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LetterCite This: ACS Photonics 2019, 6, 1356−1364

pubs.acs.org/journal/apchd5

Subcycle Transient Scanning Tunneling Spectroscopy with
Visualization of Enhanced Terahertz Near Field
Shoji Yoshida,† Hideki Hirori,‡ Takehiro Tachizaki,§ Katsumasa Yoshioka,∥ Yusuke Arashida,†
Zi-Han Wang,† Yasuyuki Sanari,‡ Osamu Takeuchi,† Yoshihiko Kanemitsu,‡
and Hidemi Shigekawa*,††

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan§
Department of Optical and Imaging Science and Technology, Tokai University, Hiratsuka-shi, Kanagawa 259-1292, Japan∥
Department of Physics, Graduate School of Engineering, Yokohama National University, Yokohama, 240-8501, Japan‡

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S Supporting Information*

ABSTRACT: The recent development of optical technology has enabled the practical use of a carrier-envelope phasecontrolled monocycle electric ﬁeld in the terahertz (THz) regime. By combining this technique with metal nanostructures suchas nanotips, which induce near-ﬁeld enhancement, the development of novel applications is anticipated. In particular, THz
scanning tunneling microscopy (THz-STM) is a promising technique for probing ultrafast dynamics with the spatial resolutionof STM. However, the modulation of the THz waveform is generally accompanied by an enhancement of the electric ﬁeld,
which is unknown in actual measurement environments. Here, we present a method enabling direct evaluation of the enhancednear ﬁeld in the tunnel junction in THz-STM in the femtosecond range, which is essential for the use of the THz near ﬁeld. In
the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluatedusing the THz-STM, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.
KEYWORDS: scanning tunneling microscopy, terahertz, near ﬁeld, ultrafast dynamics

W

The application of this technology is expected to open new
research areas, such as nanodevices that coherently control
tunneling currents,2,7 THz electron guns,8,9 and new measurement and processing techniques as a result of combining THz
waves with near-ﬁeld scanning optical microscopy (NSOM)
and scanning tunneling microscopy (STM).2,10−15 For
example, an ultrashort-pulse electron beam obtained bycontrolling the photoelectron emission from a metal probe in
the time domain of a THz subcycle has realized new ultrafast
electron microscopy, which enables the observation of ultrafast
dynamics at the atomic scale.9,16 By irradiating the tunnel
junction with THz pulses, an ultrafast tunneling current has

hen a nanoscale metal structure is irradiated byelectromagnetic waves, the near ﬁeld formed on thematerial surface is enhanced in a conﬁned space far beyond the
diﬀraction limit.1 With a metal probe (nanotip), it is possible
to obtain an intense terahertz (THz) near ﬁeld in a local region
directly beneath the nanotip owing to the ﬁeld enhancementeﬀect.2 Therefore, this near ﬁeld not only allows THz
nanoscopy of structures much smaller than the wavelength oflight beyond the diﬀraction limit3 but also provides a platform
for studying strong-ﬁeld-driven dynamics.4,5 Furthermore, thepractical use of light with a controlled carrier-envelope phase(CEP) has recently become possible. In particular, in the caseof THz waves, it is possible to spontaneously obtain CEPstable light consisting of almost monocycle pulses throughoptical rectiﬁcation by irradiating ultrashort optical pulses to a
THz antenna and various nonlinear optical crystals.6© 2019 American Chemical Society

Received: February 14, 2019
Published: May 16, 2019
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LetterFigure 1. Explanation of measurement principle. (a) Incident electric ﬁeld and its modulation by a nanotip. (b, c) Schematic illustrations ofmeasurement methods in photoemission (photoﬁeld emission and above-threshold photoemission) regime and tunneling regime, respectively. The
barrier height is modulated by the THz electric ﬁeld, and photoelectron emission (hot electron tunneling) is induced by the simultaneous
irradiation by the probe light of 517 nm (1035 nm). (d) Dependence of photocurrent on the dc bias voltage VDC applied to the HOPG sample withrespect to the nanotip while the tip was irradiated by the probe light and the THz electric ﬁeld was turned oﬀ. The photocurrent observed for VDC≤ 0 was due to above-threshold photoelectron emission caused by the process of three-photon absorption.

