www.nature.com/scientificreports

OPEN

Externally‑triggerable optical
pump‑probe scanning tunnelingmicroscopy with a time resolutionof tens‑picosecond
Katsuya Iwaya 1*, Munenori Yokota 1, Hiroaki Hanada 1, Hiroyuki Mogi 2, Shoji Yoshida 2,
Osamu Takeuchi 2, Yutaka Miyatake 1 & Hidemi Shigekawa 2*
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 forstable time-resolved measurements has remained challenging. The recent OPP-STM system, whoselaser-pulse timing is electrically controlled by external triggers, has significantly simplified thiscombination but limited its application due to nanosecond temporal resolution. Here we report anexternally-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 beamstabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing
carrier dynamics with a decay time of ∼ 170 ps and revealing local carrier dynamics at features
including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tenspicosecond resolution by the electrical control of laser pulses highlight the potential capabilities of thissystem for investigating nanoscale carrier dynamics of a wide range of functional materials.
The ability to measure carrier dynamics in nanoscale materials and devices is an important capability that requires
experimental techniques with both high spatial and high temporal ­resolutions1. To this end, many time-resolved
techniques in combination with methods such as electron m­ icroscopy2–4, photoemission electron m­ icroscopy5,6,
and X-ray d­ iffraction7 have been reported. Scanning tunneling microscopy/spectroscopy (STM/STS) is a powerful technique to probe topographic and spectroscopic properties of various material surfaces with high spatialand energy resolutions. However, the temporal resolution of conventional STM is limited to the sub-millisecond
range by the preamplifier bandwidth (∼ 1 kHz). To overcome this limitation, considerable effort has been madesince its i­ nvention8–11. Among these, the application of optical pump-probe (OPP) techniques to STM can circumvent the limitations of the circuit bandwidth, achieving higher temporal r­ esolutions12–16.
An OPP-induced tunneling current is generally weak to detect so that we need to employ a modulation
technique using a lock-in amplifier. However, the modulation of optical intensity causes severe problems such as
thermal expansions of STM tip and sample. Since changes in the tip-sample distance are exponentially multipliedin the tunneling current, such conventional OPP methods cannot be directly combined with STM. In 2004, an
exquisite delay-time modulation technique to suppress the thermal expansion effect has been ­invented17. With
subsequent noise-level and delay-time i­ mprovements18,19, the OPP-STM is now capable of probing the nonequilibrium dynamics of systems such as the atomic scale carrier dynamics around a single impurity on GaAs(110)­surface20,21, the visualization of the ultrafast carrier dynamics in a GaAs-PIN j­unction22, and the relaxation
dynamics of polarons bound to oxygen vacancies on rutile TiO 2 (110) ­surface23. Furthermore, recent studies
have realized another time-resolved STM utilizing a sub-cycle electric field as bias voltage between the STM
tip and sample, called electric-field-driven STM. By measuring an instantaneous tunneling current induced by
the sub-cycle electric field, ultrafast time-resolved measurements can be performed. The electric-field-driven
STM enables the temporal resolution faster than 1 ps and 30 fs while maintaining the spatial resolution of STM
using terahertz (THz) and mid-infrared p­ ulses24–31. These efforts have substantially expanded the possibilities of
1
UNISOKU Co., Ltd., Osaka 573‑0131, Japan. 2Faculty of Pure and Applied Sciences, University of Tsukuba,
Ibaraki 305‑8573, Japan. *email: katsuya_iwaya@unisoku.co.jp; hidemi@ims.tsukuba.ac.jp

Scientific Reports |(2023) 13:818| https://doi.org/10.1038/s41598-023-27383-z

1Vol.:(0123456789)

www.nature.com/scientificreports/
time-resolved STM. However, the use of sub-cycle pulsed electric fields still requires various expertise, including
the creation and control of electric fields.
The recent OPP-STM systems, whose laser-pulse timing is electrically controlled by external triggers, have significantly improved both the ease of use and the stability of the optical s­ ystem23,32,33, but the temporal resolutions
have been limited to the nanosecond range. For example, recent time-resolved observation of exciton dynamics
in transition metal dichalcogenides has revealed the lifetime of exciton in several tens picosecond r­ anges34. It isthus indispensable to improve the temporal resolution higher than the nanosecond range.
In addition, in most previous OPP-STM systems, the focus lens is placed at a viewport on the ultrahighvacuum (UHV) chamber while hanging the STM stage by springs for vibration isolation. This configuration
causes unstable laser illumination of the tip-sample junction because relative positions of the sample and the lens
are affected by vibration noises. The change in the light intensity due to the unstable laser illumination causes
unexpected problems such as the thermal expansion effect, making it difficult to observe physical phenomenaof interest correctly. Therefore, to utilize this experimental technique in a wider range of research fields, further
improvements in the stability of the laser spot with a high temporal resolution are strongly required.
In this study, we report the design and performance of a newly developed externally-triggerable OPP-STM
system that enables us to conduct tens-picosecond time-resolved measurements with long-term stability. To
simplify the optical system, we employ externally-triggerable picosecond laser systems (pulse width ∼ 45 ps) andcontrol the timing of laser pulses electrically. We also show that by placing the aspheric lens on the STM stage andusing an optical beam stabilization system, the position of the laser spot on the sample surface is stable for hours,
thus suitable for long-term experiments. The OPP-STM system with both high temporal resolution and highoptical stability will facilitate the widespread use of this method for understanding nanoscale carrier dynamics.

Experimental methods
System overview.

