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.

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

• 
  • 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.