This paper aims to provide a technical insight into a selection of recent developments and applications involving terahertz sensing technology.
Following an introduction, the first part of this paper considers a selection of research activities involving terahertz radiation sources and detectors. The second part seeks to illustrate how the technology is exerting a commercial impact and discusses a number of product developments and applications.
Terahertz sensing is a rapidly developing field and a strong body of research seeks to develop sources and detectors with enhanced features which often exploit novel materials, phenomena and technologies. Commercialisation is gathering pace, and a growing number of companies are producing terahertz sensing and imaging products which are finding a diversity of applications.
This provides details of recent research, product developments and applications involving terahertz sensing technology.
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Copyright © 2018, Emerald Publishing Limited
Terahertz (THz) radiation lies between the microwave and far infra-red regions of the electromagnetic spectrum in the so-called “terahertz gap” and thus effectively bridges electronics with optics. The wavelengths are generally defined as being in the range 1.0-0.1 mm (100 μm), with corresponding frequencies of 300 GHz to 3 THz. THz radiation can penetrate a wide variety of non-conducting materials such as plastics, fabrics, paper, wood and ceramics but cannot penetrate metals and is strongly attenuated by water. When interacting with these materials, variations in thickness, density or chemical composition impart unique information to the THz signal in the form of intensity and phase variations, allowing these variables to be quantified. Further, THz photon energies are four orders of magnitude less than those of x-rays and are thus non-ionising, thereby allowing safe interactions with humans in applications such as medical imaging and security screening.
In 2003, this journal published a review of the THz sensing field when it was still very much in its infancy, and in 2009, recently developed THz imaging technologies were reviewed. Since then, much technological progress has been made, and commercialisation and applications have gathered pace. This article seeks to provide an updated review of the THz sensing field by considering a selection of recent research activities and commercial developments and applications.
Unlike electromagnetic radiation on either side of the THz gap, compact, high intensity and inexpensive THz radiation sources were not readily available when interest in the sensing applications of the technology first arose. This impeded progress and sources attracted a concerted research and development effort which continues today. A diverse range of materials, techniques and phenomena have been used to generate THz radiation and fall into two major categories: electronic and photonic. Some examples are listed below, and several are large and complex, multi-component systems that may require the use of vacuum technology, external pump lasers and even cryogenic cooling. Examples of those used in commercial THz sensing and imaging products are photoconductive antennas (PCAs), IMPATT diodes, QCLs and BWOs – a diversity which reflects recent technological progress and also that each has its respective benefits and limitations in terms of cost, size, power consumption, cooling requirements, stability, power output, frequency range and tunability.
Examples of terahertz source techniques
Following are the examples of the techniques:
Backward-wave oscillators (BWOs)
Folded waveguide travelling-wave tubes
Gyrotrons and synchrotrons
Extended interaction klystrons
Gunn and IMPATT (impact ionisation avalanche transit-time) diodes
Quantum cascade lasers (QCLs)
Difference-frequency mixing in nonlinear crystals
Laser-based optical heterodyning
A major, global research effort seeks to develop sources with enhanced features. This is technologically diverse and involves emerging techniques and materials such as plasmonics, spintronics, nanomaterials and metamaterials. For example, by combining a plasmonic photomixer with a novel two-section digital distributed feedback (D-DFB) diode laser, workers from UCLA, the University of Michigan and Dublin City University, have demonstrated a high power, wavelength-tunable THz source which operates at room temperature (Yang et al., 2015). The plasmonic photomixer comprises an ultra-fast photoconductor with plasmonic contact electrodes integrated with a logarithmic spiral antenna on an ErAs:InGaAs substrate. When the beam from the D-DFB laser is incident on the photomixer’s anode electrodes, a large fraction of the photogenerated carriers is generated in close proximity to the electrodes due to the excitation of surface plasmon waves. By concentrating a major portion of the incident beam near the contact electrodes, a large number of the electrons drift to the anode electrodes on a sub-picosecond timescale. The induced photocurrent is then fed to the spiral antenna to generate radiation at the beating frequency of the two main spectral peaks of the laser. Results showed a broad frequency tuning range of 0.15-3 THz. At an average optical pump power of 100 mW, radiation powers of 1.3 mW, 106 μW and 12 μW at 0.44, 1.20 and 2.85 THz, respectively, were achieved at each continuous wave radiation cycle. The aim of future work is to investigate the inclusion of plasmonic photomixing elements inside various types of bimodal lasers to yield a single chip THz source.
Several groups have investigated the capabilities of metamaterials such as split-ring resonators (SRRs) to generate THz emissions. An example is research by workers from the Ames Laboratory, IA State University and the Karlsruhe Institute of Technology who have demonstrated broadband THz wave generation using optically excited metamaterials (Luo et al., 2014). The metamaterials were U-shaped, 40 nm-thick gold SRRs. The output from a Ti:sapphire amplifier with centre wavelength of 800 nm, pulse duration of 35 fs and a repetition rate 1 kHz was used to pump an optical parametric amplifier to produce tuneable NIR radiation (1,100-2,600 nm). This was used as a generation beam to pump the metamaterial. It was discovered that when a single layer, two-dimensional array of the metamaterial was pumped, a strong, broadband THz wave was emitted with a frequency ranging from 0.1 to 4 THz.
