pubs.acs.org/journal/apchd5

ArticleSubcycle Mid-Infrared Electric-Field-Driven Scanning TunnelingMicroscopy with a Time Resolution Higher Than 30 fs
Yusuke Arashida, Hiroyuki Mogi, Masashi Ishikawa, Ippo Igarashi, Akira Hatanaka, Naoki Umeda,
Jinbo Peng, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa*

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sı Supporting Information*

ABSTRACT: Electric-field-driven scanning tunneling microscopy (STM) thatuses 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, topave the way for studying the local dynamics of non-equilibrium states such as
elementary excitations and phase transitions in condensed materials, it isdesirable to develop an STM system with a higher time resolution. Here, wereport 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 byvisualizing the photo-induced ultrafast non-equilibrium dynamics in MoTe2. We succeeded in measuring ultrafast carrier dynamics inthe 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.
KEYWORDS: STM, subcycle electric field, mid-infrared, electric-field driven, pump-probe technique, time-resolved STM■

INTRODUCTION
Understanding and controlling the ultrafast dynamics in nonequilibrium states of condensed materials are important for
both basic and applied science and technologies.1−4 Experiments using angle-resolved photoemission spectroscopy anddiffraction methods with time resolutions in the femtosecond
region have been actively conducted, and excellent results havebeen obtained.5−10 However, these results and discussions arebasically for the reciprocal lattice space. Since these dynamics
are not spatially uniform but are strongly affected by local
structures such as atomic-level defects, for further advancements, it is highly desirable to analyze these dynamics in real
space while considering the surrounding effects of local
structures.
Scanning tunneling microscopy (STM) has an atomic-scale
resolution and is capable of evaluating local electronic
structures in comparison with environmental structures.11
However, since the time resolution of STM is in the submillisecond range, it is difficult to probe the dynamics of theultrafast phenomena described above, and many attempts havebeen made to add a high time resolution since its
invention.12,13 The two methods considered as means ofincreasing time resolution dated from the earliest STM studies
are to instantaneously give conductivity to the STM circuit
using an ultrafast optical switch and to control the bias voltage
applied between the STM tip and the sample using high-speed
electronics.14−17 The latter has been successfully used tomeasure spin dynamics in single-atomic-level research,18−20© 2022 The Authors. Published byAmerican Chemical Society

but the temporal resolution obtained using electronics isbasically limited to the sub-nanosecond region.
Under these circumstances, an STM system with a time
resolution in the femtosecond region was developed by
combining STM with the optical pump−probe method usedin ultrashort-pulse laser technologies.21−23 Similar to theoptical pump−probe method, the STM system uses theabsorption bleaching mechanism,23−25 thereby its time
resolution determined by the laser pulse width is in the
femtosecond region. Local spin dynamics have also beenmeasured with this method.26,27 Another method is electricfield-driven STM, which uses a carrier envelope phase (CEP)controlled light source to produce a near-field to apply an
instantaneous electric field between the STM tip and thesample.28 This method substitutes the high-speed electronics
with an electric field to enable a faster control of the bias
voltage between the STM tip and the sample. When a THz
electric field is used, this method is called THz-STM.28−38
In electric-field-driven STM, the change in tunnel current
corresponding to the change in bias voltage is measured.
Therefore, although care is required in analysis because the
Received: June 28, 2022
Published: September 2, 2022

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ArticleFigure 1. (a) Schematic diagram of the time-resolved MIR-STM system. BS: beam splitter, DW: diamond window, MIR: mid-infrared electric field
EMIR, and NIR: near-infrared electric field ENIR. (b) Temporal profile of the output of the NIR pulse obtained by D-scan and its spectrum. (c) EMIR
waveform measured by using a photoconductive antenna (PCA) outside the STM chamber. (d) Schematic diagram of the measurement setup. The
tunnel gap is irradiated by the combination of EMIR and ENIR with delay time td, and the corresponding change in tunnel current I(td) is measured toobtain a time-resolved signal.

electric-field waveform is not a square, it is possible to measure
the transient local density of states (LDOSs) of materials
similar to that in normal STM,28,32,33 making this method
attractive. However, since its time resolution is determined by
the bandwidth of the electric field used, the time resolution of
THz-STM is limited to the 1 ps range, which is insufficient toanalyze the non-equilibrium electronic dynamics.8
An attempt was made to develop a light source over a wider
THz region to improve the time resolution; however, theobtained pulse width of the near-field was several hundred
femtosecond.39 The transition process in the attosecond region
has recently been observed by using CEP-controlled pulses in
the near-infrared (NIR) region.40 However, since themechanism of this system is based on the interference between
two pulses of 6 fs width, unlike other laser-combined STM
systems, its range of application is limited. Recently, scanningnear-field optical microscopy (SNOM) techniques with a time
resolution of 100 fs level have been reported.41−43 Since
SNOM measures permittivity of materials, we can obtain
optically important local information. However, it is difficult to
directly measure transient LDOSs. Therefore, the developmentof an electric-field-driven STM with a higher time resolution isstrongly desired.
Here, we demonstrate electric-field-driven STM realizedusing CEP-controlled MIR pulses (MIR-STM) with a time
resolution higher than 30 fs. Using the pump−probe methodwith a coaxial arrangement of the MIR beam and an NIR beamof 8 fs, we have succeeded in observing the photo-induced
ultrafast nonequilibrium dynamics in MoTe2 in the time rangeof 0 to over 1 ps with atomically resolved STM images.

