LETTERS
PUBLISHED ONLINE: 7 NOVEMBER 2016 | DOI: 10.1038/NPHOTON.2016.205

Real-space coherent manipulation of electrons
in a single tunnel junction by single-cycle
terahertz electric ﬁelds
Katsumasa Yoshioka1, Ikufumi Katayama1*, Yasuo Minami1, Masahiro Kitajima1,2, Shoji Yoshida3,
Hidemi Shigekawa3* and Jun Takeda1*
The ultrafast coherent manipulation of electrons using waveform-controlled laser pulses1–9 is a key issue in the development of modern electronics10,11. Developing such an approach
for a tunnel junction will provide a new platform for governing
ultrafast currents on an even smaller scale, which will be indispensable for the advancement of next-generation quantum
nanocircuits12–15 and plasmonic devices16–18. Here, we demonstrate that carrier-envelope-phase-controlled single-cycle terahertz electric ﬁelds can coherently drive electron tunnelling
either from a nanotip to a sample or vice versa. Spatially conﬁned electric ﬁelds of more than 10 V nm–1 strongly modulate
the potential barrier at a nanogap in a scanning tunnelling
microscope (STM) on the subpicosecond timescale and cansteer a large number of electrons in an extremely nonlinear
regime, which is not possible using a conventional STM. Our
results are expected to pave the way for the future developmentof nanoscale science and technologies.
The latest advances in laser technology have made it possible tocontrol the carrier-envelope phase (CEP) of ultrashort laser pulses
with high accuracy. Using few-cycle CEP-controlled near-infraredlaser pulses, several studies have demonstrated the ultrafast coherent
control of electrons in a wide range of systems1–9. In these studies,
metallic nanostructures, so-called metal nanotips, were used to
produce highly localized near-ﬁelds by concentrating laser pulses
into dimensions far below the diffraction limit of the incident light.
Using a spatiotemporally tunable near-ﬁeld of up to ∼10 V nm–1,
ultrafast electron bursts from a nanotip have been realized7–9.
These sophisticated electron manipulations will open fascinating
avenues for overcoming the bandwidth limitation of signal processing
in modern information technology10,11.
A larger ﬁeld enhancement is expected to occur at a tunnel
junction12,14,18,19. The resulting strong nonlinearity will be usefulfor the manipulation of electrons at the atomic scale, which ishighly desirable for the advancement of next-generation integrated
nanocircuits12–15 and plasmonic devices16–18. However, strong nearinfrared laser pulses focused on a tunnel junction induce thermal
expansion20, leading to permanent damage of the junction.
Therefore, precise control of the motion of electrons at the atomic
scale is still a challenging task. Recently, single-cycle electric ﬁeld
transients with high intensity have been generated in the terahertz(THz) spectral range (0.1–10 THz)21,22. In contrast to few-cycle
near-infrared laser pulses, single-cycle THz electric ﬁelds with abandwidth over an octave may be useful for accurately steering electrons, and may drive electron tunnelling without heating systems
because of their low energy. Indeed, THz-ﬁeld-induced nonlinear

electron tunnelling has been observed in percolated gold nanostructures23 and metal–graphene–metal hybrid structures18 without
any thermal effects. Terahertz scanning tunnelling microscopy(THz-STM)19, which was developed after optical pump–probe
STM24,25, may allow ultrafast dynamics to be probed with fewer
thermal expansion problems.
Here, we demonstrate real-space coherent manipulation of the
motion of electrons in a single tunnel junction by utilizing CEPcontrolled single-cycle THz electric ﬁelds via the Gouy phaseshift26. Unlike the imaging of surfaces at the atomic scale using conventional STM, our THz-STM with the tunnel junction presented
here acts as an ultrafast rectifying diode or a THz ﬁeld-effect transistor7,27; the electron current is switched on and off via the CEPcontrolled single-cycle THz electric ﬁelds. The basic concept of
our experimental set-up is illustrated in Fig. 1a–c. Intense singlecycle THz electric ﬁeld transients were generated by optical rectiﬁcation of femtosecond laser pulses in a LiNbO3 prism using a tiltedpulse-front conﬁguration21. The generated THz pulses were guided
into one of two optical paths: one was used for characterizing theTHz waveforms by electro-optic sampling (EOS), while the otherwas used to deliver the THz pulses to a single tunnel junction.
The THz pulses were focused onto the apex of a Pt/Ir nanotip.
We used highly oriented pyrolytic graphite (HOPG) as the sample
because of its atomically ﬂat surface. All measurements were performed under ambient laboratory conditions. By placing either apair of spherical or cylindrical lenses in the beam path (Fig. 1b),
the CEP of the incident THz pulses (ϕCEP = 0) was shifted by
ϕCEP = π or ϕCEP = π/2, respectively, via the Gouy phase shift. As
schematically shown in Fig. 1c, the CEP-controlled single-cycle
THz electric ﬁeld modiﬁes the potential barrier between the nanotip
and the sample, leading to the unidirectional coherent motion of electrons. By irradiating the junction with THz pulses with ϕCEP = 0(Fig. 1d), a series of tunnel currents were synchronously generated
with the same repetition rate as the laser system (1 kHz), as shownin Fig. 1e. Note that the time-integrated value of each current pulse
represents the number of rectiﬁed electrons driven by a single THz
pulse through the junction (see Supplementary Section I).
The CEP dependence of the tunnel current was measured withdifferent CEPs of ϕCEP = 0, π/2 and π, while sweeping the d.c. bias
from 300 mV to –300 mV (Fig. 2a,b). The most remarkablefeature in Fig. 2b is a series of THz-induced pulse trains, which isa ﬁngerprint of the ultrafast current bursts in the tunnel junction.
In the case of ϕCEP = 0, the pulse train takes a positive value,
which corresponds to an electron tunnelling from the nanotip to
the sample. The intensity of the pulse train increases with increasing

