NW2A.3.pdf

Nonlinear Optics(NLO) © OSA 2017

Coherent Manipulation of Electrons in a Tunnel Junction
with Carrier-Envelope Phase Controlled THz Electric Fields
1

K. Yoshioka1, I. Katayama1, Y. Arashida1, Y. Minami2, M. Kitajima1,3,
S. Yoshida4, H. Shigekawa4, and J. Takeda1
Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa, 240-8501, Japan
2
Graduate School of Science and Technology, Tokushima University, Minami-josanjima 2-1, Tokushima 770-8506, Japan
3
LxRay Co. Ltd., Nishinomiya 663-8172, Japan4
Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8571 Japan
jun@ynu.ac.jp: katayama@ynu.ac.jp: hidemi@ims.tsukuba.ac.jp

Abstract: We demonstrate that single-cycle terahertz electric fields manipulate the motion ofelectrons in a single tunnel junction. The direction of the electron tunneling through the junction
strongly depends on the carrier-envelope phase of terahertz pulses.
OCIS codes: (240.7040) Tunneling; (270.1670) Coherent optical effects; (320.7110) Ultrafast nonlinear optics

1. Introduction
The latest advances in nanotechnology have made it possible to fabricate quantum nanocircuits and plasmonic
devices [1]. In these devises, electron tunneling at subnanometer gap plays a crucial role for its functionalities.
Therefore, improved control of electrons in a tunnel junction is indispensable for the development of nextgeneration quantum nanoelectronics.
Coherent control of electrons via the carrier-envelope phase (CEP) of ultrashort laser pulses is a promising way
to overcome the bandwidth limitation of the modern electronics. Developing such an approach for a tunnel junction
will provide new means to govern electrons on the nanoscale. The CEP-dependent control of electrons has beenachieved using intense few-cycle near-infrared pulses [2,3]. However, these pulses can easily damage the tunnel
junction due to thermal expansion because of its relatively high energy. Recently, we have shown that the electron
tunneling can be controlled by the single-cycle terahertz (THz) electric fields in percolated Au nanostructures [4]without any thermal effects. Because of its huge nonlinearities, a combination of the tunnel junction and the singlecycle THz electric fields is highly expected to be a new platform of the CEP-controlled electronics. In this work, wedemonstrate coherent manipulation of electrons in a tunnel junction using THz scanning tunneling microscopy(THz-STM) [5,6]. By tuning the CEP of single-cycle THz electric fields, direction of the electron tunneling wascoherently controlled either from a nanotip to a sample or vice versa.
2. Experimental Setup
Figure 1 schematically illustrates the experimental setup for theTHz-STM. The light source was a Ti-Sapphire regenerative
amplifier with center wavelength of 800 nm, pulse duration of 130
fs and repetition of 1 kHz. Intense single-cycle THz electric fields
were generated using a LiNbO3 prism in a tilted-pulse-front
configuration. The THz pulses were guided into one of two paths bytuning a removable gold-coated mirror: one was used forcharacterizing the THz waveforms by electro-optic sampling (EOS)
and the other was used for delivering the THz pulses to a tunnel
junction of the STM. For the passage to the STM, the THz pulses
were focused onto the apex of an electrochemically etched Pt/Ir
nanotip, which were polarized parallel to the nanotip. We usedhighly oriented pyrolytic graphite (HOPG) as a sample because ofits atomically flat surface. All measurements were performed underambient laboratory conditions.