MHz (Figure 1b). A pair of chirp mirrors or a BK7 plates wasused for fine adjustment to compensate for the dispersion of
the fundamental wave (not shown in the figure). A wideband
MIR pulse EMIR(t) was generated by the optical rectification
effect by splitting the fundamental wave with a beam splitter(BS) and leading one of the beams to a GaSe crystal with athickness of 30 μm.45 After that, the basic light that was notwavelength-converted was removed from the MIR beam bypassing the output from the GaSe crystal through a Ge plate.
The other NIR beam was passed through an optical delay
stage (delay) and then reflected on a Ge plate to combine itwith the MIR beam in a coaxial arrangement for pump−probe
measurement, making it easy to adjust the optical system.
Figure 1c shows the waveform of EMIR(t) obtained by using aphotoconductive antenna (PCA) outside the STM chamber.
Unipolar subcycle pulses with asymmetry between the positive
and negative regions were produced, the half period of which
was about 25 fs. The coaxial light beams were introduced into
an ultrahigh-vacuum STM chamber (5 × 10−8 Pa) through apolycrystalline diamond window (DW in Figure 1a, φ = 12.7mm) and focused onto the STM tip apex using an off-axis
parabolic mirror (f = 15 mm) at an incident angle of 55°. Boththe MIR and NIR beams were p-polarized with respect to thesample.
A mechanically polished Pt/Ir tip and a MoTe2 sample wereused for the STM measurement. It was confirmed that thethermal expansion problem could be negligible even when thepump−probe measurement was carried out with the intensity
modulation of MIR light (see Figure S1 for details). Figure 1dshows a schematic diagram of the MIR-STM measurement.
The tunnel gap was irradiated by the combination of EMIR(t)and ENIR(t) with a delay time td for the pump−probe method,
and the change in tunnel current I(td) was measured to obtaina time-resolved signal, similar to that in THz-STM.

■

EXPERIMENTAL SECTION
Time-Resolved MIR-STM System. Figure 1a shows aschematic diagram of our newly developed MIR-STM system.
A Ti:S-based optical parametric chirped-pulse amplifier(Venteon OPCPA, Laser Quantum) was used as the primarylight source of the NIR electric field ENIR(t).44 The wavelength
range of ENIR(t) was 660−940 nm, the pulse width was 8.1 fs,
the pulse energy was 1 μJ, and the repetition frequency was 4■

RESULTS AND DISCUSSION
Evaluation of the MIR Near-Field. When the tunnel gap
is irradiated by EMIR(t), an MIR near-field ENF,MIR(t) is
produced at the tip apex via the mechanism of tip-induced3157

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ArticleFigure 2. Evaluation of the MIR near field. (a) Energy diagram of the tunnel gap. Here, a bias voltage VDC is applied between the STM tip and thesample. d is the tip-sample distance. CBM and VBM represent the conduction band minimum and valence band maximum, respectively. PEE andFEE denote photoemission electrons due to NIR light irradiation and field-effect-emitted electrons, respectively. ENF,MIR(t) is the MIR near-field
produced on the STM tip via the mechanism of tip-induced enhancement of EMIR(t). (b) VDC dependence of I = PEE + FED with NIR lightirradiation and the MIR light turned off. (c) VDC dependence of ENF,MIR(t)-induced current INF,MIR(t). The MIR beam was turned on and off at
frequency of about 520 Hz with an optical chopper, and INF,MIR(t) was measured by the lock-in detection method. (d) Relationship between thenear-field voltage VNF,MIR(t) produced by ENF,MIR(t) and INF,MIR(t), shown in (c) (see Figure S3 for details). (e) EFF,MIR(t), far-field EMIR(t)measured by the PCA at the tip position (yellow, see Figure S2 for details) and VNF,MIR(t) (green). VNF,MIR(t) was calculated using the shape of
INF,MIR(t) at 6 V shown in Figure 3c. (f) Far-field (yellow) and near-field (green) Fourier spectra obtained from the data in (e). The spectrum of
ENF,MIR(t) calculated by the finite-difference time-domain (FDTD) method is also shown (blue, see Figure S4 for details).

enhancement.42,46 The transient bias voltage VNF,MIR(t) formed
by ENF,MIR(t) dynamically opens the STM circuit to detect the
time-resolved signal. Therefore, it is important to evaluate
VNF,MIR(t) under the measurement conditions. First, we
confirmed using a photoconductive antenna (PCA) that
EMIR(t) measured outside the STM chamber (Figure 1c) was
well reproduced at the tunnel gap (see Figure S2 for details).
Next, we evaluated ENF,MIR(t) produced by EMIR(t). Figure
2a shows the energy diagram of the tunnel gap. Here, a sample
bias voltage VDC is applied between the STM tip and thesample (with the tip side grounded). Owing to the differencein the band structure between the tip and the sample, thevacuum level on the sample side is as low as ∼0.5 to ∼1 eV,
even when VDC is zero.47,48 During the irradiation of the tunnel
gap with the NIR light, with a tip-sample distance d of 200 nm
to suppress the tunnel current, the current I = PEE + FEE wasmeasured, where PEE and FEE represent the photoemission
electrons and field-effect-emitted electrons, respectively. Figure
2b shows the I−VDC curve obtained by changing VDC from−10 to 10 V. No current was generated when the NIR light
was turned off (blue line), whereas I(VDC) appeared byirradiating the tunnel gap with the NIR light (red line).
Since field emission is unlikely to occur on the flat sample
surface, I(VDC) observed for VDC < 0 is due to PEE. At −4 V <VDC < 0 V, photoemission electrons from the tip PEEtip are themain component. Namely, the rapid change in I(VDC) shows
the change in PEEtip(VDC), which is excited by the lightcomponent on the high-energy side of EMIR(t), as shown in
Figure 1b. The negative current at VDC < −4 V is considered tobe the photoelectrons from the sample surface, PEEsample. Thechange in I(VDC) observed for 0 V < VDC is the change in
FEEtip(VDC) because PEEtip does not depend on VDC.
We used this mechanism to evaluate ENF,MIR(t). When thetunnel gap is irradiated with MIR pulses, the barrier height at
the tip apex changes, owing to ENF,MIR(t). Since the number of
FEEtip depends on the barrier height and the relaxation of hotelectrons is faster than the change in ENF,MIR(t), the ENF,MIR(t)waveform can be evaluated by measuring the change in
INF,MIR(td), as similarly carried out in THz-STM.32,34 Figure 2cshows the VDC dependence of INF,MIR(td), which appears towell reproduce the shape of EMIR(t) shown in Figure 1c, as
expected. The appearance of the waveform for −4 V < VDC < 0
V indicates that PEEtip is affected by ENF,MIR(t). The
momentum of these photoelectrons are strongly modified
near the tip apex within several 10 fs49 because the velocity ofthem is in the order of 1 nm/fs. In our experiment, VS = VDC +VNF,MIR(t) is applied to sample with the I−V curve, as shown in
Figure 2b, and the change in INF,MIR(td) is measured, as shownin Figure 2d. We do not know VNF,MIR (t) at this point because
ENF,MIR(td) may be different from EMIR(td). Therefore, toobtain an accurate ENF,MIR(td) waveform, it is necessary to
calculate it from the INF,MIR(td) shape shown in Figure 2c inconsideration of the characteristics of the I−V curve (seeFigure S3 for details). Figure 2e shows EMIR(t) measured by
the PCA at the tip position (yellow, see Figure S2 for detail)and VNF,MIR(t) (green) obtained from the calculation describedabove (see Figure S3 for detail). Figure 2f shows far-field(yellow) and near-field (green) Fourier spectra obtained from3158