Workers from the Fritz Haber Institute and Johannes Gutenberg University have reported THz generation from spintronic devices (Seifert et al., 2017). Spintronics differs fundamentally from traditional electronics as, in addition to charge, electron spins represent a further degree of freedom and a simple method of generating a spin-polarised current is to pass a current through a ferromagnetic material. A W/CoFeB/Pt tri-layer with a thickness of ∼6 nm on a glass substrate was excited by a laser pulse with an energy of 5.5 mJ, a 40 fs duration and an 800 nm wavelength. The pulse drives spin currents from the ferromagnetic layer (CoFeB) into the two adjacent non-magnetic layers (W and Pt) and the inverse spin Hall effect converts these spin currents into orthogonal in-plane charge currents. By design, the W and Pt layers have opposite spin Hall angles, thereby resulting in constructive superposition of the two sub-picosecond charge currents. Consequently, a broadband THz pulse is emitted into the optical far-field. This had a duration of 230 fs and an energy of 5 nJ and exhibited a gapless spectrum from 1 to 30 THz at 10 per cent of its maximum amplitude.
Detecting THz radiation has posed less of a technological challenge than its generation, being possible with several long-established techniques, notably optoacoustic Golay cells, cooled and un-cooled bolometers and pyroelectric detectors, together with newer techniques such as Schottky-barrier diodes, modified field effect transistors (FETs) and PCAs. PCAs warrant mention as they can be used both to generate and detect THz radiation and are used in a number of commercial products. Radiation is generated by firing ultra-short laser pulses at a highly resistive semiconductor thin film, generally a III-V compound semiconductor such as GaAs, GaBiAs or InGaAs, with two electrical contact pads. The laser pulses generate electron-hole pairs in the substrate, and these charge carriers are then accelerated through the substrate by a voltage applied across the electrodes, with movement of the electric field delivering a THz radiation pulse. Detection is essentially the reverse of the emission process. When a returning THz pulse is incident on the detector at the same time as an incident laser pulse, generated electron-holes are accelerated via the THz radiation to an electrode and is measured as a current. Figure 1 shows a commercial PCA source and detector pair. In these, the typical power of the emitted THz radiation exceeds 10 μW when pumped by a laser with a 30 mW output and a 150 fs pulse duration, and the detector has a usable spectral range of 0.1-4 THz. Workers from the Australian National University and the universities of Manchester and Oxford have recently reported a PCA based on a single InP nanowire (Figure 2), grown by selective area metal-organic vapour phase epitaxy (Peng et al., 2016). These detectors can provide high quality time-domain spectra and exhibited a bandwidth of 0.1-2.0 THz.
Another device capable of acting both as a THz source and a detector is the resonant tunnelling diode (RTD). These exploit quantum mechanical tunnelling, whereby electrons can tunnel through some resonant states at certain energy levels. They can be extremely compact and are capable of ultra-high-speed operation as tunnelling through very thin layers is a rapid process. Workers from Osaka University have reported a THz sensor system based on RTDs and a photonic crystal (PC) cavity in a planar waveguide which acted as a resonator (Okamoto et al., 2017). The RTD structure was grown on a semi-insulating InP substrate and a GaInAs/AlAs double-barrier quantum well constituted the tunnelling region. This yielded a compact sensing system, shown schematically in Figure 3. During evaluation, dielectric tapes of varying thicknesses were attached to the PC cavity, and the change in the resonator’s refractive index was measured. It was found that the figure of merit of refractive index sensing was an order of magnitude higher than that of metallic metamaterial resonators used in previous studies.
As with THz sources, each class of detector has its respective merits and limitations in terms of response time, sensitivity, frequency range, power consumption, size and cost, leading to an on-going body of research which has generated an extensive literature. Several groups have studied the role of nanomaterials such as carbon nanotubes, graphene films and semiconducting nanowires in FET-based detectors and an example is research by workers from Sun Yat-Sen University and the Chinese Academy of Engineering Physics (Chen et al., 2015). This involved arrays of aligned InN nanowires which were incorporated into FET structures using a transfer printing process (Figure 4). An array of the FETs was fabricated by large-scale photolithography, transfer printing and focused ion beam electrode deposition. A chip carrying 3 × 3 detectors is shown in Figure 5(a). When two or more detectors are connected in series, a stronger output photovoltage arises, and the results for different numbers of device connections are shown in Figure 5(b). The maximum responsivity was 1.1 V/W from seven interconnected devices when illuminated by a 290 GHz signal.
Some of the longer established techniques are also the topic of research. For example, a group from the University of Electronic Science and Technology of China have developed a THz imaging system based on a modified microbolometer (Wang et al., 2016). A nanostructured, titanium thin film acted as a THz absorber and was integrated with the micro-bridge structure of a VOx (vanadium oxide) microbolometer by magnetron sputtering and reactive ion etching. Continuous-wave THz detection and imaging were demonstrated by using a 2.52 THz far-IR CO2 laser as the illumination source and a 320 × 240 micro-bridge/VOx microbolometer focal plane array which was fabricated on a silicon substrate. This arrangement was successfully used to generate images of a metallic, circular washer covered by a piece of cloth and a paper clip inside an envelope.