1

Department of Physics, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan. 2 LxRay Company Limited,
Nishinomiya 663-8172, Japan. 3 Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan.
* e-mail: katayama@ynu.ac.jp; hidemi@ims.tsukuba.ac.jp; jun@ynu.ac.jp
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LETTERSd

OAP
Nanotip

EOS

0.6

Electric field (kV cm−1)a

OAP
Removable
mirrorIntensity (a.u.)

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2016.205

0.4
0.2

101
10−2
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0.0
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2

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LiNbO3

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or

or

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barrier
e−

0.08
0.06
0.04
0.02
0.00
0

1

2
Time (ms)Figure 1 | Experimental set-up of the THz-STM. a, Schematic of the experimental set-up. CEP-controlled single-cycle THz electric ﬁelds are focused onto
the apex of a Pt/Ir nanotip at an incident angle of 75°. OAP, off-axis parabolic mirror; EOS, electro-optic sampling. b, Schematic illustration of π phase-shiftof a THz electric ﬁeld via the Gouy phase shift using a pair of spherical lenses (dashed square in a). A pair of cylindrical lenses is used when a π/2 phaseshift is required. c, Schematic illustration of an electron (e–) tunnelling between a nanotip and a sample under the inﬂuence of a static electric ﬁeld (dashedcircle in a). The motion of electrons is coherently controlled by THz electric ﬁelds with different CEPs. d, Temporal proﬁle of a single-cycle THz electric ﬁeld
(ϕCEP = 0) measured using EOS. The inset shows the corresponding THz frequency spectrum. e, Pulse train generated by the THz electric ﬁeld (ϕCEP = 0)
without any d.c. bias. The feedback loop remained off during the measurement (setpoint: bias voltage Vs = 1 V; tunnel current Is = 1 nA). The decay proﬁle ofeach current pulse comes from the bandwidth of the ampliﬁer used in the STM circuits.

c

b

2

ϕCEP = 0
ϕCEP = 0

0.1
ϕCEP = π/2−4−6Current (nA)Electric field (kV cm−1)

0−2

ϕCEP = 0

ϕCEP = π/2

0.0

e−

ϕCEP = π

ϕCEP = π/2

−0.1

−8

d
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ϕCEP = π6Rectified electrons (×104)

a

E

4

ϕCEP = π

2

t

0
E

−2

ϕCEP = 0
−4

e−

−10
0 2 4 6 8 10 12Time (ps)

300

e

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100
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d.c. bias (mV)

4

f

eV

Time (ps)Potential
barrier

−300

t
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Barrier height (eV)
3 2 1 0 −1 −2 −3

s

M2

0.8

g
4

7

3

6
5
4
3

M1

−0.4
0.0
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Vacuum level

e−

−200

ϕCEP = π

5.7 ps
e−

2
1
0
−1

e−
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−2
−3

2
4 2 0 −2
Electric field (V nm−1)

0.2

0.4
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Position (nm)

