There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

https://iopscience.iop.org/article/10.1088/1361-6463/50/4/043001/pdf

The 2017 terahertz science and technology roadmap

A wireless communication system with a maximum data rate of 100 Gbit s−1 over 20 m is demonstrated using a carrier frequency of 237.5 GHz. The photonic schemes used to generate the signal carrier and local oscillator are described, as is the fast photodetector used as a mixer for data extraction.

Wireless sub-THz communication system with high data rate

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.

At the Dawn of a New Era in Terahertz Technology

Terahertz spectroscopy has rapidly progressed, and is being applied to various research fields as a new non-invasive examination method. Scientific analysis is important for the conservation of art, as it can help to reveal the history of a work and to determine the proper materials for its restoration. Mid-infrared spectroscopy and X-ray analysis are currently used to identify organic and inorganic materials, respectively. Terahertz spectroscopy is expected to identify composites themselves, and to give clear and direct information for art conservation. We have collected terahertz spectra of various historical and modern materials, and found that terahertz spectroscopy can distinguish the pigments, binders and their mixtures. The experimental results prove that terahertz spectroscopy as a new non-invasive examination method can contribute to art history and to restorations. The material database can also contribute to other fields such as earth science, the printing industry, and the food and pharmaceutical research fields.

https://www.jstage.jst.go.jp/article/elex/4/8/4_8_258/_pdf

Terahertz spectroscopy for art conservation

Terahertz (THz) Kiosk application offers ultrahigh downloads of digital information to users’ handheld devices. System configuration and multiple paths between the transmitter and receiver have important impact on the achievable data rates. In this paper, the propagation channel of THz Kiosk downloading application is investigated and a stochastic channel model is proposed. Based on channel measurements using a vector network analyzer, a 3-D ray-tracing simulator is calibrated to conduct simulations for an in-depth analysis of the different channel characteristics. Successively, the observed propagation paths are classified and the key channel parameters in time, frequency, and spatial domains are modeled for each type of ray. The resulting stochastic model is evaluated in terms of Rician K-factor and root mean square delay spread. Compared to the reference data, the mean absolute errors of these two metrics of the three investigated scenarios are less than 1.48 dB and 0.07 ns, respectively. The results show that the target 100 gigabits per second data rate is achievable at tens of gigahertz system bandwidth at proper communication distance and higher order modulation schemes. The developed channel model allows system design engineers to generate realizations of propagation channel efficiently for designing Kiosk-fashion close-proximity communication systems in the THz band.

Stochastic Channel Modeling for Kiosk Applications in the Terahertz Band

Innovative non-invasive analysis techniques for cultural heritage using terahertz technology

“High speed” in communications often means “high data-rate” and fiber-optic technologies have long been ahead of wireless technologies in that regard. However, an often overlooked definite advantage of wireless links over fiber-optic links is that waves travel at the speed of light c, which is about 50% faster than in optical fibers as shown in Fig. 17.9.1 (top left). This “minimum latency” is crucial for applications requiring real-time responses over a long distance, including high-frequency trading [1]. Further opportunities and new applications might be created if the absolute minimum latency and fiber-optic data-rates are put together. (Sub-)THz frequencies have an extremely broad atmospheric transmission window with manageable losses as shown in Fig. 17.9.1 (top right) and will be ideal for building light-speed links supporting fiber-optic data-rates. This paper presents a 105Gb/s 300GHz transmitter (TX) fabricated using a 40nm CMOS process.

Iwao Hosako

Papers

At the Dawn of a New Era in Terahertz Technology

Terahertz spectroscopy for art conservation

Stochastic Channel Modeling for Kiosk Applications in the Terahertz Band

Innovative non-invasive analysis techniques for cultural heritage using terahertz technology

17.9 A 105Gb/s 300GHz CMOS transmitter