Abstract-Terahertz Faraday and Kerr rotation spectroscopy of Bi 1 − x Sb x films in high magnetic fields up to 30 tesla

Xinwei Li, Katsumasa Yoshioka, Ming Xie, G. Timothy Noe, II, Woojoo Lee, Nicolas Marquez Peraca, Weilu Gao, Toshio Hagiwara, Ørjan S. Handegård, Li-Wei Nien, Tadaaki Nagao, Masahiro Kitajima, Hiroyuki Nojiri, Chih-Kang Shih, Allan H. MacDonald, Ikufumi Katayama, Jun Takeda, Gregory A. Fiete, and Junichiro Kono

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.115145

We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of ${\text{Bi}}_{1-x}{\text{Sb}}_x$, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition $x$ increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed minicoil magnet, we conducted measurements in magnetic fields up to 30 T, observing distinctly different behaviors between semimetallic ($x<0.07$) and topological insulator ($x>0.07$) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blueshifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields ($>20$ T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$.

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Abstract-Nonlinear terahertz dynamics of Dirac electrons in Bi thin films

Ikufumi Katayama, Yasuo Minami, Yusuke Arashida, Orjan Sele Handegard, Tadaaki Nagao, Masahiro Kitajima, Jun Takeda

https://www.spiedigitallibrary.org/conference-presentations/10756/107560P/Nonlinear-terahertz-dynamics-of-Dirac-electrons-in-Bi-thin-films/10.1117/12.2320801?contentType=Conference_Presentations&SSO=1&startYear=2018&endYear=2018&term=terahertz%7c%7c

By using both linear and nonlinear terahertz spectroscopy on epitaxial Bi and Bi1-xSbx thin films, we systematically investigated the linear and nonlinear terahertz dynamics of Dirac electrons. The linear terahertz transmittance was analyzed by the Drude model up to 50 THz, and then the plasma frequency and the damping constant were evaluated as functions of the film thickness and Sb-concentration. We found surface metallic state for Bi ultra-thin films, while semimetal to semiconductor crossover for Bi1-xSbx thin films. In the nonlinear terahertz spectroscopy, the terahertz transmittance increases with increasing the field strength, which could be assigned to the carrier acceleration along the Dirac-like band dispersion at the L point in the Brillouin zone. In addition, we observed the terahertz-induced absorption in terahertz-pump and terahertz-probe spectroscopy, which could be assigned to carrier generation due to Zener tunneling in Dirac band structure. The results demonstrate that Bi-related materials are promising candidates for future nonlinear terahertz devices.

© 2018 COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Light switch: Scientists develop method to control nanoscale manipulation in high-powered microscopes

Prof. Jun Takeda (left) and Katsumasa Yoshioka (right).

Credit: Yokohama National University

(a) Schematic of phase-controlled THz-STM. (b) Ultrafast current burst induced by phase-controlled and delay-controlled double THz near-fields. A sinusoidal THz near field produces ultrafast bidirectional current burst between a sample and a nanotip. By precisely tuning the carrier envelope phase (CEP) of THz near field, the direction and the timing of the current burst can be desirably manipulated on the femotosecond timescale. Credit: Yokohama National University

https://www.sciencedaily.com/releases/2018/10/181011112418.htm

Researchers from Japan have taken a step toward faster and more advanced electronics by developing a way to better measure and manipulate conductive materials through scanning tunneling microscopy.

The team published their results in July in Nano Letters, an American Chemical Society journal. Scientists from the University of Tokyo, Yokohama National University, and the Central Research Laboratory of Hamamatsu Photonics contributed to this paper.

Scanning tunneling microscopy (STM) involves placing a conducting tip close to the surface of the conductive material to be imaged. A voltage is applied through the tip to the surface, creating a "tunnel junction" between the two through which electrons travel.

The shape and position of the tip, the voltage strength, and the conductivity and density of the material's surface all come together to provide the scientist with a better understanding of the atomic structure of the material being imaged. With that information, the scientist should be able to change the variables to manipulate the material itself.

Precise manipulation, however, has been a problem -- until now.

The researchers designed a custom terahertz pulse cycle that quickly oscillates between near and far fields within the desired electrical current.

"The characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the nanoscale," said Jun Takeda, a professor in the department of physics in the Graduate School of Engineering at Yokohama National University. "We demonstrated that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy with a phase shifter."

According to Takeda, previous studies in this area assumed that the near and far fields were the same -- spatially and temporally. His team examined the fields closely and not only identified that there was a difference between the two, but realized that the pulse of fast laser could prompt the needed phase shift of the terahertz pulse to switch the current to the near field.