THz ﬁeld as the instantaneous bias voltage, and the ultrafast
carrier dynamics in 2H-MoTe2 excited by an optical pulse was
reproducibly probed.

been driven, and its direction can be coherently manipulated
by changing the CEP of the THz pulse.15
Recently, THz-STM has been developed using the abovedescribed mechanism, in which the tip-enhanced THz nearﬁeld is operated as a transient tunnel voltage VTHz, and theTHz-induced tunneling current ITHz is measured as a signal.
Since the tip enhancement reaches a factor of 1 × 105, it ispossible to obtain the electric ﬁeld strength necessary for theultrafast driving of tunneling current with a relatively weak
incident THz pulsed electric ﬁeld.2 In THz-STM, since thethermal expansion of the tip and sample is small,2,10 timeresolved measurement can be performed by measuring thecurrent at the moment the bias voltage is applied. The carrier
dynamics of optically pumped InAs nanodots and molecularorbitals of a vibrating pentacene single molecule werevisualized in real space with an atomic-level locality of ITHz
and a high temporal resolution of sub-picoseconds.10,11
With the precise control and utilization of the electric ﬁeld
inside optical pulses, further extension toward the extreme
limits of measurement and control can be expected. However,
although THz electromagnetic waves are enhanced bycoupling the antenna with the nanotip, as described above,the electric ﬁeld waveform is also modulated simultaneously, as
schematically shown in Figure 1a. This has been experimentally conﬁrmed in NSOM and THz-STM.15,17 Although thetheoretical description of the antenna eﬀect and other models
has been discussed,17 to realize precise measurement andcontrol, it is essential to develop a new method for directly
measuring and quantitatively evaluating the waveform in actual
measurement environments.
Here, we present a new method enabling the visualization of
the enhanced THz near-ﬁeld waveform over a nanotip and thetunnel junction by controlling the photoelectron emission andtunneling current in the femtosecond range. In the tunnelingregime, it was also demonstrated that the transient electronic
state excited by an optical pulse can be evaluated using the■

RESULTS
Experimental Setup. In this study, we performedexperiments using a system constructed by combining an
ultrashort-pulse laser and an ultrahigh-vacuum STM system(base pressure: <1 × 10−7 Pa), which enables precise
manipulation of the nanotip position (Figure S1). A tungsten(W) STM tip coated with PtIr was used as the nanotip formeasurement. As shown in Figure 1b,c, the nanotip was
irradiated with two types of optical pulses: a THz light and aprobe light. The THz pulse was generated by irradiating a
LiNbO3 crystal with an infrared (IR) pulse (1035 nm, 309 fs, 1MHz), and it was almost a monocycle with a large ﬁrst peak of
1 ps width, as shown in Figure 2g. For the probe light, IR lightand 517 nm light, which was generated by converting the IR
light into a second-harmonic wave using a β-BaB2O4 (BBO)
crystal, were prepared for the two types of measurements inwhich the photoelectron emission current (Figure 1b) and hotelectron tunneling current (Figure 1c) were detected as asignal.
In a previous work, the THz near ﬁeld at the metal tip apex
was evaluated using the THz-ﬁeld-driven photoemission(photoﬁeld emission) from the nanotip.9,18 In our case, byusing the STM setup, we realized two-dimensional (2D)
analysis of the enhanced near ﬁeld, including measurements inthe tunnel regime. As an initial stepwise comparison, highlyoriented pyrolytic graphite (HOPG), a typical material, andBi2Se3, which has been well-studied by photoelectron spectroscopy,19 were used as samples. The relaxation of the excitedelectronic states in these materials is suﬃciently fast comparedwith temporal oscillation of the THz electric ﬁeld, enablingprobing of the THz pulse waveform. In the case of the Bi2Se3
sample, there are fast (<1 ps) and slow (∼10 ps) relaxation
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LetterFigure 2. THz electric ﬁeld modulation by nanotip. (a) Optical micrograph of the ultrasharp PtIr-coated W nanotip (tip 1). (b) Map ofphotoelectrons obtained by scanning the probe light (517 nm) on the nanotip. The spot position was moved by 31 steps of 4.5 μm in the x
direction; then, after being shifted by 4.5 μm in the y direction, it was moved back along the x direction in 31 steps. This procedure was repeated,until seven steps had been completed in the y direction (the entire scanned area was 139.5 × 31.5 μm2). (c) Map of ITHz at zero delay time. (d)Optical micrograph of the PtIr-coated W nanotip fabricated by ordinary chemical etching (tip 2). (e) Map of photoelectrons obtained by scanningthe probe light (517 nm) on the nanotip. (f) Map of ITHz at zero delay time. (g) Waveform of the incident THz light. (h−j) THz near-ﬁeld
waveforms measured at A, B, and C in (c) and (f), respectively. Red and blue lines show the results for tips 1 and 2, respectively. (k) Waveform
obtained by integrating the electric ﬁeld in (g). Vs = +10 V, IVIS = 14 mW, ETHz,pk = 4.1 kV/cm (electric ﬁeld of the maximum peak), f rep = 200 kHz,and tip−sample distance ≈ 1000 nm. Here, an HOPG sample was used as the counter electrode.