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ArticleFigure 3. (a) Large-scale STM image of 2H−MoTe2. Bright spots are oxidized defects. (b) STM image of 2H−MoTe2 in an area without defects.
(c) Schematic structures of 2H−MoTe2 phases. (d)Typical I−V curves measured in areas with (blue) and without (red) defects. STM image taken
after NIR light irradiation (30.6 kW/cm2, 30 ms). TGB: twin grain boundary. Inset is the magnification of the 1T′ area. STM images were takenwith the conditions of VDC = 0.6 V and I = 50 pA.

the data in Figure 2e. The spectrum of ENF,MIR(t) calculated by
the finite-difference time-domain (FDTD) method is also
shown (blue, see Figure S4 for details). The difference
observed between EMIR(t) and VNF,MIR (t) indicates theimportance of confirming the shape of ENF,MIR(t) before
measurement, as we have pointed out in a previous paper.32
Time-Resolved MIR-STM Measurement. Sample Preparation. 2H−MoTe2, a transition-metal dichalcogenide(TMDC),50 was used as a sample. In a previous study ofMoTe2 using a THz-STM system, in which the time resolution
was 1 ps, carrier recombination dynamics of 18 and 606 ps
were observed, but the processes faster than 1 ps were beyond
its measurement limit.32 However, important non-equilibrium
dynamics, such as the modulation of the band structureassociated with ultrafast electron dynamics, appear at <∼1
ps.51 Thus, MoTe 2 is considered to be suitable fordemonstrating the performance of the newly developed MIRSTM system. However, MoTe2 exhibits a large electron latticeinteraction among the TMDCs and forms phases such as
semiconducting, metallic, charge density wave (CDW), andsuperconducting phases depending on its crystal structure andconditions.52,53 Therefore, to probe the ultrafast electronic
dynamics without such structural changes which make analysis
complicated, the NIR light intensity was sufficiently reduced to
0.56 mW measured externally, 1 order lower than that used in
ref 24. To confirm the suitability of the conditions, we carriedout time-resolved MIR-STM imaging as will be explained indetail later.
A clean surface was obtained by exfoliating bulk 2H−MoTe2
fixed with a conductive adhesive on a copper substrate in an
ultrahigh-vacuum (2 × 10−8 Pa) preparation chamber. Then, it
was moved to the STM measurement chamber (5 × 10−8 Pa)
and cooled to 78 K, and experiments were performed. 2H−
MoTe2 has a hexagonal symmetric crystal structure, and thebulk crystal is a semiconductor with a band gap of ∼1.0 eV at
77 K.54 Figure 3a shows a typical STM image of 2H−MoTe2.
Bright spots are oxidized defects.32 Figure 3b shows an STM
image of 2H−MoTe2 with atomic resolution obtained in thearea without defects. A schematic structure of 2H−MoTe2 isshown in Figure 3c. Figure 3d shows typical dI/dV curves
obtained in the areas with (blue) and without (red) defects.
Defect-related gap states are observed for the former, and timeresolved MIR-STM measurements were carried out in the area
without defects to avoid their effects.
Measurement and Analysis. After choosing an area without
defects by STM observation, time-resolved MIR-STM
measurements were performed at 78 K, as shown in Figure
1d. For the time-resolved STM measurement, a pump−probe
method was performed using NIR pulsed light as an excitationand MIR light as a probe. The MIR light was turned on and off
at a frequency of 520 Hz, and the modulated tunnel current
It,MIR(td) was detected as the time-resolved MIR-STM signalby the lock-in detection method, while STM feedback was3159

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ArticleFigure 4. (a) Time-resolved MIR-STM signal obtained in the area without defects (VDC = 150 mV, I = 50 pA, NIR pulse: 0.56 mW outside). MIR
light was turned on and off at a frequency of 520 Hz for the lock-in detection. The inset is a magnification of the signal around td = 0. The blue line
indicates the fitting curve obtained using an exponential function with two components. (b) High resolution STM image of 2H−MoTe2 shown in
Figure 3b and time-resolved MIR-STM images obtained for delay times td of −920, 210, 330, and 1080 fs (4 × 4 nm2, VDC = 31 mV, I = 40 pA,
scan speed: 1.5 s/nm). (c) Result of light-modulated scanning tunneling spectroscopy (LM-STS).55,56 The I−V curve was measured while turning
the NIR beam on and off at a frequency of 40 Hz (VDC = 800 mV, I = 50 pA, 0.56 mW outside, 512 points/0.82 s). By drawing the envelopes of thesignal, two I−V curves for with (red) and without (blue) light irradiation were obtained simultaneously. (d) Schematic band structures explaining
the time-resolved signal shown in (a).