"Our work holds enormous promise for advancing strong-field physics in nano-scale solid state systems, such as the phase change materials used for optical storage media in DVDs and Blu-ray, as well as next-generation ultrafast electronics and microscopies," Takeda said.

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Materials provided by Yokohama National University. Note: Content may be edited for style and length.

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Journal Reference:

1. Katsumasa Yoshioka, Ikufumi Katayama, Yusuke Arashida, Atsuhiko Ban, Yoichi Kawada, Kuniaki Konishi, Hironori Takahashi, Jun Takeda. Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields. Nano Letters, 2018; 18 (8): 5198 DOI: 10.1021/acs.nanolett.8b02161

Abstract-Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields

Katsumasa Yoshioka, Ikufumi Katayama, Yusuke Arashida, Atsuhiko Ban, Yoichi Kawada, Kuniaki Konishi, Hironori Takahashi,  Jun Takeda,

https://pubs.acs.org/doi/10.1021/acs.nanolett.8b02161

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip–sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription—in situ tailoring of single-cycle THz near fields in a tunnel junction—will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology

Abstract-Terahertz-induced acceleration of massive Dirac electrons in semimetal bismuth

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  • Yasuo Minami
  • , Kotaro Araki
  • , Thang Duy Dao
  • , Tadaaki Nagao
  • , Masahiro Kitajima
  • , Jun Takeda
  •  & Ikufumi Katayama

http://www.nature.com/articles/srep15870

Dirac-like electrons in solid state have been of great interest since they exhibit many peculiar physical behaviors analogous to relativistic mechanics. Among them, carriers in graphene and surface states of topological insulators are known to behave as massless Dirac fermions with a conical band structure in the two-dimensional momentum space, whereas electrons in semimetal bismuth (Bi) are expected to behave as massive Dirac-like fermions in the three-dimensional momentum space, whose dynamics is of particular interest in comparison with that of the massless Dirac fermions. Here, we demonstrate that an intense terahertz electric field transient accelerates the massive Dirac-like fermions in Bi from classical Newtonian to the relativistic regime; the electrons are accelerated approaching the effective “speed of light” with the “relativistic” beta β = 0.89 along the asymptotic linear band structure. As a result, the effective electron mass is enhanced by a factor of 2.4.

Abstract-Terahertz-Field-Induced Nonlinear Electron Delocalization in Au Nanostructures

Katsumasa Yoshioka †, Yasuo Minami *†, Ken-ichi Shudo †, Thang D. Dao ‡§, Tadaaki Nagao ‡§,Masahiro Kitajima †‡§∥⊥, Jun Takeda *†, andIkufumi Katayama *†

† Department of Physics, Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan

‡ International Center for Materials Nanoarchitectonics,National Institute for Materials Science, Tsukuba 305-0044, Japan

§ CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan

∥ LxRay Co. Ltd., Nishinomiya 663-8172, Japan

⊥ Department of Applied Physics, National Defense Academy, Yokosuka 239-8686, Japan

Nano Lett., Article ASAP

DOI: 10.1021/nl503916t

Publication Date (Web): January 5, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl503916t

*E-mail: (J.T.) jun@ynu.ac.jp., *E-mail: (Y.M.)minamiyasuo@ynu.ac.jp., *E-mail: (I.K.) katayama@ynu.ac.jp.

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond time scale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field of THz pulses; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.

Abstract-Terahertz-field-induced Nonlinear Electron Delocalization in Au Nanostructures

Katsumasa Yoshioka , Yasuo Minami , Ken-ichi Shudo , Thang Duy Dao , Tadaaki NAGAO ,Masahiro Kitajima , Jun Takeda , and Ikufumi Katayama

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl503916t

Publication Date (Web): January 5, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl503916t?journalCode=nalefd

Improved control over the electromagnetic properties of metal nanostructures is indispensable for the development of next-generation integrated nanocircuits and plasmonic devices. The use of terahertz (THz)-field-induced nonlinearity is a promising approach to controlling local electromagnetic properties. Here, we demonstrate how intense THz electric fields can be used to modulate electron delocalization in percolated gold (Au) nanostructures on a picosecond timescale. We prepared both isolated and percolated Au nanostructures deposited on high resistivity Si(100) substrates. With increasing the applied THz electric fields, large opacity in the THz transmission spectra takes place in the percolated nanostructures; the maximum THz-field-induced transmittance difference, 50% more, is reached just above the percolation threshold thickness. Fitting the experimental data to a Drude-Smith model, we found furthermore that the localization parameter and the damping constant strongly depend on the applied THz-field strength. These results show that ultrafast nonlinear electron delocalization proceeds via strong electric field acceleration; the intense THz electric field modulates the backscattering rate of localized electrons and induces electron tunneling between Au nanostructures across the narrow insulating bridges without any material breakdown.