Catálogo de publicaciones - libros

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.