Engineers demonstrate first room-temperature electrically-pumped semiconductor laser source of T-rays

CAMBRIDGE, Mass. – May 19, 2008 –Engineers and applied physicists from Harvard University havedemonstrated the first room-temperature electrically-pumpedsemiconductor laser source of Terahertz (THz) radiation, also known asT-rays. The breakthrough in laser technology, based upon commerciallyavailable nanotechnology, has the potential to become a standardTerahertz source to support applications ranging from securityscreening to chemical sensing.

Spearheaded byresearch associate Mikhail Belkin and Federico Capasso, Robert L.Wallace Professor of Applied Physics and Vinton Hayes Senior ResearchFellow in Electrical Engineering, both of Harvard’s School ofEngineering and Applied Sciences (SEAS), the findings will be publishedin the May 19 issue of Applied Physics Letters. The researchers havealso filed for U.S. patents covering the novel device.

“Terahertzimaging and sensing is a very promising but relatively new technologythat requires compact, portable and tunable sources to achievewidespread penetration. Our devices are an important first step in thisdirection,” said Capasso. “We believe our laser has great developmentpotential because the nanoscale material used was grown by MolecularBeam Epitaxy, a commercial and widely used thin film growth techniquewhich ‘spray paints’ atoms on a surface one layer at a time.”

Theability of Terahertz rays to penetrate efficiently through paper,clothing, cardboard, plastic and many other materials makes them idealfor use in many applications. For example, a device emitting T-rayscould be used to image concealed weapons, detect chemical andbiological agents through sealed packages, see tumors without causingany harmful side effects, and spot defects within materials such ascracks in the Space Shuttle’s foam insulation.

Usinglasers in the Terahertz spectral range, which covers wavelengths from30 to 300µm, has long presented a major hurdle to engineers. Inparticular, making electrically pumped room-temperature andthermoelectrically-cooled Terahertz semiconductor lasers has been amajor challenge. These devices require cryogenic cooling, greatlylimiting their use in everyday applications.

“By contrast, our device emits T-rays with several hundreds ofnanowatts of power at room temperature and microwatts of power attemperatures easily achievable with commercially availablethermoelectric coolers,” says Belkin. “Further, there is the potentialof increasing the terahertz output power to milliwatt levels byoptimizing the semiconductor nanostructure of the active region and byimproving the extraction efficiency of the terahertz radiation.”

Toachieve the breakthrough and overcome the temperature limitations ofcurrent laser designs, the researchers engineered a room temperatureinfrared Quantum Cascade Laser (QCL) that emits light at twofrequencies simultaneously. The generation of T-rays occurs at roomtemperature inside the laser material via the process ofdifference-frequency generation. The frequency of the emitted radiationis 5 THz (equal to the difference of the two infrared QCL frequencies).

QCLswere invented and demonstrated by Capasso and his team at Bell Labs in1994. The compact millimeter length semiconductor lasers operateroutinely at room temperature with high optical powers and areincreasingly used in the commercial sector for wide range ofapplications in chemical sensing and trace gas analysis. The devices,made by stacking ultra-thin atomic layers of semiconductor materials ontop of each other, are variable and tunable, allowing an engineer toadjust the energy levels in the structure to create artificial lasermedium.

Belkin and Capasso’s co-authors are Feng Xieand Alexey Belyanin, Department of Physics at Texas A&M University,College Station; and Milan Fischer, Andreas Wittmann, and Jérôme Faist,Institute of Quantum Electronics at ETH, Zürich, Switzerland. Theresearch was supported by the Air Force Office of Scientific Researchand the National Science Foundation. The authors also acknowledge thesupport of two Harvard-based centers, the Nanoscale Science andEngineering Center and the Center for Nanoscale Systems, a member ofthe National Nanotechnology Infrastructure Network.

Note: High-resolution images are available upon request.