The newly developed OPP-STM system used in this study consists of three components:
the low temperature UHV STM system, the optical system that includes two lasers and the beam stabilization
system, and the delay-time control system (Fig. 1).
The STM system is based on the UNISOKU USM1400 model and is composed of three UHV chambers: aload-lock chamber for tip and sample exchange, a tip and sample preparation chamber, and a STM observationchamber (Fig. 2a). The three chambers are mounted on a passive vibration isolation table (ADF-1311YS, Meiritz
Seiki Co., Ltd.). The preparation and observation chambers are pumped by two ion pumps (240 and 125 L/s),each equipped with a titanium sublimation pump. The base pressure of the observation chamber is 8 × 10−8 Paat room temperature and 2 × 10−8 Pa when the STM head is cooled down. The base temperature at the STM
stage when cooled with liquid nitrogen and liquid helium is 78 K and 6 K, respectively.
Both the tip and the sample are aligned horizontally in the STM head (Fig. 3a), and are transferred to the
STM head from the top using a vertical transfer rod. The flag-type sample holder is loaded onto the sample
stage which is fixed without moving capability. The tip coarse approach and the tip lateral position control are
driven by the stick and slip motion of shear piezo stacks (six stacks with travel distance of ±2.5 mm for coarse

Figure 1.  Schematic diagram of the OPP-STM system. PSD: position sensitive detector, BS: beam splitter,
VND: variable neutral density filter.
Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

2

www.nature.com/scientificreports/

Figure 2.  (a) Photograph of the STM and the optical system developed in this study. (b) Photograph of theconventional optical system used in Ref.18.

Figure 3.  (a) Three-dimensional illustration of the STM unit (top-view). Electrical wirings and copper heatlinks are not shown for clarity. (b) (Top) Optical image of the tip and its mirror image on GaAs(110) surface
with a laser spot illuminated at the tunneling junction. (Bottom) The image taken after 16 hours, showing thestability of the laser spot.

Scientific Reports |(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

3Vol.:(0123456789)

www.nature.com/scientificreports/
approach, and three stacks with travel distance of ±3 mm for lateral positioning). The STM stage is vibrationallyisolated by an eddy current damper that is composed of four stainless-steel springs and nine samarium-cobalt
magnets. The absence of significant noise peaks up to 1 kHz is confirmed in a tip-sample distance noise spectrum(Supplementary Fig. S1 online).
The aspheric lens focusing the laser light on the sample surface is located close to the sample stage (“Lens#1” in Fig. 3a, A397-A, Thorlabs, Inc.), and the light enters with the incident angle of 55◦ normal to the sample
surface. With this configuration, the numerical aperture of 0.3 is obtained (the working distance is 9.64 mm).
To precisely adjust the laser spot on the sample surface, the aspheric lens is mounted on a home-made 3D piezoelectric actuator that moves in the x, y, and z directions. The movement in each direction is driven by the stickand slip motion of three shear piezo stacks. The travel distance in the x, y, and z directions are ±3 mm, ±3 mm,
and ±2 mm, respectively. Another aspheric lens, similarly mounted on the 3D piezoelectric actuator (“Lens #2”
in Fig. 3a), is used to observe the tip apex, the sample surface, and the focused laser spot in situ through a CCD
camera (Fig. 3b). The optical magnification by the objective lens (“Lens #2”) and imaging lens (located outside
the UHV chamber, AC254-200-A, Thorlabs, Inc.) is ∼ 18x.
The sample surface under the STM tip is illuminated by a sequence of pump and probe pulses (Fig. 1), and thetunneling current is detected using a commercial current preamplifier (109 V/A, DLPCA-200, Femto Messtechnik GmbH). The change in the tunneling current induced by the delay-time modulation technique is measured
using a lock-in amplifier as a function of the delay time td between the pump and probe pulses. We discuss thislatter measurement in more detail later.

