscalematerials. Conventional scanning tunneling microscopy (STM), although providing
unparalleled spatial and energy resolutions, hits a temporal resolution ceiling at
sub-millisecond levels owing to preamplifier
bandwidth constraints. With the integrationof transformative OPP techniques into STM,
such barriers have been overcome, realizing
higher temporal resolutions. This fusion ispivotal in probing materials‘ complex nonequilibrium carrier and spin dynamics [1,2].
Another technique, the use of a subcycle
electric field as bias voltage, has been incorporated as an electric-field-driven STM.
This technique provides temporal resolutions of less than 1 ps and 30 fs, maintaining the spatial resolution of STM using terahertz (THz) and mid-infrared pulses [3-6].
This leap in technology widens the horizonof time-resolved STM; however, it still requires high expertise and simplification of
the system to make it accessible for a broad

spectrum of research endeavors. In addition,
since the application of STM is limited toconducting materials, extending this OPP
technique to atomic force microscopy (AFM)would further expand the capabilities of thistime-resolved measurement technique.

Optical Pump-Probe Scanning TunnelingMicroscopy (OPP-STM)
In the conventional OPP method, pump lightand probe light, which are delayed in time,
are irradiated onto a sample, as shown inFigure 1a. When carriers such as electrons
and holes excited by the pump light remain
in an excited state, the excitation by theprobe light is suppressed (absorption bleaching). Therefore, by measuring the reflectivityof the probe light as a function of the delaytime, one can investigate the dynamics ofthe states excited by the pump light with atime resolution corresponding to the pulse
width of the excitation light. However, thespatial resolution is limited to the diffraction and light-spot size (around micrometerorder).

Scanning Probe Microscopy

In OPP-STM, the sample surface under theSTM tip is first excited by a pump pulse andsubsequently by a probe pulse with a delay
time td, and the tunneling current is detectedusing a conventional preamplifier (Fig. 1b)[1]. When td is sufficiently long ((1) in Fig. 1c),most of the photocarriers excited by the pumppulse relax to the ground state before thesubsequent probe pulse illumination so thata similar number of carriers would be excitedby the probe pulse as with the pump pulse,
resulting in a large transient current I*probe. In
contrast, when td is short, the excited statesremain occupied with the photocarriers excited by the pump pulse when the probe pulse
illuminates the sample so that the optical absorption saturates, resulting in a small I*probe((3) in Fig. 1c). By illuminating a pair of pumpand probe pulses sequentially and by varying
td, one can detect a time-averaged tunnelingcurrent <I> as a function of td (Fig. 1d). By
fitting the time-resolved tunneling current
with an exponential function, we can obtaina decay time at the tip location, as examples
are shown in Figure 1e [1].
In the macroscopic OPP technique, themodulation of optical intensity is conventionally utilized to detect a weak OPP-induced signal. However, the optical intensity
modulation causes severe problems, such as
the thermal expansion of the STM tip. Sincechanges in tip-sample distance are exponentially multiplied in the tunneling current,
the optical intensity modulation cannot bedirectly applied to STM. To suppress the thermal expansion effect, we have developed anexcellent delay-time modulation technique[1]. In this technique, we use two delay times(td and tmax in Fig. 1b). The longer delay time
tmax is generally set to one-half of the laser
pulse interval (for example, 0.5 μs for 1 MHz
repetition rate), corresponding to the longest delay time available for the selected
repetition rate. We modulate the delay time

Fig. 1: (a) Conventional OPP configuration. (b) Schematic of OPP-STM with the delay-time
modulation method. (c) Transient tunneling current I* induced by the pump light and probe lightat the representative delay time td. Time-averaged tunneling current <I(td)> is shown for eachcase (green rectangular). (d) <I> as a function of td. The time-averaged <I> corresponding toeach case in (c) is plotted. (e) Time-resolved signals obtained for several samples. [1]between td and tmax at, for example, 1 kHz
and detect the resultant tunneling current
ΔI(td) = <I(td)> − <I(tmax)> using the lock-in
amplifier (Fig. 1b). This modulation technique enables us to keep the thermal load at
the tunnel junction constant, substantiallysuppressing the thermal expansion effect.

Compact and Stable OPP-STM
The delay time modulation technique has enabled reliable OPP-STM measurements. Since
then, many studies such as the atomic scale
carrier dynamics around a single impurity on

a GaAs surface [7,8] and the visualization of
the ultrafast carrier dynamics in a GaAs-PIN
junction [9] have been reported. However, theoptical system has been generally complex
and large in scale, hindering the widespreaduse of this technique. To overcome this difficulty, the OPP-STM system, whose laser-pulse
timing is electrically controlled by externaltriggers, has significantly improved the easeof use, but its temporal resolution has been
limited to the nanosecond range [10]. In addition, fluctuations in optical intensity cause
unexpected issues, such as the thermal expansion effect, making the accurate observationof physical phenomena challenging.

