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

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

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 present terahertz (THz) reflective spectroscopic focal-plane imaging of four explosive and bio-chemical materials (2, 4-DNT, Theophylline, RDX and Glutamic Acid) at a standoff imaging distance of 0.4 m. The 2 dimension (2-D) nature of this technique enables a fast acquisition time and is very close to a camera-like operation, compared to the most commonly used point emission-detection and raster scanning configuration. The samples are identified by their absorption peaks extracted from the negative derivative of the reflection coefficient respect to the frequency (-dr/dv) of each pixel. Classification of the samples is achieved by using minimum distance classifier and neural network methods with a rate of accuracy above 80% and a false alarm rate below 8%. This result supports the future application of THz time-domain spectroscopy (TDS) in standoff distance sensing, imaging, and identification.

Identification and classification of chemicals using terahertz reflective spectroscopic focal-plane imaging system.

We demonstrate the application of terahertz (THz) time-of-flight tomographic imaging to identify the distribution of defects in foam materials. Based on THz time-domain spectroscopy technology, THz imaging probes targets with picosecond pulses of broad-band radiation in the frequency range from 100 GHz to 3 THz. The reflected THz wave from the target is measured using electrooptic sampling, which provides two-dimensional images with phase and amplitude information, as well as the spectroscopic properties of the object. The depth information is recorded in the THz time-domain waveform. Several reconstruction models are developed for tomographic imaging of defects inside foam. Foam insulation of space shuttle fuel tanks, with prebuilt defects, are investigated with THz tomographic imaging. Most prebuilt defects are pinpointed and models used to identify different kinds of defects are discussed.

Nondestructive defect identification with terahertz time-of-flight tomography

THz radiation has been used to uncover small defects in samples of space shuttle foam. This nondestructive method could help NASA to examine the insulating foam that is applied to each shuttle's fuel tank prior to launch.

R. Reightler

Papers

Nondestructive defect identification with terahertz time-of-flight tomography

T-rays identify defects in insulating-materials