A life with terahertz waves

A fascination with electromagnetic waves at the boundary between light and radio waves

A prototype of the integrated terahertz radar. © Yasuaki Monnai, Keio University

https://research-highlights.keio.ac.jp/2020/07/a.html?fbclid=IwAR2B9o9ScG8k6U-Lpjdw98NcQZ8-hZ91KjYQ48WQ9-pC5LgMrQfiDle7OOA

First encounters with terahertz waves

"My doctoral research was on controlling beams of terahertz radiation," says Yasuaki Monnai, an associate professor at Keio University's Department of Applied Physics and Physico-Informatics. "We were working at the boundary between light and radio waves. Most likely, what led me to follow this path was meeting a professor who was transmitting electric power wirelessly using microwaves. I was impressed by his ideas and methodology, which eventually led to my interest in terahertz waves, waves that have higher frequencies than microwaves."

Monnai adds that research on light and radio waves evolved as separate fields, with few technological advances at the boundaries of these waves. Furthermore, radio waves are generated by oscillator circuits and transmitted using antennae, but light is generated by lasers and transmitted using lens systems.

Although the terahertz frequency band has the potential for communication speeds much higher than the best of today's Wi-Fi, enabling instant downloads of high-definition images, one of the issues to overcome is preventing the divergence of terahertz beams. "For example, infrared remote TV-controllers are highly directional and if they are pointed slightly away from the target, they fail to work," explains Monnai. "For my doctorate, I proposed a device to control the directionality of terahertz beams and demonstrated its principle of action. I had to visit a group in Germany to complete my research because my laboratory in Japan did not have the highly specialized terahertz wave equipment required."

Studying in Germany and the verbalization of ideas

Monnai's memories of his stay in Germany include how students approached their research and job hunting. "In Japan, it is assumed that all students work along the same timeline towards important events such as completing doctoral programs and finding jobs," says Monnai. "But in Germany, I saw that it was a case of 'people acting once they were prepared,' which means the timing for these events depends on the individual's circumstances. People take their time to think about their future without external timing constraints. This is in total contrast to the situation faced by students in Japan who all try to graduate and get jobs to start on the 1st of April of that fiscal year."

Monnai recalls being impressed by the ability of the German students to clearly explain their intentions in words. "I have heard that in physics exams, students are expected to not only solve equations but also answer questions verbally," explains Monnai. "On TV programs for children, I noticed it was common for ordinary children to explain in their own words not only what they wanted to be in the future, but also why. I felt that this kind of culture was the basis for the 'verbalization of ideas.'"

Focus on terahertz waves and remote heartbeat detection

There are a wide range of applications of electromagnetic waves. For example, microwaves are used for heating meals, infrared rays for monitoring temperature, and X-rays for looking inside matter. However, there is increasing interest in the unexplored terahertz wavelengths. "One of my research themes is focused on terahertz radar with the goal of controlling machines and supporting human cognitive behavior," explains Monnai. "We recently developed a compact, highly penetrating, high-resolution terahertz radar system for application in the fields of mobile/wearable devices and drones. We revealed the power of our technology by demonstrating non-contact and remote detection of human heartbeats where the terahertz radiation penetrated a person's clothes."

Demonstration of non-contact heartbeat detection using a terahertz radar. © Yasuaki Monnai, Keio University

The terahertz frequency band is generally recognized as covering the range from approximately 0.3 THz to around 10 THz, where 'tera' refers to one trillion (1012) cycles per second. This corresponds to the relatively long wavelengths of 1 mm to 0.1 mm when compared to around 500 nm (10-9m) for visible light. Notably, terahertz radiation is noninvasive and not harmful to the human body, thereby offering a wide range of applications ranging from inspecting defects in industrial products to whole body scanners at airports and cancer detection.

Monnai explains that the resolution of terahertz waves is significantly higher than conventional microwaves and millimeter waves, and media are more transparent to terahertz radiation than light enabling better detection. He adds that his group's technology does not require phase shifters, lenses, mechanical scanners, or other such components, thereby enabling a highly compact system without moving parts.

