PORTLAND, Ore. What's claimed to be the world's first room-temperature terahertz laser harnesses the optical equivalent of heterodyning to bridge the terahertz gap. Today, a terahertz-gap exists where most semiconductor lasers fail to operate--between microwave wavelengths (centimeters) and optical wavelengths (microns). In between are the millimeter wavelengths--terahertz frequencies (1-10 THz).
The only semiconductor lasers that run at terahertz frequencies today are supercooled quantum cascade lasers (QCL). Now, the co-inventor of the QCL (while at Bell Labs in 1994), professor Federico Capasso at Harvard University, has demonstrated a heterodyning method cast in nonlinear materials that mixes two easy-to-generate optical frequencies spaced apart at the desired terahertz frequency, resulting in a room-temperature terahertz laser.
"This class of nonlinear optical materials has the interesting property that, when illuminated by two frequencies, their constituent molecules vibrate coherently, not only at the driving frequencies, known as 'pump' frequencies, but also at their difference frequency," said Harvard professor, Federico Capasso. "As a result, at the output of the material one not only observes light at the pump frequencies, but also at the difference frequency--a process similar to the heterodyne principle widely used in radio."
By choosing optical wavelengths that are easy to generate at room temperature--but whose difference is exactly the desired terahertz frequency--Capasso and Harvard research associate Mikhail Belkin sidestepped the terahertz-gap problem, resulting in a terahertz laser that operates at room temperature. The two optical lasers used by Capasso's group in its room-temperature demonstration were at 33.7-THz (8.9-micron wavelength) and 28.5-THz (10.5-micron wavelength), which produced a difference frequency of 5.2 THz.
"Basically, electrons are driven to oscillate all in phase at this frequency, thus producing coherent terahertz emission," said Capasso. "The device structure is both a two frequency mid-infrared QCL and a nonlinear material, which generates the frequency difference. Since the two mid-infrared frequencies are generated at room temperature, their difference obviously is, as well. In this way we have circumvented the limitation of THz QCLs, which operate so far only at cryogenic temperatures."
Terahertz scanners act like x-rays, but at power levels that are completely safe to use around people. Using a terahertz scanner, airports could detect hidden weapons under clothing, as well as hazardous and toxic materials inside luggage. Terahertz lasers could also remotely detect hazardous gases floating in the air, offering a potential solution to identifying improvised explosive devices from a distance.