Abstract-Coherent detection of THz-induced sideband emission from excitons in the nonperturbative regime

K. Uchida, T. Otobe, T. Mochizuki, C. Kim, M. Yoshita, K. Tanaka, H. Akiyama, L. N. Pfeiffer, K. W. West, and H. Hirori

https://journals.aps.org/prb/accepted/43076O14Z2210330d36f6038100d0aa224fae6d98

Strong interaction of terahertz (THz) waves with excitons induces nonperturtbative optical effects such as Rabi splitting and high-order sideband generation. Here, we investigated coherent properties of THz-induced sideband emissions from GaAs/AlGaAs multi-quantum-wells, and determined the optical susceptibility of the THz dressed exciton in the nonperturbative regime. The strong dependences of both amplitude and phase of the second-order sideband emission on the THz electric field strength imply that the field ionization of the 1s exciton modifies the THz-dressed exciton's energy spectrum.

Abstract-Ultrafast Control of the Polarity of BiCoO 3 by Orbital Excitation as Investigated by Femtosecond Spectroscopy

Y. Okimoto, S. Naruse, R. Fukaya, T. Ishikawa, S. Koshihara, K. Oka, M. Azuma, K. Tanaka, and H. Hirori

Phys. Rev. Applied 7, 064016 – Published 12 June 2017

${\mathrm{https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.7.064016BiCoO}}_3$ is a perovskite-type cobalt oxide with a polar structure. We investigate the dynamics of its polar state by using photoexcitation with femtosecond visible and terahertz pulses. The intensity of the second-harmonic light, caused by a polar structure without inversion symmetry, is enhanced by more than 50% by irradiating the terahertz pulse showing an ultrafast response following the femtosecond terahertz pulse. By contrast, it is suppressed by more than 60% by exciting visible light with a 100-fs pulse width at room temperature. These results suggest not only an important role of the orbital excitation in the ${\mathrm{Co}}^{3+}$ ion but also a key to improving the nonlinear optical response, e.g., a figure of merit in nonlinear crystals on the time scale of a femtosecond.

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Abstract-Light-induced electron localization in a quantum Hall system

• T. Arikawa,
• K. Hyodo,
• Y. Kadoya
• & K. Tanaka

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4078.html

An insulating bulk state is a prerequisite for the protection of topological edge states¹. In quantum Hall systems, the thermal excitation of delocalized electrons is the main route to breaking bulk insulation². In equilibrium, the only way to achieve a clear bulk gap is to use a high-quality crystal under high magnetic field at low temperature. However, bulk conduction could also be suppressed in a system driven out of equilibrium such that localized states in the Landau levels are selectively occupied. Here we report a transient suppression of bulk conduction induced by terahertz wave excitation between the Landau levels in a GaAs quantum Hall system. Strikingly, the Hall resistivity almost reaches the quantized value at a temperature where the exact quantization is normally disrupted by thermal fluctuations. The electron localization is realized by the long-range potential fluctuations, which are a unique and inherent feature of quantum Hall systems. Our results demonstrate a new means of effecting dynamical control of topology by manipulating bulk conduction using light.

Abstract-Subcycle Optical Response Caused by a Terahertz Dressed State with Phase-Locked Wave Functions

K. Uchida, T. Otobe, T. Mochizuki, C. Kim, M. Yoshita, H. Akiyama, L. N. Pfeiffer, K. W. West, K. Tanaka, and H. Hirori

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.277402

The coherent interaction of light with matter imprints the phase information of the light field on the wave function of the photon-dressed electronic state. A driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a subcycle time scale in a GaAs quantum well in the presence of a multicycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of the excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.

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Abstract-Hydration state inside HeLa cell monolayer investigated with terahertz spectroscopy

K. Shiraga¹, T. Suzuki¹, N. Kondo¹, K. Tanaka² and Y. Ogawa^1,a)
http://scitation.aip.org/content/aip/journal/apl/106/25/10.1063/1.4922918
a) Author to whom correspondence should be addressed. Electronic mail: ogawayu@kais.kyoto-u.ac.jp
Appl. Phys. Lett. 106, 253701 (2015); http://dx.doi.org/10.1063/1.4922918


The hydration state in living cells is believed to be associated with various cellular activities. Nevertheless, in vivo characterization of intracellular hydration state under physiological condition has not been well documented to date. In this study, the hydration state of an intact HeLa cell monolayer was investigated by terahertz time-domain attenuated total reflectionspectroscopy. Combined with the extended theory of Onsager, we found 23.8 ± 7.4% of HeLa intracellular water was hydrated to biomolecules (corresponding to 1.25 g H2O/g solute); exhibiting slower relaxation dynamics than bulk water.

Abstract-Electric-Field Ionization of Gallium Acceptors in Germanium Induced by Single-cycle Terahertz Pulses

Y. Mukai, H. Hirori, K. Tanaka
http://arxiv.org/abs/1304.5825

The electric field ionization of gallium acceptors in germanium was studied by using terahertz time-domain spectroscopy after single-cycle terahertz pulse excitation. As the peak electric field of the excitation pulse increases, the distinct absorptions due to acceptor transitions centered at 2.0 and 2.2 THz decrease, and simultaneously, absorption emerges in the lower frequency region. These behaviors clearly show that the terahertz pulse ionizes neutral acceptors. The electric field dependence of the released hole density is well reproduced by a model assuming direct field-assisted tunneling of acceptors.

