Near-infrared radiation and some visible light now are used for fiber optic phone and data lines. Current optical wireless systems use very low intensity infrared radiation—the "optical" segment of the electromagnetic spectrum with wavelengths longer than those of visible light but shorter than those of radio waves — which people cannot sense. However, terahertz or far-infrared radiation on the spectrum between microwaves and mid-infrared radiation is not now used for communications. Like infrared radiation or microwaves, these waves usually travel in line-of-sight. Terahertz radiation is non-ionizing and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They can also penetrate fog and clouds but cannot penetrate metal or water. The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness.

In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s. Terahertz is a new region of the spectrum for communications because the rest of the spectrum is crowded with communications and broadcasting signals. There is a high demand for broadband communications, yet terahertz frequencies are unexplored. They are too high for electronics and there are technical obstacles in generating, manipulating and detecting terahertz radiation. For electromagnetic radiation to transmit data, the signal must be turned on and off to rapidly create the binary code of ones and zeroes. Modern optical and electronic switches cannot do that fast enough to handle signals with terahertz frequencies (1,000 billion waves per second), but can handle gigahertz signals (1 billion waves per second).

No one has built terahertz switches, but it is possible to use terahertz radiation to carry data and thus may be possible to create terahertz-speed switches for super fast wireless communications over short distances, such as between a cellular phone and headsets, a wireless mouse and a computer, and a PDA (personal digital assistant) and a computer.

Vibration spectroscopy uses emitters and detectors of terahertz radiation to detect materials such as anthrax or other biological or chemical weapons that resonate at a terahertz frequency when exposed to far-infrared light. Early terahertz devices emit numerous frequencies. Perforated films can serve as filters so future terahertz devices can use desired frequencies to zero in on specific chemical or biological weapons or concealed guns and explosives. Another method uses a terahertz emitter and a camera. Since plastics and clothing are transparent to terahertz wavelengths, metal reflects terahertz, and certain chemicals such as plastic explosives strongly absorb terahertz radiation at specific frequencies, this approach is being pursued for package inspection and whole-body imaging to look for concealed weapons or explosives. Scanners use X-rays or microwaves. But scanners using terahertz radiation should lack the risk of X-rays and be more precise than microwave scanners.

Modern technology uses many frequencies of electromagnetic radiation for communications, including radio waves, TV signals, microwaves and visible light. Far-infrared light known as TERAHERTZ region, the last unexploited part of the electromagnetic spectrum, can be harnessed to build much faster wireless communications and to detect concealed explosives and biological weapons.

Actual hardware testbed for detecting concealed explosives and biological weapons and much faster wireless communications.

• M. Kavehrad, "Broadband room Service by Light," Scientific American Journal, pp. 82-87, July 2007.
• B. Hamzeh, S. Jivkova, and M. Kavehrad, "Arbitrary Waveform Synthesis for Communication Applications," Journal of Optical Networking, Vol. 4, No. 10, pp. 647-656, October 2005.
• S. Jivkova, M. Kavehrad “Holographic Optical Receiver Front-End for Wireless Infrared Indoor Communications,” OSA Applied Optics Journal, June 2001.

Mohsen Kavehrad, Electrical Engineering