Hope for terahertz: laser operates at higher temperatures than thought possible

thinking that informs this idea about this limit.”

To address the problem of shrinking gaps between energy states at low frequencies, Hu, together with Sushil Kumar, a postdoc in his lab, Ivan Chan, a graduate student in the lab, and Sandia’s John Reno 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.

Intents and practices

“This design technique circumvents one of the problems that have been limiting the temperature,” Williams says. “Going to this new design path may give lots of benefits that will take us higher and higher in temperature. Does this result say that we’re right around the corner from getting to room temperature? Probably not. But it points a possible path, and it raises a lot of hope.”

A decade ago, it was widely believed that terahertz rays could provide a safer, more useful replacement for X-rays during airport security screening: Not only can they penetrate clothing, but, unlike X-rays, they also interact with a wide range of chemical compounds in distinctive and detectable ways.

Hu says, however, that the frequencies of terahertz rays that are good for identifying chemicals can’t penetrate materials even as thick as a suitcase wall, and they don’t reflect well off of human flesh, so even after penetrating clothing, they might never reach a detector. They could, however, detect traces of explosives — a few molecules of a chemical wafting from a shoe bomb, for instance, or clinging to the side of an abandoned vehicle — with extraordinary sensitivity. Hu believes that, when room-temperature terahertz lasers are ultimately developed, they will thus be used in conjunction with other existing and emerging technologies. “There is no single silver bullet,” he says. “There have to be as many modalities as possible to cross-correlate, in order to increase sensitivity and, more importantly, to reduce false alarms.”

—Read more in Sushil Kumar et al., “A 1.8-THz quantum cascade laser operating significantly above the temperature of ω/kB,” Nature Physics (12 December 2010) (doi:10.1038/nphys1846)