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.

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A single-pixel terahertz imaging system based on compressed sensing

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

We present a polarization sensitive terahertz detection method which is able to measure both orthogonal components of the terahertz electric field. It allows for the study of polarization dependent properties of materials. Azimuthal angle dependent transmittance in 0.2–2.6 THz frequency region for crystalline quartz is measured. Polarized terahertz transmission spectroscopy shows that birefringence can result in transmission minima. This work suggests that polarization sensitive detection is effective for removing fake absorption features caused by material birefringence.

Polarization sensitive terahertz time-domain spectroscopy for birefringent materials

A simple, compact CW sub-THz imaging system, utilizing a 0.2 and 0.6 THz Gunn diode source is presented. A silicon beam lead diode detector and a Golay cell are used for the detection. Various results are presented, which show that the CW THz imaging modality is suitable for diverse applications, such as non-destructive testing and security. The key components of the system include the Gunn diode assembly, an optical chopper, a polyethylene lens, a detector, a lock-in amplifier, and two translation stages. The beam from the Gunn diode is focused on the sample being imaged by the polyethylene lens, the transmitted or reflected beam is measured by the detector. The energy transmitted through the sample at each point in the plane of the sample is detected. Since the system has relatively few components compared to pulsed THz imaging systems, it is less expensive and easier to design and operate, although it does not provide depth or spectral information about the sample. Since no time-delay scans take place, scanning can be done quickly compared to a time-domain system, limited by the maximum velocity of the translation stages and response of the detectors. It provides information about the macroscopic features of hidden structures within materials that are transparent to sub THz radiation, such as space shuttle insulating foam, articles of clothing, and luggage.

Non-destructive sub-THz CW imaging

Pulsed THz imaging systems have a number of potential advantages in inspection applications. They provideamplitude and phase information across a broad spectral range in the far-infrared, and many common packagingmaterials are relatively transparent in this frequency range. We use T-ray imaging to allow the identication ofdierent powdered materials concealed inside envelopes. Using the terahertz spectral information we show thatdierent powders may be uniquely identied.Dierent thicknesses of the powders are imaged to investigate the inuence of scattering on the measuredTHz pulses and the classication model is extended to allow it to identify dierent materials independent of thematerial thickness.Keywords: terahertz, T-ray imaging, chirped pulse, linear-lter modelling, classication 1. INTRODUCTION Pulsed terahertz imaging is a relatively new addition to the wide array of available imaging modalities. 1 Itutilizes the terahertz, or far-infrared, region of the electromagnetic spectrum and is based upon the increasinglypopular technique of terahertz time-domain spectroscopy (THz-TDS). The unique properties of THz radiationallow terahertz imaging to ll niches that are unreachable using other techniques. THz spectroscopy systems areseeing application in semi-conductor characterization,

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Powder retection with T-ray imaging

Pulsed THz imaging is a promising non-destructive technology based on its high transmission through selected dielectric materials and its capability to provide time-of-flight and spectral information. The traditional method of the pulsed THz imaging is a point-to-point reflective scanning system. The image is acquired by analyzing the peak amplitude information of the THz pulse in the time-domain at each pixel. It requires the THz beam or sample scanned. In this paper, we present our approach of large scale, focal plane THz wave imaging. In our 2-D focal plane THz wave imaging, the THz beam is expanded to be 60 mm in diameter. The THz beam illuminates the target in a reflective manner, in which a polyethylene lens projects the image onto a 40 mm by 40 mm by 2 mm ZnTe sensor crystal. The probe beam is expanded to be 40 mm in diameter and overlap with the THz beam on the sensor. The modulated probe beam profile carrying the image information is captured by a CCD camera. This technique enables us to view the objects which are optically opaque but transparent in THz frequency and shows feasibility in remote sensing, security inspection, and military defense applications.

Hua Zhong

Papers

Nondestructive defect identification with terahertz time-of-flight tomography

Polarization sensitive terahertz time-domain spectroscopy for birefringent materials

Non-destructive sub-THz CW imaging

Powder retection with T-ray imaging

Terahertz wave imaging for landmine detection