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0
Position (nm)Figure 2 | Effect of CEP on the motion of tunnelling electrons. a, Temporal proﬁles of single-cycle THz electric ﬁelds with different CEPs (ϕCEP = 0, π/2 and π).
b, CEP dependence of tunnel current as a function of d.c. bias (Vs = 1 V, Is = 0.5 nA). The spectra with ϕCEP = 0 and π are offset by ±0.07 nA for clarity.
c, Schematic illustration of the motion of electrons driven by THz electric ﬁelds with different CEPs. The orange and blue arrows show the tunnelling direction.
d, Number of rectiﬁed electrons induced by a single THz pulse without d.c. bias as a function of the peak electric ﬁeld (Vs = 1 V, Is = 1 nA). The insets show thecorresponding incident THz waveforms with different CEPs (ϕCEP = 0, π). The dashed curve shows the best ﬁt obtained by the Simmons model. e, Potential
barrier between nanotip (M1) and sample (M2) under a bias voltage V, where s is the width of the gap between the tip and the sample. f, Time dependence ofpotential barrier under an enhanced THz electric ﬁeld. g, Potential barriers at t = 5 and t = 5.7 ps, which are indicated by the dashed lines in f. The electron
undergoes tunnelling in the smaller potential barrier at t = 5 ps rather than that at t = 5.7 ps as shown by the thicker and thinner arrows, respectively. The thicker
arrow indicates the higher probability of tunnelling.
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763

LETTERSa

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2016.205

Rectified electrons (×105)

3.0
2.5
0.1 mm

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Tip 1
Tip 2
Tip 3

1.0
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Incident electric field (kV cm−1)

b

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Tip 2
Tip 3

Rectified electrons

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105
4
2

104
4

Saturation regime
2

103
4

5

6

7

8

9

10

16

Enhanced electric field (V nm−1)

Figure 3 | Current saturation under strong electric ﬁelds. a, Number ofrectiﬁed electrons without d.c. bias as a function of incident electric ﬁeld
(ϕCEP = 0) for different tips (Vs = 1 V, Is = 1 nA). Tips 1 and 2 (sharp) were
fabricated by electrochemical etching whereas tip 3 (blunt) was formed bymechanical cutting. Insets: scanning electron microscopy images of the
sharp (top) and blunt (bottom) tips. b, Log-scale plot of number of rectiﬁed
electrons as a function of the enhanced electric ﬁeld at the tunnel junction.
The enhancement factor of tip 3 is 50,000 ± 200, which is two times
smaller than that of the sharp nanotips. Error bars correspond to 95%
conﬁdence intervals.