STM
Lens
Removable
mirrorWire gridpolarizers
LiNbO3

BalacedphotodiodEO
crystalLens
QWP
Wollastonprism

3. Results and Discussion
As shown in Fig. 2(a), the CEP-controlled single-cycle THz electricFig. 1 Schematic of the experimental setup
field was successfully generated with different CEPs (φCEP = 0, π/2and π) via the Gouy phase shift [7]. Figure 2(b) shows the CEP dependence of the tunnel current while sweeping theDC bias from 300 mV to -300 mV. The most remarkable feature in Fig. 2(b) is a series of THz-induced pulse trains,

NW2A.3.pdf

(a)

(b)
2-2e0.1
φCEP = π /2-4-6

φCEP = 0

φCEP = 0

φCEP = π /20.0e

φCEP = π

φCEP = π /2-0.1-8-10(c)φCEP = 00Current (nA)Electric Field (kV/cm)

Nonlinear Optics(NLO) © OSA 2017

φCEP = π0 2 4 6 8 10 12Time (ps)e300

2001000-100-200-300

φCEP = πDC bias (mV)Fig. 2 (a) Waveforms of single-cycle THz electric fields with different CEPs (φCEP = 0, π/2 and π) measured using EOS. (b) CEP dependence oftunnel current as a function of DC bias measured using a digital oscilloscope. The spectra with φCEP=0 and π are offset by ±0.07 nA for clarity.
(c) Schematics illustration of the electron tunneling processes through the junction. The orange and blue arrows show the tunneling direction.

which is a fingerprint of the ultrafast current bursts driven by THz electric fields. In the case of φCEP = 0, the pulse
train takes a positive value, which corresponds to an electron tunneling from the nanotip to the sample. In the case of
φCEP = π, on the other hand, the tunnel current shows the opposite behavior; the pulse train with a negative value
indicates an electron tunnelling from the sample to the nanotip. In the case of φCEP = π/2, the direction of electron
tunneling strongly depends on the DC bias; electrons undergo tunneling from the nanotip to the sample under thepositive DC bias and in the opposite direction under the negative DC bias. The experimental data were analyzed
based on the Simmons model [8] to discuss the dynamics of electron tunneling in a quantitative manner. From theanalysis, we found the extremely huge enhancement of the THz electric field at the junction with a factor of 100,000± 10,000. We also revealed that enhanced THz electric field reaches 16 V/nm at the junction which is two times
higher than the strongest THz electric field previously reported in free space [9]. This spatially confined intense
single-cycle THz electric field can steer ~300,000 electrons in an extremely nonlinear regime on the subpicosecond
timescale [6].
4. Conclusion
In conclusion, we have demonstrated coherent manipulation of electrons in a tunnel junction by utilizing CEPcontrolled THz electric fields. Direction of the electron tunneling was coherently controlled either from a nanotip toa sample or vice versa by tuning the CEP of single-cycle THz electric fields. We believe that our concept provides anew platform for the ultrafast coherent control of electrons, and may inspire a new route towards designing future
lightwave nanoelectronics.
5. References[1] W. Zhu et al., "Quantum mechanical effects in plasmonic structures with subnanometre gaps," Nat. Commun. 7, 11495 (2016).
[2] M. Krüger, M. Schenk, P. Hommelhoff, "Attosecond control of electrons emitted from a nanoscale metal tip," Nature 475, 78–81 (2011).
[3] A. Schiffrin et al., "Optical-field-induced current in dielectrics," Nature 493, 70–74 (2012).
[4] K. Yoshioka et al., "Terahertz-Field-Induced Nonlinear Electron Delocalization in Au Nanostructures," Nano Lett. 15, 1036–1040 (2015).
[5] T. L. Cocker et al., "An ultrafast terahertz scanning tunnelling microscope," Nat. Photonics. 7, 620–625 (2013).
[6] K. Yoshioka et al., "Real-space coherent manipulation of electrons in a single tunnel junction by single-cycle terahertz electric fields," Nat.
Photonics. 10, 762–765 (2016).
[7] S. Feng, H. G. Winful, "Physical origin of the Gouy phase shift," Opt. Lett. 26, 485–487 (2001).
[8] J. G. Simmons, "Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film," J.
Appl. Phys. 34, 1793–1803 (1963).
[9] M. Shalaby, C. P. Hauri, "Demonstration of a low-frequency three-dimensional terahertz bullet with extreme brightness," Nat. Commun. 6,5976 (2015).