Laser bridges infrared-microwave gap

By Eric Smalley, Technology Research News

Technologies that broadcast radio and television programming, transmit data, scan the human body and search the stars use electromagnetic radiation in the form of radio waves, microwaves, infrared light, visible light, ultraviolet light and x-rays.

These different types of radiation have waves of vastly different lengths. Various communications technologies, for instance, use waves that measure several thousand kilometers to only a few thousandths of a millimeter from crest to crest. More of the shorter waves pass a given point per second, allowing them to transmit information faster. The newest generation of cell phones, for example, transmit at 2.2 gigahertz, or billion times per second.

Where infrared, or heat waves, meet microwaves there is a gap in the wavelengths traditionally harnessed for communications and detection technologies. Devices capable of producing electromagnetic waves at these frequencies -- around a few trillion hertz -- could be the key to safer medical imaging, detectors for toxic gases and explosives, and high-speed wireless communications.

Researchers from Italy and England have taken a big step toward filling the terahertz wavelength gap by building a chip-based laser that emits light at 4.4 terahertz. The terahertz semiconductor laser currently requires cryogenic cooling, but has the potential to work at higher, more useful temperatures, said Rüdeger Köhler, a physicist at the Italian National Institute for Condensed Matter Physics (INFM).

Terahertz beams are particularly desirable for medical imaging because they produce much less energy than x-rays and so are less stressful to biological tissue, said Köhler. Terahertz radiation could also be tuned to highlight specific types of tissue to detect, for example, cavities or early-stage skin cancer, he said.

Terahertz lasers also have security applications. Scanners could penetrate clothing and plastics to detect hidden metal objects, and chemical detectors could identify toxic gases and explosives that have characteristic spectral fingerprints, or vibrations, in the terahertz region, said Köhler.

Scanners and chemical detectors work by using terahertz waves to detect the signature vibrations of the atoms in a given substance, said Köhler. "Like balls connected with springs, these atoms vibrate at characteristic frequencies, which allows [you] to distinguish the compounds," he said.

The terahertz range also promises fast wireless communications. Terahertz waves are shorter then radio waves, and so more of them are transmitted per second. Terahertz waves are long enough to penetrate walls like the radio waves used in some types of local area networks that connect computers without wiring, said Köhler. By contrast, the infrared waves used in other types of wireless networks are shorter than terahertz waves and can therefore carry more data, but are so short they cannot penetrate barriers like walls and must use unobstructed paths.

Lasers typically produce a beam of light when photons emitted by the electrons of a laser material bounce between parallel mirrors, causing them to hit other electrons that in turn release additional photons. This amplification process produces the laser's intense, monochromatic beam.

The researchers' terahertz laser is a more complicated quantum cascade device made from a stack of 1,500 alternating layers of semiconductor materials, some only a few atoms thick. The whole stack measures 12 microns, or a little more than twice the diameter of a red blood cell. Quantum cascade lasers usually emit shorter, infrared wavelengths.

The terahertz laser is made up of alternating layers of gallium arsenide and aluminum gallium arsenide. Fourteen-layer units form 104 steps in an electronic staircase that individual electrons bounce down, releasing a photon at each step.

The thicker the step, the longer the wavelength it emits. Even though some layers in the researchers' laser are extremely thin, the steps as a whole are relatively thick, which produces the longer terahertz waves. Key to the researchers design was using a large number of steps so that many electrons emit photons at once.

The researchers' terahertz semiconductor laser "is the biggest step forward in semiconductor laser technology since the original development of the quantum cascade laser at Bell Labs" in 1994, said Paul Harrison, a reader in quantum electronics and electronic and electrical engineering at the University of Leeds in England. "The researchers have pushed the technology of manipulating electron scattering to the extreme, far beyond current knowledge," he said.

The work is "very significant," said Cun-Zheng Ning, a scientist at NASA's Ames Research Center. "From past experience with semiconductor lasers, it should be expected that significant improvements and rapid commercialization will come soon," he said.

The researchers next steps are to make the terahertz laser work at higher temperatures, to make it emit light in continuous waves rather than pulses, and to make it emit light at slightly lower frequencies, said Köhler.

The researchers also plan to build devices using the laser, including chemical detectors and data communications systems, Köhler said. "On a somewhat longer time-scale, we plan to implement the laser into a detection system for chemicals and to set up a... data link using the laser as the sender and a... semiconductor detector as the receiver," he said.

The terahertz laser could be used in practical astronomy applications like detecting the chemical compositions of objects like distant stars in two to five years, said Köhler. All other applications are likely to take five to ten years, he said. "This compares to the six years it took for conventional mid-infrared quantum cascade lasers to be commercially available," he said.

Köhler's research colleagues were Alessandro Tredicucci and Fabio Beltram of INFM, Harvey E. Beere, Edmund H. Linfield, A. Giles Davies and David A. Ritchie of the University of Cambridge in England, and Rita C. Iotti and Fausto Rossi of Torino Polytechnic in Italy. They published the research in the May 9, 2002 issue of the journal nature. The research was funded by the European Commission.

Timeline:   2-5 years, 5-10 years
Funding:   Government
TRN Categories:   Optical Computing, Optoelectronics and Photonics; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Terahertz semiconductor-heterostructure laser," Nature, May 9, 2002


May 15/22, 2002

Page One

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Laser bridges infrared-microwave gap

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