Abstract-Terahertz Time Domain Spectroscopy of Gold Nanorod/Polymer Films

K. S. S. Christie1, 2, J. K. Young3, Y. Mukai4, W. S. Rockward1, K. Tanaka4, T. A. Searles1, T. Arikawa4

1Department of Physics, Morehouse College, Atlanta, Georgia 30314, USA

2 NanoJapan Program, Rice University, Houston, Texas 77005, USA

3 Department of Electrical & Computer Engineering, Rice University, Houston, Texas 77005, USA

4 Department of Physics, Kyoto University, Kyoto, 606-8502, Japan

http://nanojapan.rice.edu/Downloads/2012%20RQI/Christie,%20Kofi_RQI%20Abstract%20and%20Poster.pdf

 Nanoparticles have distinct electrical and vibrational properties from bulk materials originating from the quantum confinement and surface effect. Bioengineers are currently able to exploit these properties for applications in biosensing, using the surface plasmon resonance wavelength of gold nanorods to monitor changes in their local environment. THz-TDS provides scientists with new opportunities to study low frequency phonons, and low frequency phonons in gold nanoparticles are explicatory of their morphology. Here, terahertz time-domain spectroscopy (THz-TDS) was used to study the vibrational behavior of gold nanorods embedded in a poly(vinyl alcohol) matrix. The nanorods’ aspect ratios (diameter x length) of 30.7 x 81.6 nm, 30.7 x 84.0 nm, 16.2 x 39.5 nm, 18.7 x 52.2 nm, and 18.5 x 56.5 nm are confirmed by visible/near-infrared absorption spectroscopy and transmission electron microscopy. The frequencies of the phonon modes are expected to be proportional to the longitudinal and transverse sound velocity in the material and inversely proportional to the size of the Au nanorods. We discuss how THz-TDS offers a solid method to determine nanoparticle morphology

Terahertz pulse increases electron density 1,000-fold

http://www.eurekalert.org/pub_releases/2011-12/ific-tpi121811.php

Findings point to advances in transistor and solar cell development

		IMAGE: A picosecond terahertz pulse causes an avalanche of excitons to burst forth from semiconductor GaAs. Click here for more information.

Kyoto, Japan -- Researchers at Kyoto University have announced a breakthrough with broad implications for semiconductor-based devices. The findings, announced in the December 20 issue of the journal Nature Communications, may lead to the development of ultra-high-speed transistors and high-efficiency photovoltaic cells.
Working with standard semiconductor material (gallium arsenide, GaAs), the team observed that exposing the sample to a terahertz (1,000 gigahertz) range electric field pulse caused an avalanche of electron-hole pairs (excitons) to burst forth. This single-cycle pulse, lasting merely a picosecond (10^-12 s), resulted in a 1,000-fold increase in exciton density compared with the initial state of the sample.
"The terahertz pulse exposes the sample to an intense 1 MV/cm^2 electric field," explains Hideki Hirori, team leader and Assistant Professor at Kyoto University's Institute for Integrated Cell-Material Sciences (iCeMS). "The resulting exciton avalanche can be confirmed by a bright, near-infrared luminescence, demonstrating a three-order of magnitude increase in the number of carriers."
Research in Kyoto using terahertz waves is led by Professor Koichiro Tanaka, whose lab at the iCeMS pursues numerous applications including the development of new biological imaging technologies.
"Since terahertz waves are sensitive to water, our goal is to create a microscope that will allow us to look inside living cells in real time," says Prof. Tanaka. "These just-released results using semiconductors are an entirely different field of science, but they demonstrate the rich potential that lies in the study of terahertz waves."

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The article, "Extraordinary carrier multiplication gated by a picosecond electric field pulse" by H. Hirori, K. Shinokita, M. Shirai, S. Tani, Y. Kadoya, and K. Tanaka was published online in the December 20, 2011 issue ofNature Communications.
Acknowledgements: This work was supported by Grant-in-Aid for Young Scientists (B) (Grant No. 21760038) of the Japan Society for the Promotion of Science, and also Grant-in-Aid for Scientific Research on Innovative Area "Optical science of dynamically correlated electrons (DYCE)" (Grant No. 20104007) and Grant-in-Aid for Creative Scientific Research (Grant No. 18GS0208) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
About the iCeMSThe Institute for Integrated Cell-Material Sciences (iCeMS) at Kyoto University in Japan aims to advance the integration of cell and material sciences -- both traditionally strong fields for the university -- in a uniquely innovative global research environment. The iCeMS combines the biosciences, chemistry, materials science, and physics to capture the potential power of stem cells (e.g., ES/iPS cells) and of mesoscopic sciences (e.g., porous coordination polymers). Such developments hold the promise of significant advances in medicine, pharmaceutical studies, the environment, and industry.