components for high-energy (E − EF > 1 eV) and low-energy
regions, respectively. Hot electrons excited up to the highenergy bulk and surface states are cooled to a lower state
within 1 ps. Such an ultrafast response of the hot electrons
allows us to probe the THz near-ﬁeld waveform in the STM
tunnel junction.
In the last part of this paper, we show the result of timeresolved measurement using 2H-MoTe2 as a sample. We
excited (pumped) the sample using IR light and measured the
dynamics of the excited state using a THz pulse as theinstantaneous bias voltage to be applied between the STM tip
and sample.
Visualization of Terahertz Near-Field Waveform by
Photoelectron Emission. First, we present the method andresults when photoelectron emission was used for measurement. When irradiating a nanotip, photoelectron emission is
induced by multiphoton absorption of the probe light (517nm). The generated photoelectrons ﬂow from the nanotip toan HOPG sample upon applying direct-current (dc) bias
voltage VDC to the sample with respect to the nanotip and are
detected by a current preampliﬁer. Since the photoelectron

generation eﬃciency depends on the height of the potential
barrier at the W nanotip surface, the photocurrent intensitychanges with the proﬁle of the barrier height, which ismodulated by VDC and the THz near ﬁeld generated on the
nanotip, as shown in Figure 1b.
Figure 1d shows a current−voltage (I−V) curve obtained byvarying the applied sample bias voltage VDC, while the tip was
irradiated by the probe light with the THz electric ﬁeld turned
oﬀ. The photocurrent observed for VDC ≤ 0 was due to abovethreshold photoelectron emission caused by the process of
three-photon absorption. The photoelectron current increased
monotonically as a function of VDC. It was also conﬁrmed that
the THz-induced photoelectron current ITHz, which was
generated by the bias voltage produced by the THz pulse inaddition to VDC, linearly changed with the incident THz ﬁeld
strength, ITHz ∝ ETHz, in this bias voltage range (Figure S2).
Therefore, it is considered possible to investigate a THz nearﬁeld waveform by measuring ITHz while varying the delay time
td between the THz pulse and the probe pulse. To detect the
weak signal, the pump laser generating the THz pulses was
chopped at 430 Hz, and ITHz(td) was measured by the lock-in
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LetterFigure 3. Measurement in tunnel regime. (a) Schematic illustration of the measurement setup. The IR pulse wavelength was set to 1035 nm. The
direction of the THz pulse generated by a lithium niobate crystal was inverted by CEP control using the HWP. The THz ﬁeld strength wascontinuously controlled using a wire grid polarizer. (b) Tunnel junction band diagrams (I) without and (II) with IR pulse and THz pulse
irradiation. (c) Electrical over-stress measurement results of the incident THz waveform. By controlling the HWP angle α and adjusting the phaseof the CEP, ϕCEP, the electric ﬁeld waveform can be reversed. (d) Tip-enhanced near-ﬁeld waveform obtained by photoelectron measurement. (e)ITHz as a function of delay time (VS = 2 mV, IT = 2 pA, IIR = 5 mJ/cm2, f rep = 0.5 MHz). (f) ITHz−VTHz curves obtained for td = −5 ps and 0. (g) ITHz
as a function of delay time simulated using the ITHz−VTHz curve. (h) Schematic to explain the method of simulation to obtain the spectra shown in(g). Analysis was performed for Bi2Se3.