carried out using the total tunnel current. Since VDC is small
and ENF,MIR(t) is not completely monopolar, the direction of
the MIR-STM tunnel current signal at t = td, It,MIR(td) changes
from the STM tip to the sample [It,MIR(td) > 0] and from thesample to the STM tip [It,MIR(td) < 0)], depending on the
direction of the bias voltage VS(t) = VDC + VNF,MIR(t). Since
VNF,MIR is not a rectangular wave, It,MIR(td) is an integral over
the waveform. The relationship between the electric field
ENF,MIR and the STM gap is shown in ① in Figure 4d.
Figure 4a shows a typical result. In the region of td > 0, a
rapid increase in signal intensity (td < 0.5 ps) and its relaxation
were observed. This is the first ever time-resolved STM signal
obtained in the time range of 0 to 1 ps. The observed changein I(td) in the first 1 ps is produced by the tunneling ofelectrons near the Fermi level between the STM tip and thesample, namely, the ultrafast dynamics occurring in the MoTe2
sample were successfully probed. The blue line in Figure 4ashows the result of fitting with the double exponential function
g1exp( ) + g exp( ) where g and g are constants
t

1

t

2

2

1

2

showing the intensity of each function. A rise at τ1 = 220 fs
followed by a decay at τ2 = 720 fs was obtained.
As described above, to avoid the structural changes due tophotoexcitation, the NIR light intensity was sufficiently
reduced. To confirm the suitability of the conditions, we
carried out time-resolved MIR-STM imaging. Figure 4b shows
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a high-resolution STM image and MIR-STM images obtained
at td = −920, 210, 330, and 1080 fs. The STM tip was scanned
from left to right and from bottom to top. Since measurements
were carried out under light irradiation, there were somefluctuations, and the resolution slightly changed during
scanning. As shown in the figures, however, atomically timeresolved STM images during the ultrafast photo-induced
dynamics in MoTe2 were successfully obtained for the firsttime. A small hexagonal structure, including the second layer,
can be observed in places where the resolution is high for both
STM and MIR-STM images.
Although the appearance of 2H−MoTe2 may differ
depending on the resolution, its structure was basicallymaintained, as can be seen from the unit cell. Namely, it was
confirmed that the electronic state dynamics were measured
while maintaining the stability of the sample structure. This is
the first time-resolved STM observation of ultrafast dynamics
in the <1 ps region.
We next discuss in detail how to interpret the observed
ultrafast phenomena. First, we carried out light-modulated
scanning tunneling spectroscopy (LM-STS) measurements55,56
to examine the band structure. Figure 4c shows the result of
the LM-STS measurement obtained in the area without
defects. The I−V curve was measured while turning the NIR
beam on and off at a frequency of 40 Hz. By drawing theenvelopes of the signal, two I−V curves in cases with (red) andwithout (blue) light irradiation were obtained simultaneously.
Almost no shift was observed between the I−V curves obtainedwith and without NIR irradiation, indicating that band bending
and photovoltage are not effective for this sample. No apparent
structural changes were observed in the MIR-STM images, as
shown in Figure 4b. It is considered that no phase transition
accompanying structural changes occurred, owing to thedecreasing photoexcitation intensity, as expected.
A possible mechanism to explain the MIR-STM signal
observed in Figure 4a is band gap renormalization, which is aphenomenon where the band gap changes with increasing
carrier density. There are some reports on experiments andtheoretical analyses of this phenomenon.57,58 In the case of a
few-layer MoS2 sample with a band gap of 1.8 eV, the changes
in the band structure of MoS2 due to resonant excitation (1.83
eV) and nonresonant excitation (3.10 eV) were analyzed.59 In
the nonresonant case, the band gap first shrank in the range ofseveral 100 fs and then relaxed on the order of ∼1 ps. The
shrinkage is due to the relaxation of the excited electrons to the
conduction band edge, resulting in the appearance of thescreening effect. Subsequent relaxation was attributed to the
reduction of the screening effect due to the formation of abound state (e.g., excitons). The shrinkage magnitude
increased with the photoexcitation light intensity, reaching
about −30 meV at 110 μJ/cm2. In our system, the excitationenergy is ∼1.5 eV. Considering that the band gap of MoTe2 is∼1 eV, our experimental conditions correspond to thenonresonant excitation case in the ref 57. In addition, since
the photoexcitation light intensity is approximately 50 μJ/m2
and there is a tip enhancement effect, the band shrinkage isconsidered equal to or larger than that in our experiment.
In a report on MoTe2,51 for example, the change intransmittance of the probe light after excitation at 800 nm andthe time evolution of the absorption spectrum after excitation
at 800 nm were analyzed. The former showed changes in therising part at 100−140 fs depending on the pump lightintensity. In the latter, the red-shifted gap after excitation