Optical system. The optical system consists of the pump laser, the probe laser, and the beam stabilization
system (Fig. 1). To realize a system that is both compact and easy-to-use, we use two picosecond pulsed laser
systems (KATANA 05, NKT Photonics), whose laser-pulse timing can be electrically controlled by external triggers. The central wavelength of the lasers is 532 nm and the pulse width is ∼ 45 ps.
To perform stable OPP-STM measurements, the long-term stability of the laser spot on the sample surface iscritical. To keep both the pump and the probe pulses aligned on the same axis, we use the active beam stabilization system (Aligna, TEM Messtechnik GmbH) that controls one pair of active mirrors to stabilize each of thepump laser and of the probe laser spot position on the position sensitive detector (PSD) (Fig. 1). In addition,
the two laser heads and the beam stabilization system are placed on the vibration isolation table as shown in
Fig. 2a. The total dimension of the optical system ( 90 cm × 45 cm × 15 cm ) is substantially smaller than theprevious OPP system used in Ref.18 (Fig. 2b). We also note that placing the aspheric lens on the same stage with
the STM head is crucial to keeping the laser spot position on the sample surface unchanged for long periods oftime because the relative positions of the sample and the lens are kept unaffected by vibration noises.
To demonstrate the stability of laser spot on the sample surface, we monitor a tip and its mirror image on
GaAs(110) cleaved surface together with a laser spot focused at the tip-sample junction. Figure 3b demonstrates
that the position of the laser spot remains unchanged even after 16 h. We also monitor both the tunneling current
and tip height with the feedback loop closed when the laser is illuminated (laser power: 0.25 mW) for 12 h at
T = 6 K and confirm that the standard deviation of the tunneling current is about 0.6 % of the average set-point
current with reasonably stable tip heights (Supplementary Fig. S1 online). These results indicate that our opticalsetup is stable enough for long-term OPP-STM experiments.
Delay‑time modulation technique. In the OPP-STM measurement, the sample surface under the tip is
first excited by a pump pulse, and subsequently by a probe pulse with a delay time td (Fig. 1). When td is sufficiently long [(1) in Fig. 4a], most of the photocarriers excited by the pump pulse relax to the ground state before
the subsequent probe pulse is illuminated; thus, a similar number of carriers would be excited by the probe pulse∗
as with the pump pulse, resulting in a large transient current Iprobe.
In contrast, when td is substantially short, the excited states remain occupied when the probe pulse illuminates∗
the sample so that the saturation of optical absorption occurs, resulting in a smaller Iprobe[(3) in Fig. 4a]. By
illuminating a pair of pump and probe pulses sequentially and by varying td , a time-averaged tunneling current< I > is detected as a function of td (Fig. 4b).
When td < 0, a pump pulse and a probe pulse are swapped. A symmetric lineshape with respect to td = 0
can be observed when the intensities of the pump and probe laser pulses are finely adjusted to be equivalent,allowing us to evaluate measurement conditions such as laser intensity and delay time settings. By combining
this technique with the STM capability, the ultrafast carrier dynamics of the sample surface local structures canbe investigated.
To detect a weak OPP-induced tunneling current, we need to employ a modulation technique using a lockin amplifier. However, the modulation of optical intensity causes severe problems such as thermal expansionsof STM tip and sample as described before. Therefore, we need the delay-time modulation technique designed
to suppress the thermal expansion e­ ffect17 (Fig. 4c). In this technique, we use two delay times (tD and tmax in
Figs. 1 and 4c). The longer delay time tmax is generally set to a half of the laser pulse interval (for instance, 0.5 µs
for 1 MHz repetition rate), which corresponds to the longest delay time available for the selected repetition
rate. We modulate the delay time between tD and tmax at 1 kHz and detect the resultant tunneling current
�I(tD ) =< I(tD ) > − < I(tmax ) > using the lock-in amplifier (Fig. 4c, bottom). By sweeping tD slowly alongwith the lock-in detection, we obtain I as a function of tD, called an OPP tunneling current–delay time curve
hereafter. This technique enables us to keep the thermal load at the tunnel junction constant, suppressing thethermal expansion effect ­substantially33.

Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

4

www.nature.com/scientificreports/

Figure 4.  (a) Relationship between transient tunneling current I ∗ induced by the pump and probe light anddelay time td. Time-averaged tunneling current < I > is shown for each case. (b) < I > as a function of td. The
time-averaged < I > corresponding to each case in (a) is plotted with corresponding number. (c) Schematic ofthe delay-time modulation technique. The delay time between the pump and probe pulses is modulated betweentD and tmax at ∼ 1 kHz. Consequently, < I > is also modulated between < I(tD ) > and < I(tmax ) > at ∼ 1 kHz,and the lock-in amplifier detects �I(tD ) =< I(tD ) > − < I(tmax ) >.

Delay‑time control system. To conduct the delay-time modulation technique shown in Fig. 4c, previous
studies have either used a mechanically movable mirror to change the optical path l­ength17, or a pulse picker
such as a Pockels cell to extract specific pulse ­pairs18. These systems are inevitably large and complicated, requiring considerable skills and experience to operate. Recently, a compact tabletop OPP system, in which the timingof laser pulses is electrically controlled by external triggers using the field-programmable gate array, has been
­reported32,35. However, while this approach has significantly improved the operational ease of the optical system,
it still requires expertise in electronic circuits for modifications and maintenance. Thus, developing a simpler
optical system both in terms of ease of operation and maintenance is important for the OPP-STM method to be
widely used.
In this study, we build the delay-time control system by combining the following commercial products: adigital delay/pulse generator (DG645, Stanford Research Systems), a high-speed switch (HMC-C011, Analog
Devices Inc.) and a frequency dividing circuit (74HC4040, Toshiba) (Fig. 1). Here, the trigger pulse for the pumplight (pump trigger) is directly input from the pulse generator to the pump laser at the repetition rate of 1 MHz.
The two trigger pulses for the probe light (probe triggers) with delay time tD and tmax relative to the pump trigger are input to the switch at the repetition rate of 1 MHz (Fig. 1). Either the probe trigger with tD or with tmax
is selected at the frequency of ∼ 1 kHz in the high-speed switch. Thus, a train of probe trigger pulses with tD or
tmax are alternately input to the probe laser with a time-interval of ∼ 0.5 ms, to generate a train of probe pulses
shown in Fig. 4c.
The 1 kHz switching signal is produced by the frequency divider, which is set to 1/1024(1 MHz × 1/1024 =∼ 977 Hz). This latter frequency is also used as the reference signal for the lock-in amplifier. The time resolution of the pump-probe experiment using this system is theoretically estimated to be ∼ 70 ps,
limited by the temporal width (45 ps), the jitter of the laser pulse (15 ps), and the jitter of the electrical trigger(25 ps). We measure cross correlation between the pump and probe laser pulses using the sum-frequency generation method, and the correlation width estimated by Gaussian fitting is 77.6 ± 2.1 ps (Supplementary Fig. S2
online) and in reasonable agreement with the theoretical value.