detection method. The delay time was varied at a rate of 1 ps/
s. All experiments were performed at room temperature.
Figure 2 shows the results obtained using an HOPG sample
as the counter electrode of the nanotip. Figure 2a,d shows
optical micrographs of the two types of nanotips used in thisstudy. Both tip 1 and tip 2 were fabricated by electropolishing,but tip 1 was sharper and had a longer tapered area (see Figure
S1b for more detail). The spot position of the probe light pulse
was two-dimensionally scanned over the nanotip, and the I−V
curve (corresponding to Figure 1d) and waveform (Figure 2)
measurements were performed at each pixel position.
First, we veriﬁed that the scanning system operated
correctly. When an electric ﬁeld is applied, the distance
between the positions probed on the tip and sample aﬀects themeasurement. Therefore, we measured the photoelectron
current for VDC = 0 at each pixel, that is, the current obtainedby photoelectrons reaching the HOPG sample without a THz
electric ﬁeld and sample bias voltage VDC (corresponding to
the value at VDC = 0 in Figure 1d). Figure 2b,e shows theresults of mapping. Each map approximately reﬂects the shapeof the nanotip, but the intensity is low at the tip apex, because
the diameter of the nanotip is smaller than the light spot size.
THz waveforms were obtained by measuring the THz current
ITHz(td) at each pixel position while changing td andmaintaining VDC at +10 V. Since the photoelectron current
was almost linear with respect to the voltage around VDC = +10
V, as shown in Figure 1d, ITHz only depends on ETHz when
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Letter

STM with electronic control by an electric ﬁeld, evaluation of
the electric ﬁeld waveform under the measurement conditions
becomes indispensable. To proceed, we used the probe thatyielded the near-ﬁeld proﬁle of the tip (through thephotoelectronic emission) to perform the measurement in
the tunnel regime, that is, THz-STM measurement.
Figure 3a shows a schematic illustration of the experimental
setup. VDC and the current were set to 2 mV and 2 pA,
respectively, to bring the nanotip closer to the tunnel region.
The bias voltage was set to this low value instead of increasing
the current to avoid the possibility of the sample breaking
when the current increases. To suppress photoelectron
emission, the original IR light (1035 nm) was used for theprobe. When the tip was retracted 100 nm, the signal, which
was observed in the range of 100 μm for the case ofphotoelectrons, disappeared, thus conﬁrming that the signal
originated from the tunnel current. HOPG and Bi2Se3 wereused as samples. The signal was signiﬁcantly weaker for
HOPG, approximately 1/100 of that for Bi2Se3, and a detailed
analysis shown in Figure 3 was performed for Bi2Se3 (seeFigure S5 and its caption for more detail). The Bi2Se3 sample
was prepared by cleavage in vacuum.
Figure 3b shows schematic illustrations of the measurement
mechanism in the tunnel regime, (I) without and (II) with
irradiation. Bi2Se3 used for the analysis has a narrow band gapof ∼0.2 eV in the bulk,19 and a metallic surface state called a
Dirac cone is formed because of its topological property. Since
Bi2Se3 is a degenerate n-type semiconductor owing to Se
vacancies, the Fermi level is located above the bulk conduction
band minimum.20 Therefore, although downward band
bending occurs at the surface, it is not modulated by opticalexcitation because of the metallic characteristics of Bi2Se3.
Here, the tunnel current was measured instead of photoelectrons, which were observed for Figure 2. To detect the
weak signal of the THz-induced tunnel current, the IR pulse
used for THz generation was chopped at 430 Hz, and ITHz(td)was measured by the lock-in detection method, as was
performed for the photoelectron measurement. If an IR lightpulse is irradiated while the tunnel barrier is changed owing to
THz pulse irradiation, hot electrons are produced and enhance
the tunnel current ITHz, where ITHz depends on the delay time
between the IR pulse and the THz pulse [Figure 3b, (II)].
Since the relaxation of hot electrons in a high-energy state
occurs faster than the oscillation of the THz electric ﬁeld, thedelay time dependence of ITHz was expected to represent a
THz near-ﬁeld waveform similar to that obtained byphotoelectron emission measurement; however, the nonlinearity of the I−V curve aﬀects the spectrum, which will be
discussed later.
To probe the photoexcited electronic state, the VTHz
dependence of ITHz was measured with the delay time td
ﬁxed to zero. For this purpose, a λ/2 wave plate (HWP) for theTHz range and a wire grid polarizer (WGP) were introducedin this experiment, which enabled VTHz to be continuously
changed in both the positive and negative voltage ranges.15,21
When the angle of the HWP, α, shown in Figure 3a, is rotatedby π/2, the phase of the CEP, ϕCEP, changes by π, and the
direction of the electric ﬁeld is reversed.
Figure 3c shows the waveform of the incident THz pulse
measured at HWP angles of α = 0 and α = π. The THz
waveform was inverted vertically as expected. Thereby, thenear-ﬁeld waveform at the tip apex was also inverted, as shownin Figure 3d. These near ﬁelds worked as the transient bias