Articlerelaxed on the order of 1 ps. Another study on MoTe2 wascarried out on the basis of the measurement of absorption
spectra between the core level and the conduction/valenceband after pumping light of 800 nm on the sample.57 The
position of the red-shifted conduction band edge changed tothe high-energy side in the <1 ps region.
The timescales of the dynamics in these previous studies
correspond well to the results of our MIR-STM experiments.
In STM, the tunnel current appears positive or negative
depending on the relationship between the Fermi levels in the
STM tip and the sample. Therefore, the sign of the tunnel
current can be used as an index for evaluating the changes inthe sample band structure.
To consider the signal in Figure 4a in detail, it is divided into
areas ① to ⑥. Figure 4d shows model diagrams explaining thesignal in each region. First, in area ① of td < 0 at the left of the
figure, It,MIR(td) > 0. STM measurements were performed
under feedback conditions with sample bias VS = 31 mV.
Therefore, the integral value of the tunnel current that gives
the MIR-STM signal became positive, owing to the relationship between the shape of the I−V curve and the MIR
waveform.
Next, in the vicinity of td = 0 (area ② in Figure 4a), thewaveform of the near field shown in Figure 2e is reproduced bya signal of the tunnel current. This is due to the NIRphotoexcited hot electrons tunneling between the STM tip andthe sample, reflecting the change in the tunnel barrier in
response to the MIR waveform (ENF,MIR) when the NIR pulse
overlaps with the MIR pulse (② in Figure 4d), similar to thecase of THz-STM.32 This indicates that the relaxation of the
hot electrons excited by the NIR light occurs within a timeshorter than the NIR pulse width of 8 fs. Now tunnel current is
expected to be controlled ultra-fast and phase-sensitive.29
However, as shown by the arrow in the inset of Figure 4a,
the negative It,MIR(td) signal continued from around td = 60 fs,
deviating from the MIR waveform. This is because the hot
electrons relax and accumulate in the CBM, then tunnel to thetip side [③ in Figure 4d]. As td increases, the number ofelectrons that accumulated in the CBM increases, which
initiates band gap renormalization (band gap reduction andsubsequent relaxation). Band gap renormalization is caused by
the screening of the electric field by electrons accumulated in
the CBM and energy transfer from electrons to the sample
lattice.51,60 This process is represented by ④ to ⑥ in Figure 4d.
Previous studies have shown that the relaxation time ofnonresonantly excited hot electrons to CBM by electron−
phonon interaction was in the range of several 100 fs,51 whichcorresponds well to our result. The time scale of the reductionof the screening effect due to the formation of a bound state
was predicted to be on the order of 1 ps in a previous study,60
which is consistent with the decay time of the tunnel current inthis study, τ2 = 720 fs. Since we chose defect-free sites, theexciton formation process is more likely, but these effects may
be site-dependent.
In area ⑥, It,MIR(td) < 0 again. The direction of the tunnel
current is changed by the flow of electrons that accumulated in
the CBM with the relaxation of band renormalization. After
that, the electron density in the CBM decreases, owing to suchas electron−hole recombination, and the signal returns to theinitial state ①. This process corresponds to the long relaxation
time previously observed with THz-STM. For a sample withphotovoltage, the effect of band bending must be considered.
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Authors

As described above, the ultrafast dynamics after 8 fs NIR
light irradiation were observed using by time-resolved MIRSTM. We succeeded in measuring ultrafast carrier dynamics inthe <1 ps region, which was well explained by the change inthe band structure associated with the carrier dynamics. In
addition to the time-resolved signal measurement, timeresolved imaging was also realized. We have shown that
MIR-STM is a very promising method. It is expected that amore detailed analysis including photo-induced phasetransitions will be possible if the interaction between theelectron system and phonons is directly observed. The
development of a method for the detailed analysis of transient
I−V curves is also strongly desired for studies in the energy
space to progress further. However, electric-field-driven timeresolved STM uses subcycle pulses instead of square waves, as
instantaneously applied bias voltages: therefore, a moredetailed analysis is required to obtain correct I−V curves.
Studies have been carried out using THz-STM,32,33 but thismethod has not yet been established. If MIR-STM realizes theevaluation of transient I−V curves, it will be possible to analyze
the dynamics of the electronic structures in ultrafast
phenomena in the <30 fs region in real space. Since theseanalyses are beyond the scope of this study, we leave them for
future work. The combination of the developed system wedemonstrated in this paper with a THz electric field is expected
to open a new possibility.61

Yusuke Arashida − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Hiroyuki Mogi − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Masashi Ishikawa − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Ippo Igarashi − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Akira Hatanaka − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Naoki Umeda − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Jinbo Peng − Faculty of Pure and Applied Sciences, Universityof Tsukuba, Tsukuba 305-8573, Japan
Shoji Yoshida − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Osamu Takeuchi − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan
Complete contact information is available at:https://pubs.acs.org/10.1021/acsphotonics.2c00995
Author Contributions

Y.A. developed the system, and took data with M.I., I.I., A.H.,
and N.U. J.P., S.Y. and O.T. provided technical advice. H.S.
organized and supervised the project and edited the paper with
Y.A. and S.Y.

■

CONCLUSIONS
We have realized the first ever mid-infrared (MIR) electricfield-driven STM system enabling the pump−probe method to
be used over a wide time range with a time resolution higher
than 30 fs. We demonstrated the high potential of the new
MIR-STM system by visualizing ultrafast nonequilibrium
carrier dynamics induced by the irradiation of a sample with
an NIR pulse of 8 fs width. We succeeded in measuring
ultrafast carrier dynamics in the range of 0 to over 1 ps whichwere well explained by the change in band structure associatedwith the carrier dynamics. In addition to time-resolved signal
measurement, time-resolved imaging was also realized. It ishighly expected that this method will pave the way for studying
the local dynamics of non-equilibrium states such as
elementary excitations and phase transitions in condensed
materials.

■

Funding

We acknowledge the financial support of a grant-in-aid for
Scientific Research (17H06088, 20H00341, 20H05662) fromJapan Society for the Promotion of Science.
Notes

The authors declare no competing financial interest.

■

REFERENCES(1) Shah, J. Ultrafast Spectroscopy of Semiconductors and Semiconductor Nanostructures; Springer, 1999.
(2) Petek, H.; Ogawa, S. Femtosecond time-resolved two-photon
photoemission studies of electron dynamics in metals. Prog. Surf. Sci.
1997, 56, 239−310.
(3) Huber, R.; Tauser, F.; Brodschelm, A.; Bichler, M.; Abstreiter,
G.; Leitenstorfer, A. How many-particle interactions develop after
ultrafast excitation of an electron−hole plasma. Nature 2001, 414,
286−289.
(4) Giannetti, C.; Capone, M.; Fausti, D.; Fabrizio, M.; Parmigiani,F.; Mihailovic, D. Ultrafast optical spectroscopy of strongly correlatedmaterials and high-temperature superconductors: a non-equilibrium
approach. Adv. Phys. 2016, 65, 58−238.
(5) Rohwer, T.; Hellmann, S.; Wiesenmayer, M.; Sohrt, C.; Stange,
A.; Slomski, B.; Carr, A.; Liu, Y.; Avila, L. M.; Kalläne, M.; Mathias, S.;
Kipp, L.; Rossnagel, K.; Bauer, M. Collapse of long-range charge order
tracked by time-resolved photoemission at high momenta. Nature
2011, 471, 490−493.
(6) Reimann, J.; Schlauderer, S.; Schmid, C. P.; Langer, F.; Baierl, S.;
Kokh, K. A.; Tereshchenko, O. E.; Kimura, A.; Lange, C.; Güdde, J.;
Höfer, U.; Huber, R. Subcycle observation of lightwave-driven Diraccurrents in a topological surface band. Nature 2018, 562, 396−400.
(7) Wang, Y. H.; Steinberg, H.; Jarillo-Herrero, P.; Gedik, N.
Observation of Floquet-Bloch States on the Surface of a TopologicalInsulator. Science 2013, 342, 453−457.
(8) Gierz, I.; Petersen, R. P.; Mitrano, K. L.; Cacho, A.; Turcu, A.;
Springate, M.; Stöhr, A.; Köhler, A.; Starke, U.; Cavalleri, A.
Snapshots of non-equilibrium Dirac carrier distributions in graphene.
Nat. Mater. 2013, 12, 1119−1124.

