Recent events have accelerated the quest for ever more effective security screening to detect an increasing variety of
threats. Many techniques employing different parts of the electromagnetic spectrum from radio up to X- and gammaray
are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want,
until recently, of suitable sources and detectors. This paper describes practical techniques for Terahertz imaging and
spectroscopy which are now being applied to a variety of applications. We describe a number of proof-of-principle
experiments which show that Terahertz imaging has the ability to use very low levels of this non-ionising radiation to
detect hidden objects in clothing and common packing materials and envelopes. Moreover, certain hidden substances
such as plastic explosives and other chemical and biological agents may be detected from their characteristic Terahertz
spectra. The results of these experiments, coupled with availability of practical Terahertz systems which operate outside
the laboratory environment, demonstrate the potential for Terahertz technology in security screening and counterterrorism.
Impulsive optical excitation of the lowest two conduction or valence subbands of a GaAs/AlGaAs double quantum well can lead to coherent THz emission associated with quantum beating of subband states. We find that in the conduction band the emission arises from a time varying intersubband polarization generally dominated by the beating of continuum rather than bound exciton states. This is apparent in the electric field and excitation energy dependence of the frequency and amplitude of the THz radiation. Wavepackets made up of these continuum excitons have dephasing times of several picoseconds even for excitation an otpical phonon energy above the lowest subband edge. The long lived coherence in partly attributed to the small energy difference between the eigenstates, which substantially reduces the number of relevant scattering events, and partly to the very similar dispersion of the subbands which restricts dephasing by interference. The effect of interference is revealed in systems with significant dispersion of the intersubband gap. Two examples are presented: the valence band of a double well and the conduction band in the presence of an in-plane magnetic field.