Results

First, we demonstrate the light-modulated scanning tunneling spectroscopy ­measurement36,37 on GaAs(110)
surface, where we measure I-V curves both under dark conditions (without laser illumination), and with laser
illumination at a repetition rate of 1 MHz. The atomically flat and clean surface is obtained by cleaving a commercially available n-type (100)-oriented GaAs wafer (doped with silicon at the density of ∼ 5 × 1017 cm−3)
along the (110) plane at room temperature in UHV conditions. In this study, mechanically polished PtIr tips areused for all STM measurements. A typical constant-current STM image at a sample bias voltage V of −3 V in
Fig. 5a shows the one-dimensional rows along the [110], consisting of the atomic lattice of As where the atomic
spacing between the rows along the [110] direction is 0.565 nm and along the row is 0.4 ­nm38.
A typical I-V curve taken without laser illumination is shown in blue in Fig. 5b. A negligibly small tunnelingcurrent at V > 0 is attributed to a depletion layer formed at the surface due to the tip-induced band bending­effect39. In the case of an n-type semiconductor surface, the conduction band of the sample is bent upward near

Scientific Reports |(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

5Vol.:(0123456789)

www.nature.com/scientificreports/

Figure 5.  (a) Constant-current STM image of GaAs(110) surface. Set-point: sample bias voltage V = −3 V,and tunneling current It = 100 pA. T = 78 K. (b) I-V curves obtained with (red) and without laser illumination(blue). Set-point: V = −2 V, It = 100 pA. T = 78 K. (c) Schematic illustrations of the band structures of then-type GaAs surface when V > 0 without (top) and with laser illumination (bottom).

the surface when V > 0 (Fig. 5c, top), preventing electrons tunneling from the tip. This results in a small tunneling current at V > 0, as observed in Fig. 5b.
When the sample surface is illuminated by laser pulses, electron-hole pairs are generated; according to the
tip-induced surface potential, while the holes in the valence bands accumulate at the surface under the tip, theelectrons in the conduction bands drift back into the bulk (Fig. 5c, bottom). This redistribution of photocarriers
induces efficient screening of the tip potential and suppresses the upward band bending as shown at the bottomof Fig. 5c. The energy shift due to the screening is called surface photovoltage (SPV). Consequently, the tunnelingcurrent at V > 0 greatly increases under illumination (red in Fig. 5b), and the SPV is estimated to be about 1.1 V
at V = +3 V. This behavior is in good agreement with previous r­ esults20, and is evidence that the sample surface
under the tip is sufficiently illuminated by the laser pulses.
Next, we demonstrate an OPP tunneling current measurement at T = 78 K on the GaAs(110) surface using
the delay-time modulation technique described in an earlier section. Figure 6a shows an OPP tunneling current
versus delay time curve obtained with a laser repetition rate of 1 MHz and a maximum delay time tmax of 500 ns.
The laser power is 0.5 mW. The lineshape is well fitted by a combination of two exponential functions, where
the decay times are estimated to be 4.5 ± 0.2 and 121.3 ± 8.3 ns. The temporal resolution improved in this study
allows us to observe the faster decay time (∼ 5 ns), which is inaccessible using previous externally-triggerable
optical ­systems23,32 It is known that for an n-type semiconductor, the excited state relaxes to the original statethrough two ­processes40. One is the decay of the photocarriers in the bulk (bulk-side decay) via recombination,
drift and diffusion (Fig. 6b, top). The other is the decay of the photocarriers trapped at the surface (surface-side
decay) via thermionic emission (Fig. 6b, bottom). The surface-side decay time is generally longer than the bulkside decay time because counterpart carriers are absent near the surface due to the depletion layer, and holes
near the surface need to move into the bulk to combine with electrons. Thus, the two decay times obtained byfitting correspond to the bulk-side decay (∼ 5 ns) and the surface-side decay (∼ 120 ns), and are consistent withprevious ­results41.
Next, we demonstrate a grid-point OPP tunneling current measurement at T = 6 K. Figure 7a shows aconstant-current 50 nm × 50 nm STM image of a GaAs(110) surface where a nanoscale protrusion (possibly
produced after cleaving, referred to as “bump structure” hereafter) and a step edge are observed over flat terraces.
OPP tunneling current-delay time curves are measured at 50 × 50 grid points in the same field of view, eachpoint taking ∼ 30 seconds for a total of ∼ 21 h. Representative OPP tunneling current–delay time curves taken
at the bump structure, the step edge and the terrace are shown in Fig. 7b. For comparison, a curve averaged over
Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

6

www.nature.com/scientificreports/

Figure 6.  (a) Typical OPP tunneling current–delay time curve of GaAs(110) surface. Set-point: V = +2.8 V,
It = 100 pA. Laser power is 0.5 mW. T = 78 K. The step of delay time is 795 ps, and the averaging time ateach delay time is 30 ms. The curve is averaged over 10 sweeps and fitted with two exponential functions. (b)Schematic illustration of the band structures of the n-type GaAs during the time-resolved measurement whenV > 0. The decay processes of the photocarriers in the bulk (top) and the surface (bottom) are shown.

Figure 7.  (a) STM image of GaAs(110) surface. Set-point: V = +3 V, It = 100 pA. T = 6 K. (b) Characteristic
spatially-dependent OPP tunneling current–delay time curves. The locations where the curves are taken are
shown in (a) indicated with the same color. Laser power is 0.25 mW. (c) Line curves along A-B in (a). The stepof delay time is 3 ns, and the averaging time at each delay time is 0.15 s in (b) and (c). (d) OPP tunneling current
map at a delay time of 0 ns measured in the same field of view with (a). (e) Decay time map in the same fieldof view with (a). The decay time at each location is estimated by fitting the spectrum with a single exponential
curve.
Scientific Reports |(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

