Terahertz Frequency Detection and Identification of Materials and Objects

Various physical mechanisms leading to terahertz (THz) emission from semiconductor surfaces illuminated by femtosecond laser pulses are analyzed. Results obtained on different materials are described and relative efficiency of these materials as THz emitters is compared.

We report on the status of symmetric varactor diode multipliers for signal generation in the terahertz frequency range. The progress and basic principles of heterostructure barrier varactor (HBV) diodes are presented. Furthermore, the design methodology and electro-thermal simulation results of high-power HBV multipliers for signal generation in the millimeter and submillimeter wave region are also presented. Finally, a state-of-the-art HBV tripler with an output power of 0.2 Watt at 113 GHz is presented.

We describe our work towards THz sources which employ the “Bloch gain”, a stimulated-emission mechanism which has been predicted as early as 1971 to exist for semiconductor superlattices but which researchers – in spite of much recent work – have not yet been able to take advantage of for the implementation of THz amplifiers and lasers. From a basic-physics point-of-view, the interest in Bloch gain arises from its dynamical, second-order character, involving simultaneous scattering of an electron and emission of a THz photon. This aspect has the practically important implication that the temperature dependence of the gain is determined to a large degree by the optical-phonon energy scale and not that of the photon energy, with the consequence that there is a rather slow roll-off the gain with temperature. This feature together with the rather high-gain values which are calculated to be comparable with those of THz quantum cascade lasers at low temperature, fosters the hope that a Bloch THz laser could be the first semiconductor-based THz laser operating at room temperature.

For THz quantum cascade lasers to prove useful for applications, certain requirements for their spectral performance will have to be met. Here, we focus on the provision of single mode operation. Distributed feedback devices lasing on a single longitudinal mode are reported using both first and second order gratings. We also report the operation of terahertz master oscillator power amplifier structures with the potential to increase the output power which is available in a single mode.

We demonstrate a new waveguiding structure for terahertz (THz) radiation, in which broadband THz pulses are confined and guided along a bare metal wire. This waveguide exhibits close to the lowest attenuation of any waveguide for broadband THz pulses reported so far. It also supports propagation of broadband radiation with negligible group-velocity dispersion, making it especially suitable for use in pulsed terahertz sensing and diagnostic systems. In addition, the structural simplicity lends itself naturally to the facile manipulation of the guided pulses, including coupling, directing, and beam splitting. These results can be described in terms of a model developed by Sommerfeld, for waves propagating along the surface of a cylindrical conductor.

The paper discusses and compares the concepts, performance potential, and most recent experimental results of both classical and novel active two-terminal devices for low-noise RF power generation at submillimeter- wave frequencies up to 1 THz. These devices use transit-time, transferred-electron, and quantum-mechanical effects (or a combination of them) to create a negative differential resistance at the frequency of interest. Examples of state-of-the-art results are output power levels of more than 70 mW at 62 GHz and more than 10 mW at 132 GHz from GaAs/AlGaAs superlattice electronic devices; more than 9 mW at 280 GHz, 3.7 mW at 300 GHz, 1.6 mW at 329 GHz, and more than 40 μW at 422 GHz from InP Gunn devices; and more than 140 μW at 355 GHz from a GaAs tunnel-injection transit-time diode.

We review a selection of recent technological advances in terahertz frequency time-domain spectroscopy. We discuss the coherent generation of ultra-broadband terahertz radiation using a biased and asymmetrically excited low-temperature-grown GaAs photoconductive (PC) emitter. Using a backward collection method, terahertz radiation with frequency components over 30 THz can be collected, the highest observed for PC emitters. We outline two detection schemes, electro-optic (EO) detection using a ZnTe crystal, and PC-detection using a low-temperaturegrown GaAs PC receiver. The use of the PC receiver provides the timedomain spectroscopy system with a smooth spectral distribution between 0.3 and 7.5 THz, ideal for spectroscopic applications. We illustrate the technological developments with examples of transmission spectroscopy of polycrystalline organic materials. Specifically, we review measurements of the vibrational spectra of polycrystalline purine and adenine over the temperature range 4–290K. A number of well-resolved absorption peaks are observed, which are interpreted as originating from intermolecular vibrational modes mediated by hydrogen bonds. As the temperature is reduced, the observed absorption bands resolve into narrower peaks and some shift towards higher frequencies, which can be explained by the anharmonicity of the vibrational potentials. An empirical expression is given to describe this frequency shift.

