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

Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

Metamaterial structures are taught which provide for the modulation of terahertz frequency signals. Each element within an array of metamaterial (MM) elements comprises multiple loops and at least one gap. The MM elements may comprise resonators with conductive loops and insulated gaps, or the inverse in which insulated loops are present with conductive gaps; each providing useful transmissive control properties. The metamaterial elements are fabricated on a semiconducting substrate configured with a means of enhancing or depleting electrons from near the gaps of the MM elements. An on to off transmissivity ratio of about 0.5 is achieved with this approach. Embodiments are described in which the MM elements incorporated within a Quantum Cascade Laser (QCL) to provide surface emitting (SE) properties.

Active terahertz metamaterial devices

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

The absence of non-destructive inspection techniques for illicit drugs hidden in mail envelopes has resulted in such drugs being smuggled across international borders freely. We have developed a novel basic technology for terahertz imaging, which allows detection and identification of drugs concealed in envelopes, by introducing the component spatial pattern analysis. The spatial distributions of the targets are obtained from terahertz multispectral transillumination images, using absorption spectra measured with a tunable terahertz-wave source. The samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.

https://www.agri.tohoku.ac.jp/thz/jp/71_k02.pdf

Non-destructive terahertz imaging of illicit drugs using spectral fingerprints.

A label-free biological sensor, which is based on the resonant transmission phenomenon of a thin metallic mesh, is proposed in the terahertz wave region. By using this sensor, we demonstrate the highly sensitive detection of small amounts of protein horseradish peroxidase. For quantitative investigation of the sensitivity of our sensor, horseradish peroxidase was printed on the metallic mesh surface by using a commercial available printer. A distinct shift of the transmission dip frequency is observed for 500pg∕mm2 (11fmol) of horseradish peroxidase printed on the metallic mesh, indicating the significantly high sensitivity of our sensor.

Terahertz sensing method for protein detection using a thin metallic mesh

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

A surface-wave sensor based on the resonant transmission characteristics of metal hole arrays is demonstrated in the terahertz (THz) region. Since the frequency of the transmission peak of a metal hole array, which corresponds to the resonant frequency of the surface waves, is particularly sensitive to the refractive index in the vicinity of the metal surface, a very small change in the substances attached to the surface can be detected by monitoring the transmission spectrum. By attaching a layer of substance (thickness t<5 μm) much thinner than the wavelength of the THz wave (λTHz=1 mm at 0.3 THz) to the surface of a metal hole array, we demonstrated that the existence of such a small amount of substance can be detected more easily than without the metal hole array. This demonstration of THz sensing with metal hole arrays indicates the possibility of realizing THz surface-wave sensors for biochemical molecules in the THz region.

Terahertz surface-wave resonant sensor with a metal hole array

Terahertz time-domain attenuated total reflection spectroscopy, in combination with a two-interface model, is used to determine the complex dielectric constants of cultured human cancer cells (DLD-1, HEK293 and HeLa). Picosecond and sub-picosecond water dynamics are dominant in the measured complex dielectric constants of these cells. We demonstrate that dielectric responses below 1.0 THz best characterize the particular water dynamics of cancer cells when compared with extracellular water. Debye-Lorentz fitting revealed that this is due to a significantly attenuated slow relaxation mode and enhanced fast relaxation mode of the water in these cancer cells. These findings could lead to a new procedure to digitally evaluate cellular activities or functions, in terms of intracellular water dynamics, and remove the veil from the mysterious intracellular milieu.

/pdf/characterization-of-dielectric-responses-of-human-cancer-2u36g3xa7b.pdf

Yuichi Ogawa

Papers

Non-destructive terahertz imaging of illicit drugs using spectral fingerprints.

Terahertz sensing method for protein detection using a thin metallic mesh

Terahertz spectroscopy for art conservation

Terahertz surface-wave resonant sensor with a metal hole array

Characterization of Dielectric Responses of Human Cancer Cells in the Terahertz Region