7Vol.:(0123456789)

www.nature.com/scientificreports/
2500 grid points is also shown. As Fig. 7b demonstrates, the OPP tunneling current curves strongly depend on
the nanoscale surface structures—this spatial dependence is inaccessible using the conventional OPP methods
without STM.
To examine the spatial dependence in more detail, we compile a series of line curves across the step edge, thebump structure, and the terrace. As Fig. 7c demonstrates, the OPP tunneling current curves not only identify theinterior region of the bump structure, but they also identify the boundary between the bump and the terrace. In
fact, the OPP tunneling current image at td = 0 in Fig. 7d exhibits large amplitude I both along the perimeterof the bump structure and along the step edge.
Furthermore, it is possible to map a decay time by fitting each curve with an exponential function. In thisexperiment, a relatively low laser power (0.25 mW) is chosen so that most of curves are well fitted with a single
exponential function. The decay time map in Fig. 7e demonstrates that, inside the bump structure, the decay time(∼30-60 ns) is substantially shorter than that of its surrounding. This short decay time might be attributed to therecombination of holes in the depletion layer with electrons injected from the tip into impurity states formed
inside the bump structure, in analogy with the Co nanoparticle on GaAs(110) s­ urface18.
Additionally, we fit OPP tunneling current curves taken inside the bump structure with two exponentialfunctions, separating the fast and the slow components. Figures 8a,b show the maps of the decay time and of theamplitude of exponential function for each component, informing on the inhomogeneous spatial distributionof the fast and slow components. The histogram of the decay time shows the fast component ranges between
10 and 50 ns in a narrow temporal range (red in Fig. 8c), and the slow component ranges between 100 and
550 ns in a wider temporal range (blue in Fig. 8c). In addition, as shown in Fig. 8d, the fast component exhibits
larger amplitude values than the slow one, possibly reflecting its dominant carrier dynamics. We note that theelucidation of the origin of these results requires samples with better defined nanoscale structures, which isbeyond the scope of this work. Here, we emphasize the capability of the mapping techniques presented above
for investigating ultrafast carrier dynamics with nanometer-scale spatial resolution and applicable for various
semiconducting materials.
Finally, we conduct an OPP-STM measurement on low-temperature-grown GaAs (LT-GaAs) to directly demonstrate the tens-picosecond range temporal resolution (Fig. 9). The fast decay at around td = 0 is attributed tothe cross-correlation between the pump and probe laser pulses (FWHM of ∼ 78 ps) as shown in Supplementary
Fig. S2 online. By fitting the OPP tunneling current curve in the range of |td | > 55 ps, we obtain the decay timeof ∼ 170 ps, which could be originated from surface defect levels. The detail is beyond the scope of this work andwill not be discussed here. This result clearly shows that the OPP-STM system developed in this study enables

Figure 8.  (a) Decay time map of the fast and slow components in the bump structure in Fig. 7a. The decay timeat each location is estimated by fitting the spectrum with double exponential curves. (Left) Fast component.
(Right) Slow component. (b) Amplitude map of the double exponential curves. (Left) Fast component. (Right)
Slow component. (c) Histogram of the decay time of the fast and slow components in (a). (d) Histogram of theamplitude of the fast and slow components in (b). The numbers of counts are normalized in (c) and (d).
Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

8

www.nature.com/scientificreports/

Figure 9.  OPP tunneling current–delay time curve of LT-GaAs taken at room temperature. Set-point:
V = +5.5 V, It = 1 nA. Laser power is 4 mW. The step of delay time is 5 ps, and the averaging time at each delay
time is 78 ms. The curve is averaged over 25 sweeps. The fast decay at around td = 0 is attributed to the crosscorrelation function between the pump and probe pulses (FWHM of ∼ 78 ps) shown in Fig. S2 (plotted on theright axis). The curves in the range of |td | > 55 ps are fitted with an exponential function.

the detection of carrier dynamics in the tens-picosecond range significantly faster than the previous externallytriggerable OPP-STM s­ ystems23,32,33.

Discussion

We discuss the future possibilities of the OPP-STM technique presented in this study. First, the temporal resolution in the tens-picosecond range will greatly expand the range of applications. For example, the picosecond
range exciton dynamics in transition metal ­dichalcogenides34 can be investigated using this system. Second, theoptical system has significantly improved the stability of the laser illumination of the tunnel junction. The longterm stability of the laser illumination allows us to perform OPP tunneling current mapping, thereby visualizing
carrier dynamics in the form of, for example, decay time maps as shown in Figs. 7 and 8. This mapping technique
can be applied to reveal carrier dynamics associated with nanoscale structures such as domain boundaries intransition metal dichalcogenides and organic solar cell m­ aterials42. Photo c­ atalysis23 and photo induced phase­transition43 are also interesting phenomena to be investigated. Since a wide delay time range is available from ps
to µs by adjusting the timing of electric pulses, the current system enables the capture of various photo-induced
phenomena in these systems.
Because in this work, the laser wavelength is fixed at 532 nm (2.33 eV), the selection of a laser system according to the band energy gap of the sample of interest is still necessary. The externally-triggerable picosecond laser
systems with a wide range of wavelengths (532–1550 nm) are currently ­available44. Cutting-edge laser technology,
which has been rapidly developing, may realize a higher-performance optical system enabling a wavelengthvariable, externally-triggerable laser system with a shorter pulse width in the future.
The spatial resolution can be further improved down to the atomic level, as already demonstrated by several­groups20,21,23. We note that a time-resolved STM system (temporal resolution of faster than 30 fs) consisting ofalmost the same configuration as Fig. 3a except for the lens has recently realized atomic-scale imaging underlaser ­illumination30, indicating the potential capability of the atomic resolution with optical excitation in the
OPP-STM system developed in this study.
By using circular polarized illumination, ultrafast spin dynamics can be probed with a spatial resolution of∼ 1 nm, as demonstrated in GaAs/AlGaAs quantum ­wells45. Although we still need to either tune the laser excitation energy to a spin splitting energy, or find a suitable material possessing the spin splitting energy matchingwith the laser excitation energy, it is possible to conduct such spin dynamics measurements based on the circularpolarization modulation ­technique45 by both adding a Pockels cell and a quarter-wave plate after each pumpand probe laser, and by modifying the electronic circuit in the delay-time control system in Fig. 1. For example,
previous spin dynamics measurement of Mn on GaAs(110) surface has reported a change in the spin lifetime
with increasing Mn density, more surface-sensitive than the conventional OPP method, but spatially averaged
the spin-related tunneling current over the ­surface46. The mapping technique presented in this study may resolve
the spatial distribution of the spin lifetime of Mn atoms.
Furthermore, applying the OPP-STM technique to multiprobe systems offers us great opportunities to study
carrier dynamics of various nanoscale structures. For example, small island structures on an insulating substrate,
Scientific Reports |(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