photoelectron emission occurs. Thus, ITHz(td) provides a directreﬂection of the waveform of the THz near ﬁeld.
In ref 9, the strength of the incident THz ﬁeld was 100 kV/
cm, and the enhanced near ﬁeld obtained by time-of-ﬂight
measurement was 9 MV/cm, which was much higher than the
dc voltage (VDC = 30 V/(3 mm)). Therefore, photoelectrons
were steered back to the nanotip side, and only half the shapeof the THz ﬁeld was not obtained. They obtained the wholeshape of the THz electric ﬁeld by reversing the orientation of
the setup. In our case, the strength of the incident THz ﬁeld
was 4.1 kV/cm, and the eﬀective bias voltage between the
STM tip and sample estimated by using the I−V curve was 0.6
V, which was lower than the applied bias voltage of 10 V. In
addition, by setting the tip−sample distance close to ∼1 μm
and VDC to +10 V, the strong dc electric ﬁeld between the tip
and sample produced a photoelectron current larger thanITHz(td). Therefore, it was possible to measure the full change
in the current, while the THz electric ﬁeld direction waschanged from the nanotip to the sample and vice versa.
Figure 2h−j shows the THz near-ﬁeld proﬁles obtained forthe two types of nanotips by performing measurements at A, B,
and C in Figure 2c,f, respectively. On the one hand, whenmeasuring the THz waveform, VDC was ﬁxed at +10 V. On theother hand, as shown by the I−V curve in Figure 1d, thephotocurrent changed almost linearly with the voltage in thevicinity of VDC = +10 V. Therefore, from the slopes (dI/dV) of
the photoelectron I−V curves obtained at each point for VDC =+10 V, it was possible to convert each waveform to an eﬀective
THz voltage VTHz using the relationship of dI/dV = ITHz/VTHz,
which is plotted on the vertical axis of each ﬁgure. Figure 2c,fshows maps of ITHz at zero delay time for tips 1 and 2,
respectively. At the tip apex C, the peak values of VTHz for bothtips were larger than those at A and B owing to theenhancement eﬀect of the nanotip; in particular, the peakvalue was approximately twice as large for tip 1. Figure 2g
shows the incident THz waveform obtained by electro-optic(EO) sampling (see Figure S1). In the simple model ofantenna theory, the electric ﬁeld generated by nanotip
enhancement is obtained by integrating the original electric
ﬁeld,17 which is shown in Figure 2k. However, as shown in theﬁgures, the measured waveform is diﬀerent from the calculated
one, emphasizing the importance of directly measuring themodulated electric ﬁeld (see Figures S3 and S4 for moredetail). By using STM tips with diﬀerent tapering, it was shownthat the near-ﬁeld waveforms generated at the two diﬀerent
types of tips were diﬀerent, even for the same incident THz
electric ﬁeld. Recently, this tip dependence has been moreclearly observed using various tips of diﬀerent shapes.15
These results indicate that it is essential to directly evaluate
the near ﬁeld in the actual measurement environment to obtaina deeper understanding of the phenomena and for the practicaluse of THz techniques.
Measurement of THz Near-Field Waveform in the
Tunnel Regime. We showed that the modulation of theelectric ﬁeld can be characterized using the photoelectronic
emission. On the one hand, depending on various parameterssuch as the nanotip−sample distance, vacuum barrier height,
THz ﬁeld strength, and optical excitation energy, we cantheoretically consider the regimes of photoelectronic emission,ﬁeld emission, and tunneling;9,18 however, this has not been
suﬃciently conﬁrmed experimentally. On the other hand, since
the present setup employs an STM system, measurement in
the tunnel regime is possible. In the measurement by THz1360

