Closing the frequency gap - recent developments in submillimetre-wave techniques

The submillimetre-wavelength region, which has hitherto been almost devoid of coherent sources, is being attacked on two flanks. On the one hand, electron-beam devices are being pushed to frequencies previously thought unattainable, so that appreciable amounts of power can be generated at frequencies above 100GHz, and Gunn diodes operated in the l.s.a. mode have already nearly breached the submillimetre barrier; while, on the other hand, stimulated-emission techniques are being developed at ever increasing wavelengths, so that laser-type sources are available from the visible region down to submillimetre wavelengths. Lasers are normally fixed-frequency devices, but developments in nonlinear optical materials are likely to lead to tunable sources either by difference-frequency mixing or by parametric interactions.