Miniaturised vacuum technologies: do they have a future for terahertz frequency devices?

Summary form only given. The Terahertz (THz) frequency range (300 GHz-10 THz) is the least exploited region of the electromagnetic spectrum. At present there are no small, convenient and low-cost THz sources. The fabrication of conventional vacuum tube sources, e.g. klystrons, by traditional workshop techniques becomes difficult above 100 GHz. Sources such as Carcinotrons are expensive, bulky and require high voltages. A radical alternative is to combine micromachining and field emitter tip ("cold cathode") technologies. Micromachining technologies, especially those based on techniques used in the semiconductor processing industry can deliver complex structures such as waveguides and cavities of an appropriate length scale for THz operation i.e. a few tens of micrometres. Furthermore, the tolerances and surface obtainable makes this method entirely suitable for the realisation of low-cost, high quality components. Field emitter tips produce high current emission at relatively low voltage (tens, rather than thousands, of volts). By combining these approaches, an entirely new generation of mass-market THz devices can be realised. In this Invited Contribution, a review will be given of the present state of the art of work on THz micromachined sources. The review will include a discussion of basic principles and in particular how scaling rules apply to the working of a simple reflex klystron device. Different routes to micromachining cavities and other components will be reviewed, and their relative merits will be discussed. Comparisons will be made between nanotube and nanotip devices as sources of electrons. Latest results from our own programme will be presented. Finally, some well-known (and perhaps less well-known) vacuum tube sources, operating at lower frequency, will be "revisited" from the point of view of potential operation at THz frequencies using the new combination of approaches outlined earlier.