Terahertz waves as data interfaces between the physical world and information technology

It is becoming increasingly clear that the unique properties of terahertz waves have the potential to be extremely useful not only as communications carriers but also as measurement probes.

"Terahertz waves is a next generation technology acting as the interface for incorporating data in the physical world into the cyber world, such as autonomous driving and telepresence," says Monnai. "I believe that we must combine cutting-edge hardware and software for such applications. So currently, I am focusing my research on terahertz systems with smart hardware."

Abstract-Spatio-temporal imaging of terahertz electric-field vector: Observation of polarization-dependent knife-edge diffraction

Kenta Suzuki, Kenichi Oguchi, Yasuaki Monnai, Makoto Okano,  Shinichi Watanbe,

https://iopscience.iop.org/article/10.7567/1882-0786/ab12fc

We develop a terahertz electric-field vector imaging system that uses the rotating polarizer technique. The imaging system is used to investigate the polarization-dependent optical response of a patterned indium–tin–oxide (ITO) film deposited on a glass substrate. While ITO is transparent for the near-infrared probe light, it acts as a metal for terahertz light. Therefore, a polarization-dependent diffraction can be observed at the edge of the ITO thin film; only electric-field components perpendicular to the edge boundary behind the ITO layer. By comparison with a numerical simulation, we reveal the polarization dependence of this so-called knife-edge diffraction.

Abstract-Tutorial: Terahertz beamforming, from concepts to realizations

Daniel Headland, Yasuaki Monnai, Derek Abbott, Christophe Fumeaux, Withawat Withayachumnankul,

https://aip.scitation.org/doi/am-pdf/10.1063/1.5011063?class=chorus+notVisible

The terahertz range possesses significant untapped potential for applications including high-volume wireless communications, noninvasive medical imaging, sensing, and safe security screening. However, due to the unique characteristics and constraints of terahertz waves, the vast majority of these applications are entirely dependent upon the availability of beam control techniques. Thus, the development of advanced terahertzrange beam control techniques yields a range of useful and unparalleled applications. This article provides an overview and tutorial on terahertz beam control. The underlying principles of wavefront engineering include array antenna theory and diffraction optics, which are drawn from the neighboring microwave and optical regimes, respectively. As both principles are applicable across the electromagnetic spectrum, they are reconciled in this overview. This provides a useful foundation for investigations into beam control in the terahertz range, which lies between microwaves and infrared light. Thereafter, noteworthy experimental demonstrations of beam control in the terahertz range are discussed, and these include geometric optics, phased array devices, leaky-wave antennas, reflectarrays, and transmitarrays. These techniques are compared and contrasted for their suitability in applications of terahertz waves. 

Abstract-Waveguide Devices for Terahertz Signal Processing

Kimberly S. Reichel, Robert McKinney, Yasuaki Monnai, Nicholas J. Karl, Rajind Mendis, and Daniel M. Mittleman
https://www.osapublishing.org/abstract.cfm?uri=LAOP-2016-LW2B.1

We introduce two waveguide based devices for signal processing in future terahertz wireless communications systems: a leaky-wave antenna for frequency multiplexing and a Tjunction waveguide for broadband power splitting.

© 2016 OSA

PDF Article

Abstract-Focused terahertz waves generated by a phase velocity gradient in a parallel-plate waveguide

Robert W. McKinney, Yasuaki Monnai, Rajind Mendis, and Daniel Mittleman
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-23-21-27947

We demonstrate the focusing of a free-space THz beam emerging from a leaky parallel-plate waveguide (PPWG). Focusing is accomplished by grading the launch angle of the leaky wave using a PPWG with gradient plate separation. Inside the PPWG, the phase velocity of the guided TE1 mode exceeds the vacuum light speed, allowing the wave to leak into free space from a slit cut along the top plate. Since the leaky wave angle changes as the plate separation decreases, the beam divergence can be controlled by grading the plate separation along the propagation axis. We experimentally demonstrate focusing of the leaky wave at a selected location at frequencies of 100 GHz and 170 GHz, and compare our measurements with numerical simulations. The proposed concept can be valuable for implementing a flat and wide-aperture beam-former for THz communications systems.

