EECS faculty member Qing Hu, professor of electrical engineering, principal investigator in the Research Laboratory of Electronics (RLE) and head of the THz Quantum Cascade Laser Group has described (with his colleagues) in the most recent issue of Nature Photonics, the first practical method for tuning terahertz quantum cascade lasers. Their method is a fundamentally new in approach to laser tuning and as such promises applications in a wide range of emerging technologies.
Since joining the EECS faculty in 1990 and reaching full professor in 2002, Qing Hu has worked in what has been a 30 plus year effort by scientists in this field -- to harness the power of terahertz radiation. Over his time devoted to this research, Hu and his team have investigated THz quantum cascade lasers based on intersubband transitions in quantum wells, systematically investigating physical and engineering issues relevant to devices operating from millimeter-wave to THz frequencies.
As noted in the MIT News Office Dec. 4 article: “Since the very beginning of terahertz development in the 1970s, people have been trying to make [high-power] sources that are compact and tunable, and so far, this is really the first example of such a source,” says Peter Siegel, who leads the Submillimeter Wave Advanced Technology group at NASA’s Jet Propulsion Laboratory at Caltech. “Qing deserves a lot of credit for all the work he put in and the groundbreaking ideas he pioneered and pushed through despite lots of setbacks and competition from other groups. He really, in the end, came through with a fantastic breakthrough.”
Hu compared the tuning of a musical instrument's string (such as a violin or guitar) with tuning a laser: its pitch can be tuned by varying its length or by changing the diameter. Hu's group found success in changing the light beam's diameter.
Using a kind of quantum cascade laser called a wire laser, the wavelength of the transverse mode — the width of a large undulation, an electromagnetic-field pattern that is generated perpendicular to the laser--is actually greater than the width of the laser itself. By bringing a block of another material close enough to the laser the transverse mode is deformed which in turn changes the wavelength of the emitted light. In experiments, Hu and his colleagues found that a metal block shortened the wavelength of the light, while a silicon block lengthened it. Varying the proximity of the blocks also varies the extent of the shift.
Hu’s new tuning technique requires a particular type of quantum cascade laser called a wire laser, where the wavelength of the transverse mode — the width of the one big undulation — is actually greater than the width of the laser itself. Bringing a block of another material close enough to the laser deforms the transverse mode, which in turn changes the wavelength of the emitted light. In experiments, Hu and his colleagues found that a metal block shortened the wavelength of the light, while a silicon block lengthened it. Varying the proximity of the blocks also varies the extent of the shift.
Hu and his team have designed and are now building chips that would use electronically controlled microelectromechanical devices to bring the silicon and metal blocks in from different directions, giving the laser a precise and continuous tuning range from short to long wavelengths.
Nature Photonics 3, 732 - 737 (2009) "Tuning a terahertz wire laser" Qi Qin, Benjamin S. Williams (now at UCLA, California NanoSystems Institute), Sushil Kumar, John L. Reno(Sandia National Laboratories), Qing Hu
MIT News Office, December 4, 2009: "Fine-tuned laser - A wholly new approach to tuning a laser’s frequency brings us a step closer to airport scanners that can distinguish aspirin from explosives."
Technology Review, February 20, 2007: "T-Rays Advance Toward Airport Screening, A new laser design helps create usable terahertz radiation, which penetrates common materials but doesn't harm tissue."