9Vol.:(0123456789)

www.nature.com/scientificreports/
which are inaccessible to a single probe STM, can be observed by using one tip as an electrode, while conducting STM measurements using the other tip. Such OPP-multiprobe techniques have already been developed andapplied for investigating a monolayer island of a WSe 2/MoSe 2 ­heterostructure47, monolayer and bilayer WSe 2
islands grown on SiO2 ­substrates48, and exciton dynamics in an in-plane WS 2/WSe2 ­heterostructure34. The placement of a position-movable aspheric lens directly on the multiprobe stage will similarly improve the stability oflaser spot on the sample surface, allowing us to conduct time-resolved mapping experiments.
The optical system developed in this study is also applicable for time-resolved atomic force m­ icroscopy49, in
which the thermal expansion effect needs to be similarly suppressed. Since the long-term stability of the laser
spot on the sample surface is commonly indispensable for other time-resolved techniques, the optical system
reported in this study can be applied for various time-resolved measurements to improve data quality.

Conclusion

This work reports the development of an externally-triggerable OPP-STM system capable of conducting tenspicosecond range, long-term time-resolved measurements. We achieve this both by controlling the laser pulse
sequence electrically, and by placing the aspheric lens on the same stage with the STM head together with theuse of the beam stabilization system. The optical system enables us to conduct OPP-STM measurements witha temporal resolution of ∼ 80 ps. We successfully demonstrate the decay time of ∼ 170 ps using LT-GaAs. The
temporal resolution will be further improved by using laser systems with a shorter pulse width (currently 45 ps)
and by deliberately selecting the surrounding electronics in the future. A wide range of the laser wavelength isavailable by using a suitable externally-triggerable laser system. Our OPP-STM mapping data reveals nanoscale
carrier dynamics on GaAs(110) surface, offering the potential capabilities of this technique to a deeper understanding of carrier dynamics in various advanced functional materials.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.
Received: 30 September 2022; Accepted: 2 January 2023

References

1. Shah, J. Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures (Springer-Verlag, Berlin, 1999).
2. Zewail, A. H. Four-dimensional electron microscopy. Science 328, 187–193. https://​doi.​org/​10.​1126/​scien​ce.​11661​35 (2010).
3. Feist, A. et al. Quantum coherent optical phase modulation in an ultrafast transmission electron microscope. Nature 521, 200–203.
https://​doi.​org/​10.​1038/​natur​e14463 (2015).
4. Ideta, S. et al. Ultrafast dissolution and creation of bonds in IrTe2 induced by photodoping. Sci. Adv. 4, eaar3867. https://​doi.​org/​
10.​1126/​sciadv.​aar38​67 (2018).
5. Fukumoto, K., Yamada, Y., Onda, K. & Koshihara, S. Direct imaging of electron recombination and transport on a semiconductor
surface by femtosecond time-resolved photoemission electron microscopy. Appl. Phys. Lett. 104, 053117. https://d​ oi.o​ rg/1​ 0.1​ 063/1.​
48642​79 (2014).
6. Man, M. K. L. et al. Imaging the motion of electrons across semiconductor heterojunctions. Nat. Nanotechnol. 12, 36–40. https://​doi.​org/​10.​1038/​nnano.​2016.​183 (2017).
7. Eichberger, M. et al. Snapshots of cooperative atomic motions in the optical suppression of charge density waves. Nature 468,
799–802. https://​doi.​org/​10.​1038/​natur​e09539 (2010).
8. Mamin, H. J., Birk, H., Wimmer, P. & Rugar, D. High-speed scanning tunneling microscopy: Principles and applications. J. Appl.
Phys. 75, 161. https://​doi.​org/​10.​1063/1.​355877 (1994).
9. Weiss, S., Botkin, D., Ogletree, D. F., Salmeron, M. & Chemla, D. S. The ultrafast response of a scanning tunneling microscope.
Phys. Stat. Sol. (b) 188, 343. https://​doi.​org/​10.​1002/​pssb.​22218​80132 (1995).
10. Wintterlin, J. et al. Real-time STM observations of atomic equilibrium fluctuations in an adsorbate system: O/Ru(0001). Surf. Sci.
394, 159–169. https://​doi.​org/​10.​1016/​S0039-​6028(97)​00604-3 (1997).
11. Kemiktarak, U., Ndukum, T., Schwab, K. C. & Ekinci, K. L. Radio-frequency scanning tunnelling microscopy. Nature 450, 85–88.
https://​doi.​org/​10.​1038/​natur​e06238 (2007).
12. Hamers, R. J. & Cahill, D. G. Ultrafast time resolution in scanned probe microscopies. Appl. Phys. Lett. 57, 2031. https://​doi.​org/​
10.​1063/1.​103997 (1990).
13. Nunes, G. & Freeman, M. R. Picosecond resolution in scanning tunneling microscopy. Science 262, 1029–1032. https://​doi.​org/​
10.​1126/​scien​ce.​262.​5136.​1029 (1993).
14. Weiss, S., Ogletree, D. F., Botkin, D., Salmeron, M. & Chemla, D. S. Ultrafast scanning probe microscopy. Appl. Phys. Lett. 63, 2567.
https://​doi.​org/​10.​1063/1.​110435 (1993).
15. Keil, U. D., Ha, T., Jensen, J. R. & Hvam, J. M. Femtosecond tunneling response of surface plasmon polaritons. Appl. Phys. Lett.
72, 3074. https://​doi.​org/​10.​1063/1.​121545 (1998).
16. Grafström, S. Photoassisted scanning tunneling microscopy. J. Appl. Phys. 91, 1717. https://​doi.​org/​10.​1063/1.​14321​13 (2002).
17. Takeuchi, O. et al. Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy. Appl. Phys. Lett.
85, 3268. https://​doi.​org/​10.​1063/1.​18042​38 (2004).
18. Terada, Y., Yoshida, S., Takeuchi, O. & Shigekawa, H. Real-space imaging of transient carrier dynamics by nanoscale pump-probe
microscopy. Nat. Photon. 4, 869–874. https://​doi.​org/​10.​1038/​nphot​on.​2010.​235 (2010).
19. Shigekawa, H. & Takeuchi, O. Pump probe measuring device, and scanning probe microscope apparatus using the device. https://​www.j-​platp​at.​inpit.​go.​jp/​c1800/​PU/​JP-​48394​81/​F7AF3​1FE1E​ACB0D​E995A​E4969​E018A​800AF​2D015​57FD0​30948​268E6​399EC​
FAFD/​15/​ja (2011). (Japan/4839481), Japan Patent Office.
20. Yoshida, S. et al. Single-atomic-level probe of transient carrier dynamics by laser-combined scanning tunneling microscopy. Appl.
Phys. Express 6, 032401. https://​doi.​org/​10.​7567/​APEX.6.​032401 (2013).
21. Kloth, P. & Wenderoth, M. From time-resolved atomic-scale imaging of individual donors to their cooperative dynamics. Sci. Adv.
3, e1601552. https://​doi.​org/​10.​1126/​sciadv.​16015​52 (2017).
22. Yoshida, S., Terada, Y., Oshima, R., Takeuchi, O. & Shigekawa, H. Nanoscale probing of transient carrier dynamics modulated ina GaAs-PIN junction by laser-combined scanning tunneling microscopy. Nanoscale 4, 757–761. https://​doi.​org/​10.​1039/​C2NR1​
1551D (2012).

Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

10www.nature.com/scientificreports/
23. Guo, C. et al. Probing nonequilibrium dynamics of photoexcited polarons on a metal-oxide surface with atomic precision. Phys.
Rev. Lett. 124, 206801. https://​doi.​org/​10.​1103/​PhysR​evLett.​124.​206801 (2020).
24. Cocker, T. L. et al. An ultrafast terahertz scanning tunnelling microscope. Nat. Photon. 7, 620–625. https://​doi.​org/​10.​1038/​nphot​
on.​2013.​151 (2013).
25. Cocker, T. L., Peller, D., Yu, P., Repp, J. & Huber, R. Tracking the ultrafast motion of a single molecule by femtosecond orbital
imaging. Nature 539, 263–267. https://​doi.​org/​10.​1038/​natur​e19816 (2016).
26. Li, S., Chen, S., Li, J., Wu, R. & Ho, W. Joint space-time coherent vibration driven conformational transitions in a single molecule.
Phys. Rev. Lett. 119, 176002. https://​doi.​org/​10.​1103/​PhysR​evLett.​119.​176002 (2017).
27. Garg, M. & Kern, K. Attosecond coherent manipulation of electrons in tunneling microscopy. Science 367, 411–415. https://​doi.​
org/​10.​1126/​scien​ce.​aaz10​98 (2019).
28. Yoshida, S. et al. Terahertz scanning tunneling microscopy for visualizing ultrafast electron motion in nanoscale potential variations. ACS Photonics 8, 315–323. https://​doi.​org/​10.​1021/​acsph​otoni​cs.​0c015​72 (2021).
29. Wang, L., Xia, Y. & Ho, W. Atomic-scale quantum sensing based on the ultrafast coherence of an H2 molecule in an STM cavity.
Science 376, 401–405. https://​doi.​org/​10.​1126/​scien​ce.​abn92​20 (2022).
30. Arashida, Y. et al. Subcycle mid-infrared electric-field-driven scanning tunneling microscopy with a time resolution higher than30 fs. ACS Photonics 9, 3156–3164. https://​doi.​org/​10.​1021/​acsph​otoni​cs.​2c009​95 (2022).
31. Arashida, Y. et al. Phase-controllable multi-laser system with coaxially combined near-infrared and subcycle mid-infrared andTHz pulsed beams. Appl. Phys. Express 15, 092006. https://​doi.​org/​10.​35848/​1882-​0786/​ac88b6 (2022).
32. Mogi, H. et al. Externally triggerable optical pump-probe scanning tunneling microscopy. Appl. Phys. Express 12, 025005. https://​doi.​org/​10.​7567/​1882-​0786/​aaf8b2 (2019).
33. Kloth, P., Thias, O., Bunjes, T., von der Haar, J. & Wenderoth, M. A versatile implementation of pulsed optical excitation in scanning tunneling microscopy. Rev. Sci. Instrum. 87, 123702. https://​doi.​org/​10.​1063/1.​49711​89 (2016).
34. Mogi, H. et al. Ultrafast nanoscale exciton dynamics via laser-combined scanning tunneling microscopy in atomically thin materials. NPJ 2D Mater. Appl. 6, 72 (2022).
35. Shigekawa, H., Takeuchi, O. & Wang, Z. Optical output system, measurement system, optical pump-probe scanning tunnelingmicroscope system, computing device, program, and computing method. https://​www.j-​platp​at.​inpit.​go.​jp/​c1800/​PU/​JP-​69643​
77/​A8DAD​1F1A6​DF330​F18DD​8FE26​CA442​967F1​9D3A1​08F09​241E5​1F268​752DA​D784/​15/​ja (2021). (Japan/6964377), Japan
Patent Office.
36. Takeuchi, O., Yoshida, S. & Shigekawa, H. Light-modulated scanning tunneling spectroscopy for nanoscale imaging of surface
photovoltage. Appl. Phys. Lett. 84, 3645. https://​doi.​org/​10.​1063/1.​17370​63 (2004).
37. Yoshida, S., Kanitani, Y., Takeuchi, O. & Shigekawa, H. Probing nanoscale potential modulation by defect-induced gap states on
GaAs(110) with light-modulated scanning tunneling spectroscopy. Appl. Phys. Lett. 92, 102105. https://​doi.​org/​10.​1063/1.​28887​
33 (2008).
38. Feenstra, R. M., Stroscio, J. A., Tersoff, J. & Fein, A. P. Atom-selective imaging of the GaAs(110) surface. Phys. Rev. Lett. 58, 1192.
https://​doi.​org/​10.​1103/​PhysR​evLett.​58.​1192 (1987).
39. de Raad, G. J., Bruls, D. M., Koenraad, P. M. & Wolter, J. H. Interplay between tip-induced band bending and voltage-dependent
