Abstract-Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses

Yasuyuki Sanari, Takehiro Tachizaki, Yuta Saito, Kotaro Makino, Paul Fons, Alexander V. Kolobov, Junji Tominaga, Koichiro Tanaka, Yoshihiko Kanemitsu, Muneaki Hase, and Hideki Hirori

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.165702

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ upon irradiation with an intense terahertz (THz) pulse. THz-pump–optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Abstract-Terahertz microscopy assisted by semiconductor nonlinearities

François Blanchard, Xin Chai, Tomoko Tanaka, Takashi Arikawa, Tsuneyuki Ozaki, Roberto Morandotti, and Koichiro Tanaka

Fig. 1. Experimental setup. (a) THz transmission scheme using wire-grid polarizer and a LN sensor. (b) Illustration of the sample used for near-field investigations. (c) Visible image of a gold ring array structure patterned on a In0.53Ga0.47As${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ epilayer thin film.

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-20-4997#articleFigures

Terahertz (THz) imaging is currently based on linear effects, but there is great interest on how nonlinear effects induced by terahertz radiation could be exploited to provide extra information that is unobtainable by conventional imaging schemes. In particular, at field strengths on the order of 100  kV cm−1$100\mathrm{kV}{\mathrm{cm}}^{-1}$ to 1  MV cm−1$1\mathrm{MV}{\mathrm{cm}}^{-1}$, transmission properties inside semiconductor materials are largely affected at the picosecond time-scale, which raise the prospect of interesting nonlinear imaging applications at THz frequencies. Here, we experimentally investigate a method to map the two-dimensional nonlinear near-field distribution of an intense THz pulse passing through a thin film-doped semiconductor. By inserting a metamaterial structure between the electro-optic sensor and the doped film, the nonlinear near-field dynamics shows a different and enhanced contrast of the sample when compared to its linear counterpart.

© 2018 Optical Society of America

Abstract-Control of Spoof Localized Surface Plasmons Using Terahertz Near-field Microscope

Takashi Arikawa, Shohei Morimoto, Tomoki Hiraoka, François Blanchard, Kyosuke Sakai, Keiji Sasaki, and Koichiro Tanaka

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=2704300894288992135

We performed time-resolved terahertz near-field imaging of a gold disk with sub-wavelength periodic grooves and successfully observed spoof localized surface plasmons. A selective excitation method is also demonstrated with orbital angular momentum of light.

© 2018 The Author(s)

Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water As Revealed by Terahertz Spectroscopy

Keiichiro Shiraga , Yuichi Ogawa, Koichiro Tanaka, Takashi Arikawa, Naotaka Yoshikawa, Masahito Nakamura, Katsuhiro Ajito, Takuro Tajima,

http://pubs.acs.org/doi/10.1021/acs.jpcb.7b11839

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broad-band THz spectroscopy (0.2–12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2, 4, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of urea concentrations up to 5 M, and this number is in close agreement with that of water constrained by strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to urea’s NH2 inversion. These experimental results point to the coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the −CO group and slight structural disturbance (the chaotropic aspect) by the −NH2 group.

Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water as Revealed by Terahertz Spectroscopy

Keiichiro Shiraga, Koichiro Tanaka, Yuichi Ogawa, Takuro Tajima

https://www.researchgate.net/publication/322148885_Coexistence_of_Kosmotropic_and_Chaotropic_Impacts_of_Urea_on_Water_as_Revealed_by_Terahertz_Spectroscopy

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broadband THz spectroscopy (from 0.2 to 12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2 THz, 4 THz, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of the urea concentrations up to 5 M and this number is close agreement with that of water constrained by the strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen-bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to the urea’s NH2 inversion. These experimental results point to coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the –CO group and slight structural disturbance (the chaotropic aspect) by the –NH2 group.

Abstract-Focusing light with orbital angular momentum by circular array antenna

Takashi Arikawa, Shohei Morimoto, and Koichiro Tanaka

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-12-13728&origin=search

We experimentally demonstrated focusing of light with orbital angular momentum (OAM) using an 8-element circular array of linear antennas. A spiral phase plate was used to generate a vortex beam with an OAM of ħ in the terahertz (THz) frequency region. We used THz near-field microscope to directly measure the phase vortex. A beam profile with a center dark spot and 2π phase rotation was observed in the small center gap region of the circular array antenna after the vortex beam excitation. The beam size is reduced by a factor of 3.4 ± 0.2. Half-wave resonance of the antenna element is responsible for the focusing function, indicating the scalability of this method to other frequency regions. This method will enable deep subwavelength focusing of light with OAM and eliminate the obstacle for the observation of the dipole forbidden transition with finite OAM of the vortex beam.