© 2015 Optical Society of America

Full Article  |  PDF Article

Abstract-Frequency-division multiplexing in the terahertz range using a leaky-wave antenna

• Nicholas J. Karl,
• Robert W. McKinney,
• Yasuaki Monnai,
• Rajind Mendis
• & Daniel M. Mittleman

• 
  

Nature Photonics

 

(2015)

 

doi:10.1038/nphoton.2015.176

Received

 

09 February 2015 

Accepted

 

11 August 2015 

Published online

 

14 September 2015

http://www.nature.com/nphoton/journal/vaop/ncurrent/full/nphoton.2015.176.html

The idea of using radiation in the 0.1–1.0 THz range as carrier waves for free-space wireless communications has attracted growing interest in recent years, due to the promise of the large available bandwidth^1, 2. Recent research has focused on system demonstrations^3, 4, as well as the exploration of new components for modulation⁵, beam steering⁶ and polarization control⁷. However, the multiplexing and demultiplexing of terahertz signals remains an unaddressed challenge, despite the importance of such capabilities for broadband networks. Using a leaky-wave antenna based on a metal parallel-plate waveguide, we demonstrate frequency-division multiplexing and demultiplexing over more than one octave of bandwidth. We show that this device architecture offers a unique method for controlling the spectrum allocation, by variation of the waveguide plate separation. This strategy, which is distinct from those previously employed in either the microwave⁸ or optical⁹regimes, enables independent control of both the centre frequency and bandwidth of multiplexed terahertz channels.

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Researchers develop key component for terahertz wireless

All communications networks need some form of multiplexing -- the ability to send multiple data streams through a single medium. Researchers from Brown have used a leaky wave antenna to separate terahertz waves by frequency. The work provides a viable multiplexing and demultiplexing strategy for future terahertz data networks, which have the potential to deliver data many times faster than today's cellular or Wi-Fi networks.
CREDIT: MITTLEMAN LAB / BROWN UNIVERSITY

http://www.eurekalert.org/pub_releases/2015-09/bu-rdk091015.php

PROVIDENCE, R.I. [Brown University] -- Terahertz radiation could one day provide the backbone for wireless systems that can deliver data up to one hundred times faster than today's cellular or Wi-Fi networks. But there remain many technical challenges to be solved before terahertz wireless is ready for prime time.

Researchers from Brown University have taken a major step toward addressing one of those challenges. They've developed what they believe to be the first system for multiplexing terahertz waves. Multiplexers are devices that enable separate streams of data to travel through a single medium. It's the technology that makes it possible for a single cable to carry multiple TV channels or for a fiber optic line to carry thousands of phone calls at the same time.

"Any terahertz communications application is going to need some form of multiplexing and demultiplexing," said Daniel Mittleman, professor of engineering at Brown and senior author of a paper describing the new device. "This is, to our knowledge, the first time anyone has demonstrated a viable strategy for multiplexing in the terahertz range."

The research was published September 14 in Nature Photonics.

Today's cellular and Wi-Fi networks rely on microwaves to carry voice conversations and data. But the increasing demands for data transfer are quickly becoming more than microwaves can handle. Terahertz waves have a much higher frequency and therefore more potential bandwidth. Scientists and engineers have only recently begun exploring the potential of terahertz waves, however. As a result, many of the components for a terahertz wireless network -- including multiplexers -- have not yet been developed.

The multiplexer that Mittleman and his colleagues have been working on makes use of what's known as a leaky wave antenna. In this case, the antenna is made from two metal plates placed in parallel to form a waveguide. One of the plates has a small slit in it. As terahertz waves travel down the waveguide, some of the radiation leaks out of the slit. It turns out that terahertz waves leak out a different angles depending on their frequency.

"That means if you put in 10 different frequencies between the plates -- each of them potentially carrying a unique data stream -- they'll come out at 10 different angles," Mittleman said. "Now you've separated them and that's demultiplexing."

On the other end, a receiver could be tuned to accept radiation at a particular angle, thus receiving data from only one stream.

