http://phys.org/news/2012-07-terahertz.html
By Bill Steele
(Phys.org) -- Cornell researchers have developed a new method of generating terahertz signals on an inexpensive silicon chip, offering possible applications in medical imaging, security scanning and wireless data transfer.
Terahertz radiation, the portion of the electromagnetic spectrum between microwaves and infrared light, penetrates cloth and leather and just a few millimeters into the skin, but without the potentially damaging effects of X-rays. Terahertz scanning can identify skin cancers too small to see with the naked eye. Many of the complex organic chemicals used in explosives absorb terahertz radiation at particular frequencies, creating a "signature" that detectors can read. And because higher frequencies can carry more bandwidth, terahertz signals could make a sort of super-Bluetooth that could transfer an entire high-definition movie wirelessly in a few seconds.
Current methods of generating terahertz radiation involve lasers, vacuum tubes and special circuits cooled near absolute zero, often in room-sized apparatus costing thousands of dollars. Ehsan Afshari, assistant professor of electrical and computer engineering, has developed a new method using the familiar and inexpensive CMOSchip technology, generating power levels high enough for some medical applications. With further research, higher power will be possible, Afshari said, enabling such devices as handheld scanners for law enforcement.
Afshari and graduate students Yahya Tousi and Vahnood Pourahma describe the new approach in the June 8 issue of the journal Physical Review Letters.
Schematic of a ring of oscillators (gray circles) coupled to generate terahertz frequencies. Coupling circuits (blue triangles) shift the phase of the oscillations to reinforce the fourth harmonic. (Ehsan Afshari)
The ability of solid-state devices to generate high frequencies is limited by the characteristics of the material -- basically, how fast electrons can move back and forth in a transistor. So circuit designers make use of harmonics -- signals that naturally appear at multiples of the fundamental frequency of an oscillator. That fundamental frequency is usually set by a circuit that uses a variable capacitor called a varactor, but at terahertz frequencies varactors don't tune sharply. Afshari has come up with a new way of tuning by coupling several oscillators in a ring, producing what engineers call a high-quality signal, where all the power goes into a very narrow frequency band.
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