As noted in the story on the FCC white paper on unlicensed technology, there is a demand for more spectrum for new technologies. The white paper lists ways to increase the efficiency of spectrum currently being used. Another option, which it touches on briefly, is to take advantage of little used spectrum.
Throughout the history of radio communications, frequencies originally thought to be unusable turned out to be quite usable with appropriate technology. Ham radio operators were allowed to use those useless "short-wave" frequencies and found they were able to span the globe with far less power than that required for "long-wave" low frequencies. Of course, those big alternators that were used to generate high power long wave signals wouldn't work at short waves. You could argue tubes were needed to take advantage of these frequencies. The upward march continued. Satellites went from 4 GHz to 12 GHz and recently to Ka band frequencies over 24 GHz.
What is the upper limit? Frequencies in the terahertz range--over 100 GHz--approach the frequency of light. In the upper terahertz range, devices begin to look more like optics than RF components.
What could terahertz spectrum be used for? If we discount the complexity of generating terahertz waves, any of the applications that use infrared for communications should work well in the terahertz range. Terahertz frequencies should also work better than lasers for high bandwidth short distance point to point links. Current applications include radio astronomy, spectroscopy, and radiometric sensors. Terahertz waves have also been used in the development of complex radar systems. Since terahertz wavelengths are proportionately smaller than gigahertz wavelengths, accurate data can be collected using scale models. "T-rays" are attracting interest as a possible, less damaging, alternative to X-rays.
There are different methods of generating terahertz frequencies. Some techniques use frequency multiplexers, others use negative resistance diode oscillators and more exotic techniques use lasers and optical down-converters.
The equipment used to generate and sense terahertz waves is quite interesting, blending RF and optical techniques. The University of Bath Terahertz Technology Group web site
has great photos of extremely small RF components
. Technical descriptions, drawings and photos of solid-state terahertz devices are available on the University of Michigan Center for Space Terahertz Technology Web site
The most detailed description of terahertz technology and applications is in a paper by Peter H. Siegel. Terahertz Technology
is hidden on NASA's Single Aperture Far-Infrared Observatory Web site
If you wade through these documents, you will probably come to the conclusion that terahertz frequencies are of little use for broadcasting or communications. Of course, people thought the same thing about shortwave and UHF frequencies not too many years ago!