Fig. 2: (a) Photograph of the compact OPP-STM system. (b) Three-dimensional illustration of the OPP-STM unit. (c) Optical image of the tip andits mirror image on GaAs(110) surface with a laser spot illuminated at the tunneling junction before (top) and after 16 h (bottom), showing thestability of the laser spot. [11]Imaging & Microscopy 1/2024 • 35

Scanning Probe Microscopy

Fig. 3: (a) Grid-point time-resolved tunneling current measurement on GaAs(110) cleaved surface [11]. Characteristic features along the step edge and the perimeter of the nanoscale bump
structure are identified in the time-resolved tunneling current map (right). By fitting a time-resolved tunneling current curve at each grid point, we can obtain the nanoscale relaxation timemap (top left). (b) Snapshots of ultrafast motion of photoinjected electrons in a C60 multilayer/
Au sample obtained by THz-STM. [5]electrically controlled delay-time modulation with tuning-fork-type frequency modulation (FM) AFM (Fig. 4b) [14]. By focusing
optical pulse pairs onto the apex of the AFM
probe, we were able to detect the amplitudeof the frequency shift Δf as a time-resolved
signal. The markedly stable time-resolved
measurements on bulk WSe2, a layered semiconductor, have unveiled dynamics withtwo decay components through an ultrafast
photoinduced force (Fig. 4c). These signals,
decoded as the surface recombination anddiffusion of photocarriers through tunnelingcurrent and force spectroscopy, mark a significant stride in our understanding of material properties. This innovative approach not
only resolves the limitations encountered intime-resolved STM due to tunneling current
but also paves the way for its application
across a diverse array of materials, setting a
new standard in nanoscale imaging.

Conclusion

Fig. 4: (a) Dipole-dipole interaction mechanism. αt, αs, and E0 are the complex effective polarizabilities of the tip and sample, the incident light electric field vector, respectively. (b) Schematicof tuning-fork-type time-resolved FM AFM setup. (c) Time-resolved signal obtained for a multilayer WSe2 sample and fitting results. [14]To improve the temporal resolution and thestability of laser illumination based on acompact electrically controlled laser system(Fig. 2a), the OPP-STM system with a temporal resolution of tens of picoseconds has
recently been developed [11]. The long-term
stability of laser illumination was realizedalso by placing the focus lens on the STM
stage (Fig. 2b) and confirmed by monitoring
the tip and its mirror image on the sample
surface together with a laser spot focused on
the tip-sample junction (Fig. 2c).
To demonstrate the performance of thesystem, we conducted a grid-point time-resolved tunneling current measurement on
GaAs(110) surfaces at T = 6 K (Fig. 3a). By
measuring the time-resolved tunneling current at each grid point, we can compile thetime-resolved tunneling current maps at eachdelay time and determine characteristic features along the nanostructure (Fig. 3a, right).
Furthermore, it is possible to map a relaxation
time by fitting each curve with an exponential function. The relaxation time map (Fig.
3a, top left) demonstrates that the relaxation
time inside the nanoscale bump structure

36 • Imaging & Microscopy 1/2024

is substantially shorter than that of its surroundings. This mapping technique is powerful in visualizing carrier dynamics associated with nanoscale structures. Figure 3b
shows an example of a time-resolved signal
map obtained by THz-STM. The improvementin the usability of the optical system can beextended to electric-field-driven STM, which
is currently our ongoing investigation.

OPP Atomic Force Microscopy
The optical system we have developed is also
applicable to AFM, expanding its potential
beyond conductive materials such as semiconductors and metals, traditionally targetedin STM. Integrating AFM into our repertoire
significantly broadens the horizons of nanoscale time-resolved microscopic techniques. Previous studies have clarified thephotoexcited dynamics of forces originatingfrom surface photovoltage and dipole-dipoleinteractions (Fig. 4a) based on optical intensity modulation (Fig. 1a) [12,13]. We haveadvanced these achievements by combining

Our streamlined optical system markedly eases researchers‘ path to harness the transformative power of the OPP-STM technique. With
the multiprobe (MP) system, features such as
small islands on an insulating substrate canbe observed by using one tip as an electrodewhile conducting STM measurements withthe other tip. The OPP-MP measurements
have already been demonstrated on monolayer transition metal dichalcogenides to investigate nanoscale exciton dynamics [15,16].
Regarding the limitations of wavelength andtemporal resolutions, cutting-edge laser technology, which has been rapidly developing,may overcome these limitations, thereby realizing a higher-performance optical systemenabling a wavelength-variable, externallycontrollable laser system with a smaller pulse
width in the future. The development of easyto-use optical systems and their applications
to various scanning probe techniques will expand the capability of OPP-SPM techniquesand contribute to a deeper understanding ofvarious photo-induced phenomena.
Affiliation
1UNISOKU Co., Ltd., Hirakata, Osaka, Japan
2University of Tsukuba, Tennnodai, Tsukuba,
JapanContactProf. Dr. Hidemi Shigekawa
University of Tsukuba
Tsukuba, Ibaraki, Japanhidemi@ims.tsukuba.ac.jp

References:https://bit.ly/IM-Shigekawa