One aspect of the field of THz radiation is the marriage of microwave
and optical techniques. By its very nature, THz radiation bridges the
gap be- tween the microwave and optical regimes. The former can be
characterized by the fact that most devices are comparable in size to
the wavelength of the radiation. As a result, the propagation of energy
in these devices is generally in the form of single-mode or
low-order-mode guided waves. In contrast, the optical and infrared
ranges are generally characterized by beams containing many modes, with
dimensions much larger than the wavelength. Of course, there are
exceptions to these rules, notably the single-mode propagation of
optical radiation in fibers. Nonetheless, the general description holds
true. Because of these fundamental differences, it is natural that the
techniques used in their implementation are quite distinct. Much of the
research in the THz field has been based on the melding of these
disparate ideas.