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

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

According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

We describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high-speed image acquisition. The image formation is based on the theory of compressed sensing, which permits the reconstruction of a N-by-N pixel image using much fewer than N2 measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector. We demonstrate the concept using a pulsed terahertz time-domain system and show the reconstruction of both amplitude and phase-contrast images. The idea of compressed sensing is quite general and could also be implemented with a continuous-wave terahertz source.

/pdf/a-single-pixel-terahertz-imaging-system-based-on-compressed-2zw4tugxrl.pdf

A single-pixel terahertz imaging system based on compressed sensing

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technologies are promising for the standoff detection and identification of explosive targets.

Terahertz Spectroscopy and Imaging for Defense and Security Applications

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers inno- vative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technol- ogies are promising for the standoff detection and identifica- tion of explosive targets.

Terahertz Spectroscopy and Imaging for Defense and Security Applications Laboratory experiments indicate that, with more powerful sources and more sensitive detectors, terahertz techniques have great potential for detection and identification of concealed explosives.

We report the use of a compact continuous-wave sub-terahertz system for inspection applications, using electronic generation and detection methods. A combination of a Gunn diode emitter, a Schottky diode detector, and a polyethylene Fresnel lens provides line-scan images at 0.2m∕s with a data acquisition rate of 512points∕s. Examples of the measurement of NASA’s insulating panels and applicability of the technology to other nondestructive testing applications are presented and discussed.

/pdf/compact-continuous-wave-subterahertz-system-for-inspection-4neq3m3kde.pdf

Compact continuous-wave subterahertz system for inspection applications

We report an evaluation of pulsed terahertz (THz) time-domain measurement and continuous wave (CW) terahertz measurement for non-destructive testing applications. The strengths and limitations of the modalities are explored via the example of the detection of defects in space shuttle foam insulation. It is decided that CW imaging allows for a more compact and simple system, while pulsed measurements yield a broader range of information.

https://iopscience.iop.org/article/10.1088/0268-1242/20/7/021/pdf

Comparison between pulsed terahertz time-domain imaging and continuous wave terahertz imaging

We report the characterization of the emission profile of terahertz waves generated by four-wave mixing in the presence of laser-induced plasma. Highly directional terahertz waves with a divergence angle smaller than 10° are measured in the presence of long plasmas (>10mm). Frequency-dependent interference structures in the angular distribution of the radiation are observed under tighter focusing conditions and are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz generation from four-wave mixing in air plasmas is a promising source for spectroscopy and imaging in the terahertz range.

Hua Zhong

Papers

Terahertz Spectroscopy and Imaging for Defense and Security Applications

Terahertz Spectroscopy and Imaging for Defense and Security Applications Laboratory experiments indicate that, with more powerful sources and more sensitive detectors, terahertz techniques have great potential for detection and identification of concealed explosives.

Compact continuous-wave subterahertz system for inspection applications

Comparison between pulsed terahertz time-domain imaging and continuous wave terahertz imaging

Terahertz emission profile from laser-induced air plasma