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-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, Hidemi Shigekawa,

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.9b00266

The recent development of optical technology has enabled the practical use of a carrier-envelope phase-controlled monocycle electric field in the terahertz (THz) regime. By combining this technique with metal nanostructures such as 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 resolution of 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 enhanced near 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 evaluated using the THz-STM, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.

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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Story Source:

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-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.