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T-ray: King of Lasers?
9 May 2002 (All day)
A new microscopic laser uses a sandwich of semiconductors to generate a beam of t-rays, a rarely used--but much coveted--type of radiation. The inventors say these laser beams could have a tremendous variety of uses--from communications to medicine to astronomy.
T-rays, or terahertz radiation, lie between the infrared and microwave swaths of the electromagnetic spectrum. T-ray laser beams can go right through plastic, concrete, and other common materials, which makes them extremely well-suited for wireless communications, inspecting packages, and astronomy. In addition, terahertz radiation could determine the structures of many complex chemicals, yet is safe for biological use--desirable properties for medical imaging. Unfortunately, everyday lasers made with silicon and other semiconductors are extremely bad at converting electricity into t-rays. Previous terahertz lasers had to rely on gases stored in bulky containers or high-energy magnetic fields.
The new t-ray laser, described in the 9 May issue of Nature, is only about 10 micrometers thick, or a tenth the width of a human hair. The heart of the device is a microscopic sandwich of quantum wells--crystalline films of semiconductors only a few billionths of a meter thick, each with a different electrical conductivity. When this stack is charged with electricity, the electrons trapped between the more electrically resistant layers pump up each other's energy levels and emit photons. By delicately weaving 1500 layers together, a team of researchers created a "quantum cascade" that amplified this effect into a laser beam. "The most surprising part is that our laser actually worked," says team member Alessandro Tredicucci of the National Enterprise for Nanoscience and Nanotechnology in Pisa, Italy.
"This is very surprising and very good news," says physicist Daniel Mittleman of Rice University in Houston. Mittleman says some theorists had thought such a laser was impossible. Still, a lot of kinks need to be worked out before the new laser is ready for mass-production. The laser works best at about -265 degrees Celsius, and commercial applications usually require -195 degrees Celsius--the boiling temperature of cheaply available liquid nitrogen--if not room temperature. Even so, terahertz radiation expert David Citrin, an electrical engineer at the Georgia Institute of Technology in Atlanta, says the new laser is "an outstanding achievement."
Bell Labs Introduction to Quantum Cascade Lasers