Terahertz (THz) time-domain spectroscopy (TDS) is used to study two types of amorphous materials: glasses and polymers. The theory of far-infrared (IR) absorption in amorphous materials is used to analyse the results, and to understand the differences in THz absorption among the sample materials. A family of related borosilicate glasses has been examined along with silica glass, and their THz absorption coefficients and refractive indices are compared. Two chalcogenide glasses are also studied. Three types of polymer plates have been examined, and their THz transmission properties are compared with those of glasses. Polymerisation in SU8 films has been studied by exposing samples to UV for different lengths of time and comparing their THz transmission properties.

The vibrational modes corresponding to protein tertiary structural motion lay in the far-infrared or terahertz (THz) frequency range. These collective large-scale motions depend on global structure and thus will necessarily be perturbed by ligand-binding events. We discuss the use of THz dielectric spectroscopy to measure these vibrational modes and the sensitivity of the technique to changes in protein conformation, oxidation state and environment. A challenge of applying this sensitivity as a spectroscopic assay for ligand binding is the sensitivity of the technique to both bulk water and water bound to the protein. This sensitivity can entirely obscure the signal from the protein or protein–ligand complex itself, thus necessitating sophisticated sample preparation making the technique impractical for industrial applications. We discuss methods to overcome this background and demonstrate how THz spectroscopy can be used to quickly assay protein binding for proteomics and pharmaceutical research.

Spectroscopic imaging with terahertz (THz) or submillimeterwave (SMM) sources holds great promise for both defense and dual-use applications, such as for detection of chemical/biological weapons (CBW), concealed explosives, and other weapons (particularly nonmetallic varieties), and even through-the-wall imaging. To perform spectroscopy with active illumination of the target, either multiple or tunable continuous-wave (CW) sources or broadband pulsed sources are needed; passive illumination (e.g. using the cold sky) is limited to outdoor settings. While using incoherent (or intentionally decohered) illumination, either from the sky, a noise source, or a frequency-modulated CW source helps to reduce the interference caused by standing-wave phenomena (analogous to laser speckle), all such approaches have severe limitations in that they cannot perform accurate ranging, they are limited to a narrow range of frequencies, or are relatively weak. They are also all fundamentally limited to incoherent detection, which has limited signal-to-noise ratio (SNR) performance, lacking the advantages of heterodyne downconversion and detection. Using pulsed, broadband, coherent THz or SMM sources, and detectors is ideal for spectroscopic imaging and detection.

There is a fundamental difference between the dielectric spectra of crystalline systems as well as amorphous and liquid systems in the THz range. Here we discuss recent theoretical progress on the calculation of the lowest vibrational modes in crystalline compounds. With density-functional perturbation theory it is possible to simulate, in a quantitative manner, the THz vibrational spectrum of hydrogen-bonded molecular crystals. In contrast to crystalline systems, aqueous systems have a featureless dielectric spectrum in the low-THz range. We will show that it is possible to use reflection THz spectroscopy to measure the alcohol concentration in aqueous solutions in a manner that is independent of the contents of other ingredients such as sugar, yeast, and organic particles.

The terahertz (THz) region is beginning to be exploited for many “real world” applications. The development of pulsed photoconductive THz generation and detection has already yielded a range of successful products, and further research into both applications and complementary THz technologies promises much more. In security screening, THz radiation has potential to image through clothing to detect concealed objects. In medical imaging, THz shows promise in both diagnosis roles and as a surgical tool. There are also many hitherto unexploited applications in both analytical spectroscopy and gas phase sensing. However, the demonstration of performance and functionality is only the first step in a product development process, which must also address commercial and physical constraints. Continuous-wave (cw) THz systems based on photomixer technology are attracting increasing interest, and appear to fulfill many of these criteria. In this chapter, an overview of this technique and practical implementation considerations for “real world” applications are discussed, with demonstrations of our cw technology and routes to future commercial development.

The direct use of terahertz (THz) spectral features in screening for explosives, drugs, and other contraband is limited since adjacent materials may obscure the signature of a suspect material. This paper describes a system that overcomes this problem by combining tomographic imaging with THz spectroscopy. This paper identifies a set of 30-GHz-wide windows in the atmospheric water vapour absorption up to 3 THz: data collected in these bands permit the reconstruction of an object with 1-cm resolution. Images of the distribution of innocuous and suspect material within the object are generated by an algorithm providing an optimised filter. The spectral characteristics of any region within the object may also be examined. Practical issues associated with implementing this system are discussed.