© 2017 Optical Society of America

Abstract-Strong Terahertz-Field Effect on Electron-Hole System in Quantum Wells

Koichiro Tanaka

https://www.osapublishing.org/abstract.cfm?URI=ls-2015-LM3H.2

We present strong terahertz light can modulate electron-hole pair creations in 2D-semiconductors. By means of THz pump and optical probe spectroscopy, we observed a strong spectral modulation of the 1s heavy-hole exciton peak of GaAs quantum wells due to Rabi splitting below 10 kV/cm. We also confirmed Franz-Keldysh type spectral modulation and its novel time-dependence in the higher field as high as 50 kV/cm.

© 2015 OSA

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Abstract-Terahertz Nonlinear Magnetic Response in in Antiferromagnets

Koichiro Tanaka, Y Mukai, H Hirori, T Yamamoto, and H Kageyama
https://www.osapublishing.org/abstract.cfm?uri=CLEO_SI-2015-STu2H.1

We report on the nonlinear magnetization dynamics of a HoFeO3 crystal induced by a strong terahertz magnetic field resonantly enhanced with a split ring resonator. The terahertz magnetic field induces a large magnetization change of 40% of the spontaneous magnetization. The frequency of the antiferromagnetic resonance decreases in proportion to the square of the magnetization change.

© 2015 OSA

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Abstract-Ultrafast Carrier Dynamics in Graphene under a High Electric Field

http://prl.aps.org/abstract/PRL/v109/i16/e166603

Shuntaro Tani^1,2,*, François Blanchard^2,3, and Koichiro Tanaka^1,2,3,†

¹Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
²CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

³Institute for Integrated Cell-Material Sciences (WPI-iCeMS),Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan

We investigated ultrafast carrier dynamics in graphene with near-infrared transient absorption measurement after intense half-cycle terahertz pulse excitation. The terahertz electric field efficiently drives the carriers, inducing large transparency in the near-infrared region. Theoretical calculations using the Boltzmann transport equation quantitatively reproduce the experimental findings. This good agreement suggests that the intense terahertz field should promote a remarkable impact ionization process and increase the carrier density.

Terahertz radiation induced ballistic electron transport in graphene

Shuntaro Tani, François Blanchard, Koichiro Tanaka

http://arxiv.org/abs/1206.1392

(Submitted on 7 Jun 2012)

We investigated ultrafast carrier dynamics in graphene with near-infrared transient absorption measurement after intense half-cycle terahertz pulse excitation. The terahertz electric field efficiently drives the carriers, inducing large transparency in the near-infrared region. Theoretical calculations using the Boltzmann transport equation quantitatively reproduce the experimental findings. This good agreement suggests that the intense terahertz field should promote remarkable impact ioniza- tion process, which leads to suppression of optical phonon emission and results in efficient carrier transport in graphene.

Comments:	4 pages, 3 figures
Subjects:	Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Cite as:	arXiv:1206.1392v1 [cond-mat.mes-hall]

Submission history

From: Shuntaro Tani [view email]
[v1] Thu, 7 Jun 2012 04:09:09 GMT (468kb)

Terahertz Pulse Generates 1,000-Fold Increase in Electron Density

http://int.saci.kyoto-u.ac.jp/?p=2336
The study of carrier multiplication has become an essential part of many-body physics and materials science. Assistant Prof Hideki Hirori and co-workers observed that when exposed to a single-cycle electric field pulse at the 1000 GHz (terahertz) frequency range, a sample of standard semiconductor material (gallium arsenide, GaAs) burst an avalanche of electron-hole pairs (excitons) 1,000-times more abundant than initial states only on the picosecond (10-12 s) time scale. The observed bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations. These just-released results with the world strongest terahertz pulses demonstrate the rich potential that lies in the study of terahertz radiation.

This carrier multiplication directly affects nonlinear transport phenomena in ultra-high-speed transistors and plays a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors.

Related Information 1. H. Hirori, K. Shinokita, M. Shirai, S. Tani, Y. Kadoya, and K. Tanaka: Nature Commun. 2, 594 (2011).
2. H. Hirori, A. Doi, F. Blanchard, and K. Tanaka: Appl. Phys. Lett. 98, 091106 (2011).

Photo-designed terahertz devices

http://www.nature.com/srep/2011/111018/srep00121/full/srep00121.html

Technologies are being developed to manipulate electromagnetic waves using artificially structured materials such as photonic crystals and metamaterials, with the goal of creating primary optical devices. For example, artificial metallic periodic structures show potential for the construction of devices operating in the terahertz frequency regime. Here we demonstrate the fabrication of photo-designed terahertz devices that enable the real-time, wide-range frequency modulation of terahertz electromagnetic waves. These devices are comprised of a photo-induced, planar periodic-conductive structure formed by the irradiation of a silicon surface using a spatially modulated, femtosecond optical pulsed laser. We also show that the modulation frequency can be tuned by the structural periodicity, but is hardly affected by the excitation power of the optical pump pulse. We expect that our findings will pave the way for the construction of all-optical compact operating devices, such as optical integrated circuits, thereby eliminating the need for materials fabrication processes.