d.c bias. In the case of ϕCEP = π, on the other hand, the tunnel
current shows the opposite behaviour; the pulse train with a negativevalue indicates an electron tunnelling from the sample to thenanotip. In the case of ϕCEP = π/2, the direction of electron tunnelling strongly depends on the d.c. bias; electrons undergo tunnellingfrom the nanotip to the sample under a positive d.c. bias and in the
opposite direction under a negative d.c. bias. The current pulse disappears as the d.c. bias approaches 0 (see Supplementary Section II).
As schematically summarized in Fig. 2c, the coherent motion of electrons at the junction is controlled by the CEP-locked THz pulses witha given d.c bias.
Figure 2d shows the number of rectiﬁed electrons induced by asingle THz pulse as a function of the peak electric ﬁeld with different
CEPs (ϕCEP = 0 and ϕCEP = π). The nonlinear increase in thenumber of rectiﬁed electrons with the THz electric ﬁeld was numerically evaluated on the basis of the Simmons model28 assuming a
potential barrier between the nanotip (M1) and the sample (M2)under a bias voltage V, as illustrated in Fig. 2e. As shown by thedashed line in Fig. 2d, the experimental data are accurately reproduced with the adjustable parameters of an effective workfunctionof 3.8 ± 0.1 eV, a gap width of 1.00 ± 0.01 nm and an enhancement
factor of 100,000 ± 10,000. These values were also conﬁrmed bycurrent–distance (I–Z) and d.c. current–voltage (I–V ) experiments(see Supplementary Section III). The large ﬁeld enhancement originates from the extremely tight focusing of the THz electric ﬁeld
onto the single junction with a 1 nm gap between the sample andthe nanotip induced by the broadband antenna effect of thenanotip and/or the plasmonic effect at the tunnel junction8,14. The
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time-dependent modulation of the potential barrier was also calculated using the Simmons model. As shown in Fig. 2f,g, the potential
barrier is coherently distorted by the enhanced THz electric ﬁeld at
the junction. For example, the barrier height is reduced to 0.98 eV
whereas the barrier width decreases to 0.41 nm at the ﬁeld strengthof +5.3 V nm–1. In contrast, a ﬁeld strength of −2.0 V nm–1 causes
the barrier to shrink less, with the height and width reduced to1.78 eV and 0.75 nm, respectively. This potential asymmetry
driven by the CEP-locked single-cycle THz electric ﬁeld leads tounidirectional electron tunnelling through the junction on the
subpicosecond timescale.
By further increasing the THz electric ﬁeld, we can implement a
new regime for electron tunnelling that is inaccessible by conventional STM. The results are shown in Fig. 3a for three different
tips: two sharp nanotips (tips 1, 2) and one blunt tip (tip 3) asshown in the inset. The curves plotting the number of rectiﬁed electrons as a function of the incident electric ﬁeld exhibit different
behaviours for the tips. However, after converting the incident
electric ﬁeld into the enhanced electric ﬁeld, the curves exhibit
almost the same behaviour (Fig. 3b), indicating that the tip geometry
affects only the enhancement factor, and that the sharp tip cantightly focus the THz electric ﬁeld.
The striking feature in Fig. 3b is the strong saturation of the tunnelling electrons observed above the enhanced electric ﬁeld of 6 V nm–1,
which was not predicted by the conventional Simmons model. A
recent self-consistent calculation29 predicted a new regime that deviatesfrom the Simmons model at extremely high voltages, the so-called
space-charge-limited regime, in which an additional space-chargepotential plays an important role in limiting the current ﬂow in thejunction. In this calculation, the saturation of the tunnel currents
occurred above 6 V nm–1 with a workfunction of 4.08 eV; thesevalues are in reasonably good agreement with our results. Note that
this regime cannot be realized with a d.c. bias voltage or near-infrared
pulses because the junction is easily damaged by Joule heating andhigh-energy photons, respectively20. Finally, we stress that an enhanced
THz electric ﬁeld of 16 V nm–1 was achieved at the junction, which is
two times higher than the strongest THz ﬁeld previously reported infree space30. This spatially conﬁned large single-cycle THz electricﬁeld can coherently drive the motion of as many as ∼300,000 electrons
on the subpicosecond timescale.
In summary, we have demonstrated real-space coherent manipulation of electrons in a single tunnel junction. By utilizing CEPcontrolled single-cycle THz electric ﬁelds, electron tunnelling canbe induced either from the nanotip to the sample or vice versa.
The extremely large ﬁeld enhancement at the junction strongly
modulates the potential barrier between the nanotip and thesample, leading to the strong saturation of rectiﬁed electrons in aspace-charge-limited regime. We believe that this concept provides
a new platform for the ultrafast coherent control of electrons, andmay inspire a new route towards designing future nanoelectronics.
Furthermore, our CEP-controlled THz-STM with an extremely
high electric ﬁeld is expected to be a powerful tool for exploring
the ultrafast nonlinear control of matter22 at the atomic scale.

Methods
Methods and any associated references are available in the
Supplementary Information.
Received 19 April 2016; accepted 22 September 2016;
published online 7 November 2016

