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

Published 17 December 2010

Terahertz rays — radiation between microwaves and infrared rays on the electromagnetic spectrum — are a promising means of detecting explosives, but they have proven hard to generate cost effectively. So far, solid-state lasers — the cheap, miniature type of laser found in CD players — have been unable to produce terahertz rays unless they are super-cooled, which makes them impractical for mass deployment; now a group of researchers report a solid-state terahertz laser that operates at nearly twice the temperature that putative proportionality would have predicted

Terahertz rays — radiation between microwaves and infrared rays on the electromagnetic spectrum — are a promising means of detecting explosives, but they have proven hard to generate cost effectively. So far, solid-state lasers — the cheap, miniature type of laser found in CD players — have been unable to produce terahertz rays unless they are super-cooled, which makes them impractical for mass deployment.

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.

That hypothesis, however, turns out to be wrong. In the latest issue of the journal Nature Physics, a group of researchers at MIT and Sandia National Laboratories report a solid-state terahertz laser that operates at nearly twice the temperature that that putative proportionality would have predicted.

That temperature is still too low to be practical for airport scanners or devices in a bomb squad’s tool kit, but it suggests that the quest for room-temperature terahertz lasers shouldn’t be called off just yet.

“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.”

Energy gap

Solid-state lasers are made from semiconductors, materials such as gallium arsenide that can act as either conductors or insulators. Applying a voltage to the semiconductor causes its electrons to jump into a higher-energy state, and when the electrons fall back into their original state, they release their excess energy as photons, or particles of light.

 

At low frequencies, however, the gaps between an electron’s energy states become smaller, which makes it harder to coax electrons into exactly the right state for photon emission. Lower temperatures, in turn, allow for more precise control of the electrons’ energy levels.

“In physics, there’s a standard way of thinking, that temperature equals energy, and if you want to go to smaller energies, and see quantum effects, you better go to smaller temperatures,” says Benjamin Williams, director of UCLA’s Terahertz Devices and Intersubband Nanostructures Laboratory, who wasn’t involved in the research. “And that’s the type of