Qing Hu, EECS professor and principal investigator in the MIT Research Laboratory of Electronics (RLE) has reported his latest work with his MIT and Sandia Labs research team members on developing terahertz laser technology that has potential for use in bomb detection. The temperature barrier, as explained in MIT News Office article, December 16, 2010 by Larry Hardesty, has prevented terahertz rays -- radiation between microwaves and infrared rays on the electromagnetic spectrum -- from being practically produced without supercooling, an impractical answer for widespread deployment.
In fact, the News Office reports, some researchers had even begun to suspect that a room-temperature, solid-state terahertz laser was physically impossible. The performance of experimental terahertz lasers built in the lab has suggested a linear correlation between operating temperature and frequency, in which halving the frequency requires roughly halving the temperature. This led some scientists to speculate that frequency and temperature are linked by some fundamental physical law, a strict proportionality that couldn’t be violated.
But Qing Hu, together with Sushil Kumar, a postdoc in his lab, Ivan Chan, a graduate student in the lab, and Sandia Laboratory's John Reno have built a laser in which the applied voltage causes electrons to jump into an even higher-energy state than usual. Through a phenomenon called “scattering,” the electrons then release some of that energy as physical vibration rather than as light. They remain in an excited state, however, and release most of their remaining energy as photons.
The new laser is built from the same materials used in existing terahertz lasers, gallium arsenide and aluminum gallium arsenide, which are deposited in alternating layers. Each loss of energy occurs in a different layer, and the thickness of the layer determines how much energy the electron loses.
This latest work is reported in the journal Nature Physics giving hope to the possibility that terahertz lasers may yet be possible for practical applications.
"There are many naysayers saying that they can never be made operational at room temperature." says Qing Hu, "We break this psychological, empirical barrier by a factor of two. No one will say that it’s a barrier anymore."
MIT News Office, Dec. 16, 2010. Larry Hardesty. ("New hope for terahertz: A laser that generates terahertz rays — which can detect explosives — operates at higher temperatures than some thought possible."
Nature Physics (2010): doi:10.1038/nphys1846, Published online 12 December 2010 "A 1.8-THz quantum cascade laser operating significantly above the temperature of planckω/kB"