Products and applications
A growing number of companies now offer THz sensing products which are finding applications in such diverse fields as non-destructive testing, industrial inspection, security, chemical and biological analysis, remote sensing and medical imaging such as cancer screening.
Reflecting the ever-present threat of terrorism, several companies have developed sensing and imaging systems for a variety of security screening applications. For example, TeraSense produces the “Terahertz Imaging Scanner” which finds uses such as checking postal packages for concealed objects. The product operates in transmission mode and allows imaging through envelopes, packages, parcels and small bags and can identify potentially hazardous items. The THz source is based on IMPATT diode technology and the detector is a 256-element linear array, fabricated on a single wafer from a GaAs high mobility heterostructure using standard semiconductor processing and conventional optical lithography. The company has also developed a body scanner (Figure 6) for the stand-off detection of items such as firearms, bombs and explosive belts hidden beneath clothing (Figure 7). Intended for use in locations such as airports, train stations and border crossings, it operates in reflection mode at a frequency of 100 GHz and can scan at a distance of up to 3 m from the target with a 3 cm resolution and has a field of view of approximately 70 × 70 cm. It uses a 32 × 32 or 64 × 64-pixel 2D imaging detector array to create the images.
Non-destructive thickness measurement is a growing field of application. For the inspection of surface coatings, such as paint on vehicle bodies, TeraView, a spin-out from Toshiba Research and Cambridge University and one of the longest established companies in this field, has developed the “TeraCota”, a portable, non-contacting instrument that relies on the reflection of THz pulses from the various paint layers. It can determine the thickness of individual layers in multiple layer systems on both metallic and non-metallic substrates with an accuracy of about 1.5 µm. Founded in 2010, Canadian TeTechS offer the “TeraGauge” which was developed in partnership with the University of Waterloo and uses PCAs as the source and detector. It forms the heart of the “PlastiMeasure”, a thickness measurement system designed specifically for plastic and thin-walled packaging. This can conduct measurements of up to ten layers ranging in thickness from 0.01 to 5 mm, while also locating and measuring the barrier layer within the structure. TeTechS has worked with IMD Ltd. of Switzerland to develop a dedicated plastic bottle thickness testing system, the “IMDvista Layer”.
Several applications are emerging in the microelectronics industry. For example, TeraView, together with Intel Corporation, developed an IC package fault analysis and quality assurance system, the “EOTPR 2000” (electro optical terahertz pulse reflectometry – EOTPR) which has been used for manual fault isolation and inspection. It was superseded by the fully automated “EOTPR 5000” in 2016. These products use a technique termed “EOTPR” to detect weak or marginal interconnect quality and other defects in advanced IC packages. The system represents the world’s first use of THz sensing to isolate and detect IC package faults. With ever-falling IC feature sizes and the growing use of nanomaterials in electronics and other applications, a need exists for non-destructive, sub-surface inspection and imaging techniques with nm/sub-nm resolution. This has been addressed by Applied Research and Photonics who have developed an imaging system that uses its proprietary dendrimer dipole excitation-based continuous wave THz source (Rahman et al., 2016) which, through reflection-mode scanning in three orthogonal directions can create sub-surface 2D and 3D images with nm resolution. The technique has been used to generate images of a film of silver iodide quantum dots (QDs) with a size of ∼7.7 nm, deposited on a silicon substrate. A 5 µm3 volume was scanned on a layer-by-layer basis in the X, Y and Z directions and a 100 nm3 segment was extracted from the top, creating the image shown in Figure 8. Figure 9 shows three separate layers extracted from the image. The company has also demonstrated time-domain spectroscopy and high resolution reconstructive imaging via THz reflectometry of epitaxially grown semiconductors, notably SiGe/Ge/Si, with nm resolution, using the arrangement shown in Figure 10 (Rahman and Rahman, 2016). Fourier transform of the time-domain signal was used to compute the absorbance spectra of the samples and it was found that these spectra yield unique signatures for Ge and SiGe epitaxial layers, as well as for the Si substrate. This technique has potential to play an important role in quality inspection at the production line as well as in various process development purposes.
THz sensing technology is a rapidly developing field and a strong body of research seeks to develop improved sources and detectors with enhanced features which often exploit novel materials, phenomena and technologies. Commercialisation is rapidly gathering pace and a growing number of companies are producing THz sensing products which are finding uses in a diversity of applications which will doubtless increase as the technology matures.
(a) Schematic of the transfer printing process, (b) SEM of InN nanowires in array groups. Scale Bar: 5 μm, (c) SEM image of the aligned nanowires with source and drain electrodes deposited. Scale Bar: 2 μm, (d) SEM of the device with a gate of 300 nm Al2O3 deposited, (e) SEM of the FET based on five aligned nanowires. Scale Bar: 2 μm, (f) SEM of the whole device. Scale Bar: 100 μm
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