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.

Making steps toward improved data storage

Terahertz electromagnetic pulse controlling the physical structure of data-storage material. Credit: Kyoto University/Hirori Lab
 https://phys.org/news/2018-11-storage.html#jCp

A team of scientists has created the world's most powerful electromagnetic pulses in the terahertz range to control in fine detail how a data-storage material switches physical form. This discovery could contribute to scaled-down memory devices, eventually revolutionizing how computers handle information.

Compact discs might be out of fashion, but they may have inspired the next generation of computer nanotechnology. A glass layer in CDs consists of a phase-change material that can be encoded with information when light pulses cause crystals in small regions of the layer to either grow or melt.

Phase-change materials triggered by electrical impulses—rather than light—would offer new memory technologies with more stable and faster operation than that possible in many current types of memory devices. In addition, downscaling memory sites in phase-change materials could increase memory density. But this remains challenging because of the difficulty of controlling the crystallization and amorphization (melting) processes.

Addressing this issue in an article in Physical Review Letters, a team of scientists led by Kyoto University observed nanometer-scale growth of individual crystals in a phase-change material composed of germanium, antimony and tellurium—or GST—after applying high-powered terahertz pulses as a trigger.

"One reason crystallization and amorphization of GST under an electric field are difficult to control is the heat diffusion effects in the micrometer scale associated with electrical inputs, which also contribute to the crystallization," explains group leader Hideki Hirori. "Fortunately, terahertz technologies have matured to the point where we can use short pulses to generate strong electric fields while suppressing heating effects."

Hirori and his coworkers developed a terahertz pulse generator that delivered ultra-short and highly intense terahertz pulses across a pair of gold antennas. These pulses created an electric field in the GST sample comparable to that of an electrically switched device. Importantly, this approach greatly reduced the heat diffusion because of the extremely short duration of terahertz pulses—around 1 picosecond, or 10-12 seconds—enabling fine control over the rate and direction of GST crystallization. A region of crystallization grew in a straight line between the gold antennas in the direction of the field, at a few nanometers per pulse.

When the team tracked stepwise changes in crystallization while increasing the number of terahertz pulses, they were surprised to find that after a certain point, crystal conductivity rapidly sped up instead of rising in line with the increase in terahertz strength. The researchers hypothesize that electrons jumping between states in the crystal added an unexpected source of heat to the system, boosting crystallization.

Hirori explains: "Our experiment reveals how nanoscale and direction-controlled growth of crystals in GST can be achieved. We also identified a phenomenon which should assist in the design of new devices and ultimately realize the fast and stable digital information handling potential that this material promises."

 Explore further: Asymmetric plasmonic antennas deliver femtosecond pulses for fast optoelectronics

More information: Yasuyuki Sanari et al, Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.121.165702

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.

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).

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.