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsphotonics.2c00995.
Analysis of thermal expansion problem; evaluation of
MIR far-field; reconstruction of the near field voltage
VNF,MIR(t) from INF,MIR(t); FDTD calculation of MIR
near-field waveform (PDF)■

ArticleAUTHOR INFORMATION

Corresponding Author

Hidemi Shigekawa − Faculty of Pure and Applied Sciences,
University of Tsukuba, Tsukuba 305-8573, Japan;
orcid.org/0000-0001-9550-5148; Email: hidemi@
ims.tsukuba.ac.jp
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ACS Photonics

pubs.acs.org/journal/apchd5(9) Morrison, V. R.; Chatelain, R. P.; Tiwari, K. L.; Hendaoui, A.;
Bruhács, A.; Chaker, M.; Siwick, B. J. A photoinduced metal-like
phase of monoclinic VO2 revealed by ultrafast electron diffraction.
Science 2014, 346, 445−448.
(10) Sokolowski-Tinten, K.; Blome, C.; Dietrich, C.; Tarasevitch, A.;
Horn von Hoegen, M.; von der Linde, D.; Cavalleri, A.; Squier, J.;
Kammler, M. Femtosecond x-ray measurement of ultrafast melting
and large acoustic transients. Phys. Rev. Lett. 2001, 87, 225701.
(11) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. 7×7
Reconstruction on Si(111) Resolved in Real Space. Phys. Rev. Lett.
1983, 50, 120−123.
(12) Tian, Y.; Yang, F.; Guo, C.; Jiang, Y. Recent Advances in
Ultrafast Time-Resolved Scanning Tunneling Microscopy. Surf. Rev.
Lett. 2018, 25, 1−19.
(13) Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Lasercombined scanning tunnelling microscopy for probing ultrafast
transient dynamics. J. Phys. Condens. Matter 2010, 22, 264008.
(14) Weiss, S.; Ogletree, D. F.; Botkin, D.; Salmeron, M.; Chemla,
D. S. Ultrafast scanning probe microscopy. Appl. Phys. Lett. 1993, 63,
2567−2569.
(15) Nunes, G.; Freeman, M. R. Picosecond Resolution in Scanning
Tunneling Microscopy. Science 1993, 262, 1029−1032.
(16) Moult, I.; Herve, M.; Pennec, Y. Ultrafast spectroscopy with ascanning tunneling microscope. Appl. Phys. Lett. 2011, 98, 233103.
(17) Saunus, C.; Raphael Bindel, B. J.; Pratzer, M.; Morgenstern, M.
Versatile scanning tunneling microscopy with 120 ps time resolution.
Appl. Phys. Lett. 2013, 102, 051601.
(18) Loth, S.; Etzkorn, M.; Lutz, C. P.; Eigler, D. M.; Heinrich, A. J.
Measurement of Fast Electron Spin Relaxation Times with Atomic
Resolution. Science 2010, 329, 1628−1630.
(19) Yan, S.; Choi, D. J.; Burgess, J. A. J.; Rolf-Pissarczyk, S.; Loth, S.
Control of quantum magnets by atomic exchange bias. Nat.
Nanotechnol. 2015, 10, 40−45.
(20) Friedlein, J.; Harm, J.; Lindner, P.; Bargsten, L.; Bazarnik, M.;
Krause, S.; Wiesendanger, R. A radio-frequency spin-polarizedscanning tunneling microscope. Rev. Sci. Instrum. 2019, 90, 123705.
(21) 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. 2017, 119, 176002.
(22) Yoshida, S.; Yokota, M.; Takeuchi, O.; Oigawa, H.; Mera, Y.;
Shigekawa, H. Single-Atomic-Level Probe of Transient Carrier
Dynamics by Laser-Combined Scanning Tunneling Microscopy.
Appl. Phys. Express 2013, 6, 032401.
(23) Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Real-space
imaging of transient carrier dynamics by nanoscale pumpg-probe
microscopy. Nat. Photonics 2010, 4, 869−874.
(24) Mogi, H.; Wang, Z. H.; Bamba, T.; Takaguchi, Y.; Endo, T.;
Yoshida, S.; Taninaka, A.; Oigawa, H.; Miyata, Y.; Takeuchi, O.;
Shigekawa, H. Development of laser-combined scanning multiprobe
spectroscopy and application to analysis of WSe2/MoSe2 in-plane
heterostructure. Appl. Phys. Express 2019, 12, 045002.
(25) Guo, C.; Meng, X.; Fu, H.; Wang, Q.; Wang, H.; Tian, Y.; Peng,
J.; Ma, R.; Weng, Y.; Meng, S.; Wang, E.; Jiang, Y. Probing
Nonequilibrium Dynamics of Photoexcited Polarons on a MetalOxide Surface with Atomic Precision. Phys. Rev. Lett. 2020, 124,
206801.
(26) Yoshida, S.; Aizawa, Y.; Wang, Z. H.; Oshima, R.; Mera, Y.;
Matsuyama, E.; Oigawa, H.; Takeuchi, O.; Shigekawa, H. Probing
ultrafast spin dynamics with optical pump-probe scanning tunnelling
microscopy. Nat. Nanotechnol. 2014, 9, 588−593.
(27) Wang, Z. H.; Yoon, C. H.; Yoshida, S.; Arashida, Y.; Takeuchi,O.; Ohno, Y.; Shigekawa, H. Surface-mediated spin dynamics probed
by optical-pump-probe scanning tunneling microscopy. Phys. Chem.
Chem. Phys. 2019, 21, 7256−7260.
(28) Cocker, T. L.; Jelic, V.; Gupta, M.; Molesky, S. J.; Burgess, J. A.
J.; Reyes, G. D. L.; Titova, L. V.; Tsui, Y. Y.; Freeman, M. R.;
Hegmann, F. A. An ultrafast terahertz scanning tunnelling microscope.
Nat. Photonics 2013, 7, 620−625.