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LetterFigure 4. Transient electronic dynamics obtained by THz-STM for 2H-MoTe2. (a) STM image of the sample (sample bias VS = 1.2 V, tunnel
current It = 10 pA). Bright spots indicated by D in the magniﬁed image are defects. (b) Delay-time dependence of ITHz (red line). Near-ﬁeld
waveform at the STM tip apex used for measurement, which was obtained by the photoelectron method (blue line) is superimposed. (insets) The
relationship between the pump and probe lights for td < 0 and td > 0. (c) Example of spectrum obtained over a wide delay-time range. The blackline is a ﬁtting curve with an exponential function of two components. (d) Light-intensity dependence of ITHz. ITHz has two components at 1.5 mJ/
cm2, but a one-component ﬁtting is in good agreement with ITHz at 1.1 and 0.7 mJ/cm2. (VS = −3 mV, IT = 0.6 pA, f rep = 1 MHz). (e) Schematic
illustrations of (left) without illumination, (I) THz illumination without IR, (II) THz illumination just after IR excitation, and (III) THz
illumination after IR excitation with rather large td in the range of τc (carrier recombination lifetime).

voltages of V1THz (ΔϕCEP = 0) and V2THz (ΔϕCEP = π) applied tothe tunnel junction. Since the tip was diﬀerent from thoseshown in Figure 2, the waveform was slightly diﬀerent fromthose shown in the ﬁgures. However, a common feature is thatthe near-ﬁeld waveform is modiﬁed from the incident THz
waveform. The strength of the near ﬁeld shown in Figure 3d isslightly asymmetric, which may be due to the slight change inthe spot position while changing the angle. However, a linear
relationship was obtained between the strength of the incident
THz ﬁeld and that of the obtained near ﬁeld (see Figure S6).
Next, the delay time dependence of ITHz induced by these
two near-ﬁeld waveforms was measured in the tunnel region,and the results are shown in Figure 3e. Measurements were
performed for VTHz = V1THz and V2THz. The THz-pulse-induced
tunnel current ITHz ﬂowed from the sample to tip when α = 0
and from the tip to the sample when α = π/2, but a spectral
shape similar to the near-ﬁeld waveform was obtained. The factthat a delay time dependence of ITHz was observed implies thatthe tunneling probability transiently changed with the change
in the electronic state induced by the IR pulse irradiation. To
conﬁrm this eﬀect, transient I−V curve measurements were

performed for td = −5 ps and 0 by measuring ITHz while
continuously sweeping VTHz by rotating the WGP and HWP.
The obtained results are shown in Figure 3f. Since thetransient I−V curve for td= −5 ps represents the transient
electronic state 5 ps after IR excitation, where the photoexcitedstate of both the tip and sample are almost completely relaxed,
the shape of the transient I−V curve is considered to be similar
to that of the static I−V curve without the eﬀect of the opticalpulse (Figure S7). In fact, a semiconducting electronic state
was obtained. In contrast, the slope of the I−V curve increased
for the case of td = 0, and the I−V curve became similar to thatfor a metallic electronic structure. As shown in the band
diagram (II) in Figure 3b, this change corresponds to the
reduction of the eﬀective vacuum barrier by the excitation of
hot electrons; the I−V curve reﬂects the optically excitedtransient electronic state.
By using the ultrafast electronic response in the tunnel
junction, we can evaluate the electric ﬁeld waveform by thetunnel current. The transient I−V curve depends on the delay
time in accordance with the dynamics of the photoexcitedelectronic state. Since the relaxation of the hot electrons was
fast, the excited state was almost relaxed at td = −5 ps, and1361

