Unless you work with VHF/UHF radio, microwave technologies, or perhaps radar, you probably don't deal with higher frequency signals much, although now that 2.5 Gbit/s and 5 Gbit/s serial data streams are a part of many systems, that picture is changing for circuit designers.
At UHF (whether it’s 5 GHz or 5 Gbit/s) and into the conventional microwave domain, up to perhaps a few hundred GHz or so, transmission line principles come into play. Striplines, tuned cavities, plumbing-like connectors, waveguides, horns and corner reflector antennas, ultra-low-C probes, and critical-dimension circuit elements are the rule.
If you're lucky enough to work with so-called millimeter-wave systems, this is routine stuff. If you're just making the transition into the Gbit/s domain, confer with the folks at Wavecrest, LeCroy, Tektronix, Agilent, and the like. They'll help you along and sell you some intriguing test-and measurement instrumentation in the bargain.
Somewhere above a few hundred GHz, though, conventional physics changes. Between the communications and radar frequencies we know a lot about, and the infrared regions of visible light, there exists a no-man's land. These wavelengths, shorter than a millimeter (measured in micrometers), are referred to as terahertz (THz) signals. A THz is equal to one trillion Hz.
At THz frequencies, signals are subject to weird effects such as molecular absorption and quantum-level interactions. A lot of THz technologies border on, and merge with, optical and phototonics sciences.
Until recently, not much was known about how signals play in the THz domain. This picture is changing, however, with longer-wave optical time-domain spectral instruments emerging, and with the advent of reasonably practical CW and pulsed THz sources such as backed-wave oscillators, optically pumped THz lasers, DM (direct multiplied) sources,
http://www.virginiadiodes.com and the like.
Thinking about THz signals, I decided to dust off a book sitting on my shelf since the late 1970s. It looks at THz signals, but from a remarkable and unique point of view. The author is Dr Philip S Callahan. As a professor of entomology (the study of insects), Callahan's
Tuning Into Nature subtitled
Solar Energy, Infrared Radiation and the Insect Communication System, (1975, Devin-Adair, ISBN 0-8159-6309-2) is a thought-provoking work.
In his dissertation, Callahan shows how insects communicate in the 17 octaves of energy that exist between 1-mm wavelengths starting above the highest microwave bands, and the IR region. Most amazing is his description of the antenna-like structures on the wings and spines of many insects. These electret structures operate like microwave dish and horn antennas.
Drawing upon the work and advice of his colleagues at NASA, Georgia Institute Of Technology, Willow Run Infrared Radiation Lab, and elsewhere in the scientific and engineering communities, Callahan describes how an electromagnetic wave energy converter, or
EWEC, could vastly boost the efficiency of solar energy conversion.
Callahan explains that for wavelengths between 1 µm (micrometer) and 30 µm in IR, there is almost total absorption of energy from the sun between about 5 µm and 7 µm. But, he points to excellent windows between 1 µm and 2.5 µm, between 3.5 µm and 5 µm, and between 7 µm and 14 µm.
The EWEC makes use of these bands. Patented by University of Florida professor Robert Bailey in 1973, the EWEC resembles the wax-coated antenna-like sensors of many insects. In
Tuning Into Nature Callahan postulates that an EWEC converter could have a conversion efficiency of 50% to 70%.
More than thirty years have elapsed since the EWEC patent was issued and Callahan's book appeared. I wonder if anyone is working on a practical solar converter based on the EWEC?
Have you any experience, knowledge, or opinions about THz energy, THz solar converters, or the EWEC? If so, please write me at
amm at en-genius dot net, or post your comments here.
Some References:
- Arnone, D D et al. Application of terahertz technology to medical imaging. SPIE Terahertz Spectroscopy Applications II; International Society for Optical Engineering: Bellingham, WA, 1999; pp 209 – 219
- Köhler, Tredicucci, Beltram, Beere, Linfield, Davies, Ritchie, Iotti, Rossi. Terahertz semiconductor-heterostructure laser. Nature 2002, 417, 156
- Mueller, Fontanella, Henschke. Stabilized, Integrated, Far-Infrared Laser System for NASA/Goddard Space Flight Center. 11th International Symposium on Space Terahertz Technology, Ann Arbor, MI, May 1 – 3, 2000
- Mueller, Waldman, Power and Spatial Mode Measurements of Sideband Generated, Spatially Filtered, Submillimeter Radiation. IEEE MTT 1994, 42 (10), 1891
- Rochat, Ajili, Willenberg, Faist, Beere, Davies, Linfield, Ritchie. Low-threshold terahertz quantum- cascade lasers. Applied Physics Letter. 2002, 81, 1381
- Siegel, P H Terahertz Technology. IEEE MTT 2002, 50 (3), 910
- Williams, Callebaut, Kumar, Hu, Reno. 3.4-THz quantum cascade laser based on LO-phonon scattering for depopulation. Applied Physics Letter 2003, 82, 1015
