Terahertz QCL tuning method could help terahertz systems distinguish aspirin from explosives

Terahertz rays can penetrate clothing, plastic, and human tissue, and since they're absorbed to different degrees by different molecules, they can also tell chemicals apart. MIT researchers have developed a new tuning method for a terahertz quantum-cascade laser (QCL) that could, for example, be incorporated into an airport laser scanner to determine whether a vial in a closed suitcase contained aspirin, methamphetamines, or an explosive.

In 1994, researchers at Bell Labs invented a new kind of small but powerful semiconductor laser called a quantum cascade laser, and in 2002, it was shown to be able to operate at terahertz frequencies. But accurately assessing an object's chemical composition requires exposing it to a continuous range of frequencies, which are absorbed to different degrees.

In a paper appearing in the most recent issue of Nature Photonics, Qing Hu, a professor of electrical engineering at the Massachusetts Institute of Technology's (MIT's; Cambridge, MA) Research Laboratory of Electronics, and his colleagues describe the first practical method for tuning terahertz quantum-cascade lasers. What's more, the method is a fundamentally new approach to laser tuning that could have implications for other emerging technologies.

One way to change the pitch of a guitar string is to change its diameter: the lower-pitched strings on a guitar are thicker than the higher-pitched ones. And Hu's tuning technique is, roughly speaking, to change the diameter of the light beam. Hu's new tuning technique requires a particular type of QCL called a wire laser, where the wavelength of the transverse mode 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 points out that his technique could also be applied to a new type of tiny laser that can be used for extremely fine-scale sensing. Ordinarily, visible-light lasers cannot be narrower than the wavelength of the light being used, but researchers have found ways around that fundamental limit by using a virtual particle called a plasmon, which is like a wave passing through a cloud of electrons. Some new types of plasmon lasers could also be tuned through manipulation of their transverse modes.

In its experiments, Hu's group used a mechanical lever to bring a block of either silicon or metal close to a QCL from a single direction. But they've 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, which could ultimately improve the detection sensitivity of terahertz sensing systems.

For more information, see the full story at web.mit.edu/newsoffice/2009/tunable-terahertz.html.