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ΔIexc (VTHz) [=ITHz (VTHz, 0 ps) − ITHz (VTHz, −5 ps)]corresponds to the change in the I−V curve owing tophotoexcitation. Therefore, ITHz (td) shown in Figure 3g was
simulated from the relationship shown in Figure 3h, with thenear ﬁeld being considered to be the voltage applied between
the probe and sample. (See Figure S8 for more detail.) Unlike
the case of photoelectrons, the time-resolved waveform
measured by the tunneling current has a diﬀerent form fromthe near-ﬁeld waveform, because the I−V curve is nonlinear.
The peak around td = 0 is slightly wider in the experimental
curve than in the simulated curve. This is thought to be due tothe relaxation time of hot electrons not being taken into
consideration in the simulation. It is considered that we canmeasure the relaxation process of hot electrons by measuring
the delay-time dependence of the transient I−V curve in moredetail, which is diﬃcult at present. Since the THz near-ﬁeld
waveform has a tail of over 1 ps in the region of td < 0, theresponse of an electronic state with a similar relaxation timecannot be measured separately from the eﬀect of the tail. It is
expected to be possible to evaluate the excited state of a sample
by conducting experiments using light pulses and THz pulsesof a shorter time width in the future.
In addition, as clearly shown in Figure 3e, the waveform
obtained by the tunnel current has a large positive−negative
asymmetry (e.g., the ratio between the intensities of the ﬁrst
and second peaks) compared with the waveforms obtained byphotoelectron emission shown in Figure 2. This is because theTHz near ﬁeld itself has positive−negative asymmetry, as
shown in Figure 3d, and the tunnel current induced by thenear ﬁeld is further rectiﬁed (the asymmetry is enhanced) by
the nonlinearity in the I−V curve, as schematically shown in
Figure 3h. Although accurate characterization of the THz
waveform with the tunnel current requires a correction using
the I−V curve, as shown here, it was conﬁrmed that the THz
near-ﬁeld proﬁle can be observed even in the tunnel current
regime.
As has been shown, transient I−V measurement ispotentially a very powerful technique for elucidating theexcited electronic states and dynamics of materials with atomic
spatial resolution. For this purpose, it is indispensable toevaluate the tip-enhanced near ﬁeld waveform. Since the tipenhanced near ﬁeld can be changed by adjusting theexperimental conditions, environmental conditions, and theshape of the tip, it is extremely important to directly conﬁrm
the near-ﬁeld waveform using the method described in thispaper, in addition to by simulation such as by the ﬁnite
diﬀerence time domain (FDTD) method.
Measurement of Time-Resolved Signal using THz
Near Field as a Bias Voltage. In the last part, we show theresult of time-resolved measurement using 2H-MoTe2 for asample. We excited (pumped) the sample using IR light andmeasured the dynamics of the excited state using the THz
electric ﬁeld as the instantaneous bias voltage applied between
the STM tip and sample. The bulk 2H-MoTe2 used in thisstudy has an indirect band structure, but since it has direct
band gap of 1.1 eV at the k point,22 it is possible to induce theoptical transition with the IR laser of our system (1035 nm).
Figure 4 shows the experimental scheme and a typical result.
Figure 4a shows an STM image of the sample surface. The
bright spots observed in the image are attributed to intrinsic
defects such as vacancies.23 Figure 4b shows the delay-timedependence of ITHz (red line). The enhanced near ﬁeld of theTHz probe pulse used for measurement, whose waveform wasevaluated by the photoelectron method (blue line), is
superimposed. The insets show the relationship between thepump and probe lights for td < 0 and td > 0. On the one hand,for td > 0, similar to the case of Bi2Se3, the THz near-ﬁeld
waveform was observed through the fast response of thesample such as hot electrons, although the signal was small. On
the other hand, for td < 0, since the THz pulse was irradiatedafter the IR pulse, as shown in the inset, ITHz reﬂects the
dynamics of the excited state due to photocarrier generation.
Since 2H-MoTe2 is a conventional semiconductor, in contrast
to Bi2Se3, photocarriers excited by optical pulses induce asurface photovoltage,24,25 leading to a change in ITHz. Thus, thedelay-time dependence of ITHz for td < 0 represents the carrier
decay process in 2H-MoTe2. Figure 4e illustrates energy band
diagrams of the STM tunneling junction in the steady state(without illumination) and the THz pulse-induced nonequilibrium state at the peak of the THz ﬁeld (I−III). In thenonequilibrium state, when both THz and IR pulse illuminate
the tunnel junction, photocarriers generated by IR pulse screen
the THz ﬁeld in semiconductor, and THz ﬁeld is fully applied
to the vacuum gap, as shown in II. Without photocarriers, THz
near ﬁeld partially penetrates the semiconductor surface andinduces surface band bending shown in I, which reduces
eﬀective THz ﬁeld applied to the vacuum gap and suppresses
ITHz. Therefore, in the case of III, ITHz decreases as td becomes
longer where photocarriers were decreases due to carrier
recombination.
Figure 4c shows the results of measurement over a wide
delay-time range, focusing on the td < 0 side. Because of thelimited range of the delay time, the appearance of the entire
long decay component was not clear. Here, we assumed asimple model, and the relaxation of the excited state was wellﬁtted with a two-component exponential function with the
lifetimes of τfast = 18 ps and τslow = 606 ps. On the one hand,τslow, which was considered to be the carrier recombination
lifetime, was also conﬁrmed by optical pump−probe measurement.22 On the other hand, the fast relaxation time τfast
appeared when the pump ﬂuence was increased more than
1.5 mJ/cm2 as shown in light intensity dependence (Figure4d). The result suggests that the fast carrier recombination was
due to the capture of electrons and holes by defects via Auger
processes, as was observed in other transition-metal
dichalcogenide monolayer (TMDC) materials,26 because themechanism is dominant when the photocarrier density ishigh.27 Since the signal originated from the hot electrons in thecase of the Bi2Se sample, the conductance increased for
positive and negative bias voltages to give similar results. In thecase of 2H-MoTe, since band bending and other surface
characteristics are involved, the bias-voltage dependence of
signal may become more complicated. We leave a detailed
analysis to future work.