References

1. Paulus, G. G. et al. Measurement of the phase of few-cycle laser pulses. Phys. Rev.
Lett. 91, 253004 (2003).
2. Corkum, P. B. & Krausz, F. Attosecond science. Nat. Phys. 3, 381–387 (2007).
3. Schiffrin, A. et al. Optical-ﬁeld-induced current in dielectrics. Nature 493,
70–74 (2013).
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NATURE PHOTONICS DOI: 10.1038/NPHOTON.2016.205
4. Paasch-Colberg, T. et al. Solid-state light-phase detector. Nat. Photon. 8,
214–218 (2014).
5. Luu, T. T. et al. Extreme ultraviolet high-harmonic spectroscopy of solids.
Nature 521, 498–502 (2015).
6. Hohenleutner, M. et al. Real-time observation of interfering crystal electrons inhigh-harmonic generation. Nature 523, 572–575 (2015).
7. Krüger, M., Schenk, M. & Hommelhoff, P. Attosecond control of electrons
emitted from a nanoscale metal tip. Nature 475, 78–81 (2011).
8. Wimmer, L. et al. Terahertz control of nanotip photoemission. Nat. Phys. 10,
432–436 (2014).
9. Piglosiewicz, B. et al. Carrier-envelope phase effects on the strong-ﬁeld
photoemission of electrons from metallic nanostructures. Nat. Photon. 8,
37–42 (2014).
10. Caulﬁeld, H. J. & Dolev, S. Why future supercomputing requires optics. Nat.
Photon. 4, 261–263 (2010).
11. Krausz, F. & Stockman, M. I. Attosecond metrology: from electron capture to
future signal processing. Nat. Photon. 8, 205–213 (2014).
12. Ward, D. R., Hüser, F., Pauly, F., Cuevas, J. C. & Natelson, D. Optical rectiﬁcationand ﬁeld enhancement in a plasmonic nanogap. Nat. Nanotech. 5,
732–736 (2010).
13. Vincent, R., Klyatskaya, S., Ruben, M., Wernsdorfer, W. & Balestro, F. Electronic
read-out of a single nuclear spin using a molecular spin transistor. Nature 488,
357–360 (2012).
14. Yoshida, K., Shibata, K. & Hirakawa, K. Terahertz ﬁeld enhancement andphoton-assisted tunneling in single-molecule transistors. Phys. Rev. Lett. 115,
138302 (2015).
15. Sharma, A., Singh, V., Bougher, T. L. & Cola, B. A. A carbon nanotube opticalrectenna. Nat. Nanotech. 10, 1027–1032 (2015).
16. Savage, K. J. et al. Revealing the quantum regime in tunnelling plasmonics.
Nature 491, 574–577 (2012).
17. Tan, S. F. et al. Quantum plasmon resonances controlled by molecular tunnel
junctions. Science 343, 1496–1499 (2014).
18. Bahk, Y.-M. et al. Electromagnetic saturation of angstrom-sized quantum
barriers at terahertz frequencies. Phys. Rev. Lett. 115, 125501 (2015).
19. Cocker, T. L. et al. An ultrafast terahertz scanning tunnelling microscope. Nat.
Photon. 7, 620–625 (2013).
20. Grafström, S. Photoassisted scanning tunneling microscopy. J. Appl. Phys. 91,
1717–1753 (2002).
21. Hirori, H., Doi, A., Blanchard, F. & Tanaka, K. Single-cycle terahertz pulses withamplitudes exceeding 1 MV/cm generated by optical rectiﬁcation in LiNbO3.
Appl. Phys. Lett. 98, 091106 (2011).
22. Kampfrath, T., Tanaka, K. & Nelson, K. A. Resonant and nonresonant
control over matter and light by intense terahertz transients. Nat. Photon. 7,
680–690 (2013).

23. Yoshioka, K. et al. Terahertz-ﬁeld-induced nonlinear electron delocalization in
Au nanostructures. Nano Lett. 15, 1036–1040 (2015).
24. Terada, Y., Yoshida, S., Takeuchi, O. & Shigekawa, H. Real-space imaging oftransient carrier dynamics by nanoscale pump–probe microscopy. Nat. Photon.
4, 869–874 (2010).
25. Yoshida, S. et al. Probing ultrafast spin dynamics with optical pump–probe
scanning tunnelling microscopy. Nat. Nanotech. 9, 588–593 (2014).
26. Feng, S. & Winful, H. G. Physical origin of the Gouy phase shift. Opt. Lett. 26,
485–487 (2001).
27. Higuchi, T., Maisenbacher, L., Liehl, A., Dombi, P. & Hommelhoff, P.
A nanoscale vacuum-tube diode triggered by few-cycle laser pulses. Appl. Phys.
Lett. 106, 051109 (2015).
28. Simmons, J. G. Generalized formula for the electric tunnel effect between
similar electrodes separated by a thin insulating ﬁlm. J. Appl. Phys. 34,
1793–1803 (1963).
29. Zhang, P. Scaling for quantum tunneling current in nano- and subnano-scale
plasmonic junctions. Sci. Rep. 5, 9826 (2015).
30. Shalaby, M. & Hauri, C. P. Demonstration of a low-frequency three-dimensional
terahertz bullet with extreme brightness. Nat. Commun. 6, 5976 (2015).

Acknowledgements
This work was supported in part by the Grants-in-Aid for Scientiﬁc Research (numbers
15H05734, 16H03820, 16H04001 and 16H06010) from the Japan Society for the
Promotion of Science and the Ministry of Education, Culture, Sports, Scienceand Technology, and by the Strategic Information and Communications R&D
Promotion Programme (SCOPE #145003103) of Japan Ministry of Internal Affairs
and Communications.

Author contributions
I.K., M.K., H.S. and J.T. conceived and coordinated this project. K.Y. designed and built the
THz-STM set-up. K.Y. and Y.M. developed the intense THz generation system, and K.Y.
and S.Y. constructed the operation program of the STM. K.Y. carried out the experiments
and simulations with support from I.K., Y.M. and S.Y., and H.S., M.K. and J.T. contributed
to the initial concept of the experiments. K.Y. and J.T. wrote the manuscript with
contributions from all authors.

Additional information
Supplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondence andrequests for materials should be addressed to I.K., H.S. and J.T.

Competing ﬁnancial interests

The authors declare no competing ﬁnancial interests.

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