ABSTRACT
The spectroscopy in the terahertz (THz) region of the electromagnetic spectrum is an important and growing field of research with a wide range of applications. In contrast to incoherent techniques which detect the intensity of the THz radiation, the time-resolved approach is intrinsically coherent and hence allows the detection of amplitude and phase of the radiation, enabling one to derive a wider range of information. An interesting alternative emission mechanism of coherent THz radiation was recently observed, namely the coherent emission from optically excited plasmons. Electrically-pumped sources of coherent radiation would then make widespread application of THz spectroscopy systems feasible. It is important to distinguish several types of temporal coherence after the photoexcitation of charge carriers by optical transitions. The basic theoretical description for the emission characteristics of a coherent ensemble of emitters is based on the multiplication principle of conventional antenna theory.