■

CONCLUSION

The combination of CEP-controlled monocycle pulses in theTHz regime with metal nanostructures such as nanotips has
interesting novel applications, which require a precise understanding of the THz waveform. However, when a THz electricﬁeld is irradiated to a nanostructure, the electric ﬁeld is
ampliﬁed, although its waveform is also modulated. Therefore,
it is urgently necessary to develop a new method to directly
measure and evaluate near-ﬁeld waveforms in actual measurement environments.
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ACS Photonics 2019, 6, 1356−1364

ACS Photonics

LetterJPMJPR1427) grants from JST and Murata Science
Foundation. We thank Y. Kawada, H. Takahashi, andHamamatsu Photonics K. K. for fabricating the achromatic
terahertz quarter-wave plate.

We have succeeded in developing a new spectroscopy
technique capable of visualizing the spatially resolved THz
near ﬁeld at a nanotip by controlling the photoelectron
emission from the nanotip with subcycle time resolution. In
addition, using the STM system in our experimental setup, we
successfully demonstrated that the waveform can also be
observed in the tunnel regime. In the tunnel regime, it wasdemonstrated that the transient electronic state excited by an
optical pulse can be measured using the THz ﬁeld as theinstantaneous bias voltage. The ultrafast carrier dynamics in
2H-MoTe2 excited by an optical pulse was reproduciblyprobed.
Considering the rapid development of current research on
the measurement and control of the extreme quantum limit via
light, CEP technology will be a key to obtaining a deeper
understanding of quantum processes and for the developmentof new devices such as to investigate the dynamics of hotelectrons28 and spins29−32 and to perform single-molecule
measurement,33,34 for example. Our method, which enables theevaluation and accurate control of the CEP, is thus expected toplay an essential role in the further advancement of science andtechnology.

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ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsphotonics.9b00266.
Schematic of experimental setup, data plots showing
dependence of THz waveform at tip apex on electricﬁeld strength, frequency dependence of THz electricﬁeld modulated by the two types of nanotips, FDTD
simulations of near ﬁeld, measurement results for
HOPG, relationship between tip-enhanced near ﬁeld
strength and THz intensity, tunnel spectrum of Bi2Se3,
tip−sample distance dependence of the near-ﬁeld
waveform at the tip apex (PDF)■

REFERENCES

AUTHOR INFORMATION

Corresponding Author*E-mail: hidemi@bk.tsukuba.ac.jp.
ORCID

Yoshihiko Kanemitsu: 0000-0002-0788-131X
Hidemi Shigekawa: 0000-0001-9550-5148
Author Contributions

S.Y., H.H., and T.T. contributed equally to this research byadvancing the THz-STM system and performing experiments.
K.Y. contributed to the transient spectroscopy measurement,
and FDTD simulations were performed by H.H., T.T., andY.S., while Z.W. performed OPP measurement for comparison.
Y. A., O. T. and Y.K. provided technical advice. H.S. organized
and supervised the project and edited the paper with S.Y. andthe other authors.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
H.S. and T.T. acknowledge the ﬁnancial support of a Grant-inAid for Scientiﬁc Research (17H06088 and 19H02562,
respectively) from Japan Society for the Promotion of Science.
H.H. acknowledges the ﬁnancial support of PRESTO (No.
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