surface corrugation on GaAs(110) surfaces. Phys. Rev. B 66, 195306. https://​doi.​org/​10.​1103/​PhysR​evB.​66.​195306 (2002).
40. Terada, Y., Yoshida, S., Takeuchi, O. & Shigekawa, H. Laser-combined scanning tunnelling microscopy for probing ultrafast transient dynamics. J. Phys.: Condens. Matter 22, 264008. https://​doi.​org/​10.​1088/​0953-​8984/​22/​26/​264008 (2010).
41. Yokota, M. et al. Bases for time-resolved probing of transient carrier dynamics by optical pump-probe scanning tunneling microscopy. Nanoscale 5, 9170–9175. https://​doi.​org/​10.​1039/​C3NR0​2433D (2013).
42. Takeuchi, O. et al. Microscopic description of the current-voltage characteristics of a bulk-heterojunction organic solar cell underillumination. Appl. Phys. Express 7, 021602. https://​doi.​org/​10.​7567/​APEX.7.​021602 (2014).
43. Terada, Y. et al. Optical doping: Active control of metal-insulator transition in nanowire. Nano Lett. 8, 3577–3581. https://​doi.​org/​
10.​1021/​nl801​350p (2008).
44. For example, KATANA series, NKT Photonics and SID series, Irisiome Solutions.
45. Yoshida, S. et al. Probing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopy. Nat. Nanotechnol. 9,
588–593. https://​doi.​org/​10.​1038/​nnano.​2014.​125 (2014).
46. Wang, Z. et al. Surface-mediated spin dynamics probed by optical-pump-probe scanning tunneling microscopy. Phys. Chem.
Chem. Phys. 21, 7256–7260. https://​doi.​org/​10.​1039/​C8CP0​7786J (2019).
47. Mogi, H. et al. Development of laser-combined scanning multiprobe spectroscopy and application to analysis of WSe2/MoSe2
in-plane heterostructure. Appl. Phys. Express 12, 045002. https://​doi.​org/​10.​7567/​1882-​0786/​ab09b9 (2019).
48. Mogi, H. et al. Laser-combined multiprobe microscopy and its application to the materials with atomic layer thickness. Jpn. J. Appl.
Phys. 61, SL1011. https://​doi.​org/​10.​35848/​1347-​4065/​ac6a3c (2022).
49. Schumacher, Z., Spielhofer, A., Miyahara, Y. & Grutter, P. The limit of time resolution in frequency modulation atomic force
microscopy by a pump-probe approach. Appl. Phys. Lett. 110, 053111. https://​doi.​org/​10.​1063/1.​49756​29 (2017).

Acknowledgements

The authors are grateful to Masaharu Sakai and Kazuhiko Kurita (UNISOKU) for their technical assistance.
This work was supported by Adaptable and Seamless Technology Transfer Program through Target-driven R&D(A-STEP), Japan Science and Technology Agency (JST), and a Grant-in-Aid for Scientific Research (17H06088,
20H00341, 22H00289) from Japan Society for the Promotion of Science.

Author contributions

K.I. and M.Y. carried out the experiments and the data analyses. H.H., H.M., S.Y., and O.T. designed the delaytime control system. Y.M. and H.S. supervised the project. K.I. wrote the manuscript with input from all authors.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary Information The online version contains supplementary material available at https://​doi.​org/​
10.​1038/​s41598-​023-​27383-z.
Correspondence and requests for materials should be addressed to K.I. or H.S.
Reprints and permissions information is available at www.nature.com/reprints.

Scientific Reports |(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

11Vol.:(0123456789)

www.nature.com/scientificreports/
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps andinstitutional affiliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreative Commons licence, and indicate if changes were made. The images or other third party material in thisarticle are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to thematerial. If material is not included in the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly fromthe copyright holder. To view a copy of this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.
© The Author(s) 2023

Scientific Reports |Vol:.(1234567890)(2023) 13:818 |https://doi.org/10.1038/s41598-023-27383-z

12