Article(29) Yoshioka, K.; Katayama, I.; Minami, Y.; Kitajima, M.; Yoshida,S.; Shigekawa, H.; Takeda, J. Real-space coherent manipulation ofelectrons in a single tunnel junction by single-cycle terahertz electricfields. Nat. Photonics 2016, 10, 762−765.
(30) Wang, L.; Xia, Y.; Ho, W. Atomic-scale quantum sensing basedon the ultrafast coherence of an H2 molecule in an STM cavity.
Science 2022, 376, 401−405.
(31) Cocker, T. L.; Peller, D.; Yu, P.; Repp, J.; Huber, R. Tracking
the ultrafast motion of a single molecule by femtosecond orbital
imaging. Nature 2016, 539, 263−267.
(32) Yoshida, S.; Hirori, H.; Tachizaki, T.; Yoshioka, K.; Arashida,Y.; Wang, Z. H.; Sanari, Y.; Takeuchi, O.; Kanemitsu, Y.; Shigekawa,
H. Subcycle Transient Scanning Tunneling Spectroscopy with
Visualization of Enhanced Terahertz Near Field. ACS Photonics
2019, 6, 1356−1364.
(33) Jelic, V.; Iwaszczuk, K.; Nguyen, P. H.; Rathje, C.; Hornig, G.
J.; Sharum, H. M.; Hoffman, J. R.; Freeman, M. R.; Hegmann, F. A.
Ultrafast terahertz control of extreme tunnel currents through single
atoms on a silicon surface. Nat. Phys. 2017, 13, 591−598.
(34) Yoshioka, K.; Katayama, I.; Arashida, Y.; Ban, A.; Kawada, Y.;
Konishi, K.; Takahashi, H.; Takeda, J. Tailoring Single-Cycle Near
Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled
Terahertz Electric Fields. Nano Lett. 2018, 18, 5198−5204.
(35) Luo, Y.; Jelic, V.; Chen, G.; Nguyen, P. H.; Liu, Y. J. R.;
Calzada, J. A. M.; Mildenberger, D. J.; Hegmann, F. A. Nanoscaleterahertz STM imaging of a metal surface. Phys. Rev. B 2020, 102,
205417.
(36) Yoshida, S.; Arashida, Y.; Hirori, H.; Tachizaki, T.; Taninaka,
A.; Ueno, H.; Takeuchi, O.; Shigekawa, H. Terahertz Scanning
Tunneling Microscopy for Visualizing Ultrafast Electron Motion in
Nanoscale Potential Variations. ACS Photonics 2021, 8, 315−323.
(37) Ammerman, S. E.; Jelic, V.; Wei, Y.; Breslin, V. N.; Hassan, M.;
Everett, N.; Lee, S.; Sun, Q.; Pignedoli, C. A.; Ruffieux, P.; Fasel, R.;
Cocker, T. L. Lightwave-driven scanning tunnelling spectroscopy ofatomically precise graphene nanoribbons. Nat. Commun. 2021, 12,6794.
(38) Abdo, M.; Sheng, S.; Rolf-Pissarczyk, S.; Arnhold, L.; Burgess, J.
A. J.; Isobe, M.; Malavolti, L.; Loth, S. Variable Repetition Rate THz
Source for Ultrafast Scanning Tunneling Microscopy. ACS Photonics
2021, 8, 702−708.
(39) Müller, M.; Martín, S. N.; Kampfrath, T.; Wolf, M. PhaseResolved Detection of Ultrabroadband THz Pulses inside a Scanning
Tunneling Microscope Junction. ACS Photonics 2020, 7, 2046−2055.
(40) Garg, M.; Martin-Jimenez, A.; Pisarra, M.; Luo, Y.; Martín, F.;
Kern, K. Real-space subfemtosecond imaging of quantum electronic
coherences in molecules. Nat. Photonics 2021, 16, 196−202.
(41) Nishida, J.; Johnson, S. C.; Chang, P. T. S.; Wharton, D. M.;
Dönges, S. A.; Khatib, O.; Raschke, M. B. Ultrafast infrared nanoimaging of far-from-equilibrium carrier and vibrational dynamics. Nat.
Commun. 2022, 13, 1083.
(42) Eisele, M.; Cocker, T. L.; Huber, M. A.; Plankl, M.; Viti, L.;
Ercolani, D.; Sorba, L.; Vitiello, M. S.; Huber, R. Ultrafast multiterahertz nano-spectroscopy with sub-cycle temporal resolution. Nat.
Photonics 2014, 8, 841−845.
(43) Plankl, M.; Faria Junior, P. E.; Mooshammer, F.; Siday, T.;
Zizlsperger, M.; Sandner, F.; Schiegl, F.; Maier, S.; Huber, M. A.;
Gmitra, M.; Fabian, J.; Boland, J. L.; Cocker, T. L.; Huber, R.
Subcycle contact-free nanoscopy of ultrafast interlayer transport in
atomically thin heterostructures. Nat. Photonics 2021, 15, 594−600.
(44) Yoshioka, K.; Igarashi, I.; Yoshida, S.; Arashida, Y.; Katayama,
I.; Takeda, J.; Shigekawa, H. Subcycle mid-infrared coherent
transients at 4 MHz repetition rate applicable to light-wave-driven
scanning tunneling microscopy. Opt. Lett. 2019, 44, 5350.
(45) Kübler, C.; Huber, R.; Leitenstorfer, A. Ultrabroadband
terahertz pulses: generation and field-resolved detection. Semicond.
Sci. Technol. 2005, 20, S128−S133.
(46) Lu, F.; Jin, M.; Belkin, M. A. Tip-enhanced infrarednanospectroscopy via molecular expansion force detection. Nat.
Photonics 2014, 8, 307−312.
3163

