This paper reviews earlier studies on superluminal wave propagation in anomalously dispersive media that have been carried out in the electronic, microwave, and optical regimes. Those studies are relevant to observation of modulated Gaussian pulses transmitted through various media at speeds apparently faster than c without distortion. This paper also presents the condition for superluminal propagation that is established based on the magnitude-phase relation of a causal and minimum-phase filter. Since the condition is modeled on the basis of filter theory, it is applicable to all types of media. A terahertz experiment with a periodic bandgap structure is also included to illustrate superluminal propagation.

THz science is developing rapidly in Europe as well as the rest of the world. There is a strong interest in the exploitation of optical technologies in the THz frequency range in virtually all fields of basic and applied sciences of physics, chemistry, biology as well as medicine. Commercial interest in this field has also been growing, spurred by the potential of THz tools in quality control and the biotechnology sector. We will review some contrast mechanisms, which form the basis for real-world applications of THz technology, in particular in the fields of applied chemistry and biotechnology. Whereas narrow bandwidth THz technology may become important for, e.g., real-time imaging at larger standoff distances, we will concentrate on broad bandwidth THz technologies for spectroscopic identification of various substances. It has recently been established that the 0.1-5 THz spectral range contains unique fingerprints of a very large number of crystalline materials, including explosives, illicit drugs as well as most other chemicals in powder form. Since many packaging materials are transparent to THz radiation this fundamental property of crystalline compounds allows remote (contact- free) sensing combined with chemical recognition. On the other hand, the THz spectrum of amorphous systems, including aqueous solutions, contains very little information about the detailed composition of the system. However, under certain conditions it is still possible to learn a great deal about amorphous systems with broadband THz spectroscopy. Amorphous systems of great biotechnological importance include DNA and proteins, both in aqueous solution and as dried matter. We will discuss methods for THz science and technology to attack the very complex problems involved in the extraction of useful new information, which may be difficult, expensive, or impossible to obtain with other methods, from minute amounts of biomaterial.

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.