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."

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."

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

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