"We think it's definitely a reasonable solution to meet the needs of a terahertz communication network," said Nicholas Karl, a graduate student at Brown and the paper's lead author. Karl led the experiments on the device with fellow graduate student Robert McKinney. Other authors on the study are Rajind Mendis, a research professor at Brown, and Yasuaki Monnai from Keio University in Tokyo.

One of the advantages to the approach, the researchers say, is that by adjusting the distance between the plates, it's possible to adjust the spectrum bandwidth that can be allocated to each channel. That could be especially useful when such a device is deployed for use in a data network.

"For example, if one user suddenly needs a ton of bandwidth, you can take it from others on the network who don't need as much just by changing the plate spacing at the right location," Mittleman said.

The group plans to continue its work to refine the device. A research group from Osaka University is collaborating with Mittleman's group to implement the device in a prototype terahertz network they're building.

"This is a first-generation, proof-of-concept device," Karl said. "There are still things we can do to improve it and we'll continue to study it."

Mittleman hopes that the work will challenge other researchers to start developing components for terahertz networks.

"The biggest impact this may have is it may just be the kick that people need to start thinking about this issue," Mittleman said. "That means they'll start coming up with clever ideas that are entirely different from this one."

###

The work was supported by the National Science Foundation and the W.M. Keck Foundation.

Note to Editors:

Editors: Brown University has a fiber link television studio available for domestic and international live and taped interviews, and maintains an ISDN line for radio interviews. For more information, call (401) 863-2476.

New device electrically steers and focuses terahertz waves

http://www.rdmag.com/news/2013/01/new-device-electrically-steers-and-focuses-terahertz-waves

Yasuaki Monnai (right) and Kristian Altmann with the device mounted to the characterization setup at Marburg University. Photo: Bastian Reitemeier, University of MarburgIn a close collaboration, researchers from the University of Marburg/Germany and of the University of Tokyo/Japan have demonstrated a device which allows for an electric and flexible focusing and steering of terahertz (THz) waves.

The ability to redirect and focus THz beams will be of particular importance for THz communication systems, which will work with directed links between emitters and receivers. Yet, the position of THz emitters and/or receivers or the distance between them are likely to change from time to time, as we move with a laptop or other mobile devices freely in a room. Moreover, walking persons or moving objects might block the link. Hence, it is crucial to have the ability to redirect THz beams or to vary their divergence. Other application fields include remote sensing and the inspection of industrial goods.

The device was developed by Yasuaki Monnai in the group of Prof. Hiroyuki Shinoda at the University of Tokyo. It is based on a sub-wavelength array of metal cantilevers which can be micromechanically actuated by electrostatic forces such that tunable gratings of different periodicity can be created.  Tuning the grating pattern allows for a shaping of the wavefront of the diffracted radiation and, hence, to vary the direction of the THz beams. Furthermore, the divergence of the THz beam can be controlled.

The characterization experiments have been performed by Kristian Altmann and Yasuaki Monnai in the group of Prof. Martin Koch at the University of Marburg. In the first proof of concept, the steerable range at 0.3 THz exceeded an angle of 40 degrees. The accomplished beam directions and the field profiles agree well with theoretical expectations.

Terahertz beam steering and variable focusing using programmable diffraction gratings

Source: University of Marburg

Abstract-Terahertz beam focusing based on plasmonic waveguide scattering

Yasuaki Monnai¹, Kristian Altmann², Christian Jansen², Martin Koch², Hartmut Hillmer³, and Hiroyuki Shinoda¹

¹Department of Information Physics and Computing, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
²Department of Physics, Philipps University of Marburg, Renthof 5, 35032 Marburg, Germany
³Institute of Nanostructure Technologies and Analytics, University of Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany

http://apl.aip.org/resource/1/applab/v101/i15/p151116_s1

We demonstrate free-space focusing of terahertz (THz) radiation by scattering plasmonic surface-waves into the air. We use a grating of shallow holes which contains non-equidistant defects which act as scattering centers. The scattering occurs with defined phase delays such that the waves emitted in free-space interfere constructively to form a focus above the waveguide surface. In contrast to conventional lenses, this structure does not require any free-space on its backside and has great potential for integrated THz optics.

© 2012 American Institute of Physics