Since early 2002, HMGCC has been monitoring developments in Terahertz (THz) technology in government, academia, and industry, as applied to counterterrorism and wider security applications, both at home and abroad. In addition it has funded proof of concept studies in UK industry and research in UK universities (Leeds and Durham) to help form a view as to its likely performance. This paper summarizes the results of work that HMGCC and other UK Government Departments have undertaken to date and draws a positive conclusion as to the prospective operational performance of THz systems in the security domain. Suggestions as to the way the technology needs to be developed, to enable both cost and functionally effective systems to be produced that will satisfy operational needs, are made.

This paper describes the main parameters – contrast, spatial resolution, and thermal sensitivity – which define the performance of any stand-off imaging system. The origin of the signature for both metal and dielectric objects hidden under clothing in the frequency range from 100 GHz to 500 GHz is discussed. At 100 GHz the signature is dominated by reflection whilst at 500 GHz it is dominated by emission. A 94-GHzpassive millimetre-wave imaging system has been designed and fabricated to image objects under clothing. This imager is based on a Schmidt camera folded using polarisation techniques.

We have developed compact THz-wave parametric generators with different characteristics that operate at room temperature. One generates high energy and broadband THz waves, being suitable for detecting the transmission of absorptive or diffusive samples, and the other has a potential of wide tunability and narrow linewidth, useful for spectroscopic measurements. In our laboratory, THz waves continue to broaden their range of applications. We have developed a basic technology for THz imaging which allows detection and identification of drugs concealed in envelopes by introducing the component spatial pattern analysis.

Terahertz (THz) radiation offers innovative sensing technologies that can provide information unavailable through other conventional electromagnetic techniques. With the advancement of THz technologies, THz sensing will impact a broad range of areas. This chapter focuses on the use of THz spectroscopy for sensing applications in three aspects: explosives detection, pharmaceutical identification, and biological characterization. A THz spectral database in the 0.1–20 THz range was established using both THz time-domain spectroscopy (THz-TDS) and Fourier transform farinfrared spectroscopy. The calculated spectra based on density functional theory and the experimental results are in good agreement in the 3–20 THz range, but not in the 0.1–3 THz range. It is also demonstrated that THz spectroscopy is capable of detecting and identifying the explosive Royal Demolition Explosive (RDX) in diffuse reflection geometry. THz-TDS was applied successfully for pharmaceutical identification, such as identifying hydrated and anhydrous drugs, probing the reaction kinetics of dehydrations, and solid-state reactions of pharmaceutical materials. It was found that most solid-state amino acids, purines, and other small biocompounds have THz absorption features in the 0.1–3 THz range. Solid-state proteins and bioactive protein microsuspensions in organic media also exhibit THz absorption features which may reflect their collective vibrational modes and could be used to probe their functional 3D conformation states. Additionally, owing to the high sensitivity of differential THz-TDS, it was successfully used to sense the minute change of biological cell monolayers. These studies have pointed to new ways for using THz spectroscopy in pharmaceutical and biological sensing.

We discuss basic considerations for potential short-range THz communication systems which may replace or supplement present WLAN systems in 10–15 years from now. On the basis of a few fundamental estimations we show that such a system will need a line-of-sight connection between receiver and emitter. To circumvent the blocking of the direct line-of-sight connection indoor THz communication systems will also have to rely on non-line-of-sight paths which involve reflections off the walls. The reflectivity of the walls can be enhanced by dielectric mirrors. This new scheme makes steerable high-gain antennas a necessity. Hence, a wireless THz communication system can not be a simple extension of the existing technology of today’s local area networks. Instead it involves completely new concepts and ideas that have not yet been worked upon.

It is now over a decade since the first papers were published on Terahertz (THz) pulsed imaging. Much was then claimed for this field, and many applications were confidently foreseen in medicine, biology, process engineering, surveillance, and security. During the intervening 10 years, a variety of developments have taken place in both THz science and technology and the subject is maturing rapidly. In this final chapter, the central Themes of this workshop will be briefly reviewed. In addition, a summary will be presented of the key questions that must be addressed in the short term if applicable. THz science is to realise some of the potential which has been claimed for this new window on the electromagnetic spectrum.