https://doi.org/10.1021/acsphotonics.2c00995
ACS Photonics 2022, 9, 3156−3164

ACS Photonics

pubs.acs.org/journal/apchd5(47) Shimada, T.; Ohuchi, F. S.; Parkinson, B. A. Work Function
and Photothreshold of Layered Metal Dichalcogenides. Jpn. J. Appl.
Phys. 1994, 33, 2696−2698.
(48) Yu, E. T.; Barmak, K.; Ronsheim, P.; Johnson, M. B.;
McFarland, P.; Halbout, J.-M. Two-dimensional profiling of shallow
junctions in Si metal-oxide-semiconductor structures using scanningtunneling spectroscopy and transmission electron microscopy. J. Appl.
Phys. 1996, 79, 2115.
(49) Herink, G.; Solli, D. R.; Gulde, M.; Ropers, C. Field-driven
photoemission from nanostructures quenches the quiver motion.
Nature 2012, 483, 190−193.
(50) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.;
Strano, M. S. Electronics and optoelectronics of two-dimensionaltransition metal dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712.
(51) Chi, Z.; Chen, H.; Zhao, Q.; Weng, Y. X. Ultrafast carrier andphonon dynamics in few-layer 2H-MoTe2. J. Chem. Phys. 2019, 151,
114704.
(52) Wang, L.; Wu, Y.; Yu, Y.; Chen, A.; Li, H.; Ren, W.; Lu, S.;
Ding, S.; Yang, H.; Xue, Q. K.; Li, F. S.; Wang, G. Direct Observationof One-Dimensional Peierls-type Charge Density Wave in Twin
Boundaries of Monolayer MoTe2. ACS Nano 2020, 14, 8299−8306.
(53) Qi, Y.; Naumov, P. G.; Ali, M. N.; Rajamathi, C. R.; Schnelle,W.; Barkalov, O.; Hanfland, M.; Wu, S.-C.; Shekhar, C.; Sun, Y.; Süß,V.; Schmidt, M.; Schwarz, U.; Pippel, E.; Werner, P.; Hillebrand, R.;
Förster, T.; Kampert, E.; Parkin, S.; Cava, R. J.; Felser, C.; Yan, B.;
Medvedev, S. A. Superconductivity in Weyl semimetal candidate
MoTe2. Nat. Commun. 2016, 7, 11038.
(54) Lezama, I. G.; Ubaldini, A.; Longobardi, M.; Giannini, E.;
Renner, C.; Kuzmenko, A. B.; Morpurgo, A. F. Surface transport andband gap structure of exfoliated 2H-MoTe2 crystals. 2D Mater. 2014,
1, 021002.
(55) Takeuchi, O.; Yoshida, S.; Shigekawa, H. Light-modulated
scanning tunneling spectroscopy for nanoscale imaging of surface
photovoltage. Appl. Phys. Lett. 2004, 84, 3645−3647.
(56) Yoshida, S.; Kikuchi, J.; Kanitani, Y.; Takeuchi, O.; Oigawa, H.;
Shigekawa, H. Tip-induced band bending and its effect on local
barrier height measurement studied by light-modulated scanningtunneling spectroscopy. e-J. Surf. Sci. Nanotechnol. 2006, 4, 192−196.
(57) Attar, A. R.; Chang, H. T.; Britz, A.; Zhang, X.; Lin, M. F.;
Krishnamoorthy, A.; Linker, T.; Fritz, D.; Neumark, D. M.; Kalia, R.
K.; Nakano, A.; Ajayan, P.; Vashishta, P.; Bergmann, U.; Leone, S. R.
Simultaneous observation of carrier-specific redistribution andcoherent lattice dynamics in 2H-MoTe2 with femtosecond corelevel spectroscopy. ACS Nano 2020, 14, 15829−15840.
(58) Meckbach, L.; Hader, J.; Huttner, U.; Neuhaus, J.; Steiner, J. T.;
Stroucken, T.; Moloney, J. V.; Koch, S. W. Ultrafast band-gap
renormalization and build-up of optical gain in monolayer MoTe2.
Phys. Rev. B 2020, 101, 075401.
(59) Chi, Z.; Chen, H.; Chen, Z.; Zhao, Q.; Chen, H.; Weng, Y.-X.
Ultrafast Energy Dissipation via Coupling with Internal and External
Phonons in Two-Dimensional MoS2. ACS Nano 2018, 12, 8961−
8969.
(60) Britz, A.; Attar, A. R.; Zhang, X.; Chang, H. T.; Nyby, C.;
Krishnamoorthy, A.; Park, S. H.; Kwon, S.; Kim, M.; Nordlund, D.;
Sainio, S.; Heinz, T. F.; Leone, S. R.; Lindenberg, A. M.; Nakano, A.;
Ajayan, P.; Vashishta, P.; Fritz, D.; Lin, M. F.; Bergmann, U. Carrierspecific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray
absorption spectroscopy using an x-ray free-electron laser. Struct. Dyn.
2021, 8, 014501.
(61) Arashida, Y.; Umeda, N.; Mogi, H.; Ishikawa, M.; Hatanaka, A.;
Takeuchi, O.; Yoshida, S.; Shigekawa, H. Phase-controllable multilaser system with coaxially combined near-infrared and subcycle midinfrared and THz pulsed beams. Appl. Phys. Express 2022, 15, 092006.

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