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Harvard prof develops room-temp THz laser

Posted: 23 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:room-temperature laser? terahertz laser? semiconductor laser?

The first room-temperature terahertz laser is claimed to harness the optical equivalent of heterodyning to bridge the terahertz gap. 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- to 10THz).

The only semiconductor lasers that run at terahertz frequencies today are supercooled quantum cascade lasers (QCL). Harvard University professor, and co-inventor of the QCL, Federico Capasso, 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," Capasso explained. "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 33.7THz (8.9? wavelength) and 28.5THz (10.5? wavelength), which produced a difference frequency of 5.2THz.

"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.

Quantum cascade
Conventional lasers energize electrons, which then emit a single photon by jumping from the semiconductor's conduction band to its valence band. Quantum cascade lasers, on the other hand, arrange a stair-step of quantum wells!each at a progressively lower energy level!that allow electrons to cascade down an energy staircase, emitting a photon at each step. Today, quantum cascade lasers lose their ability to work in the terahertz gap without supercooling. But by using a heterodyning architecture, the Harvard researchers demonstrated twin quantum cascade lasers, whose mixed output is in the terahertz gap.

The heterodyning principle is well known in nonlinear optics as difference frequency generation (DFG). Most materials act like linear harmonic oscillators when light impinges on them, oscillating only when the frequency matches their own internal natural resonant frequency. Nonlinear materials like vacuum tubes and transistors, on the other hand, can be made to resonate at the sum and difference frequencies of two inputs, enabling radios to move signals between bands, or to encode and decode them.

Others have demonstrated the feasibility of terahertz lasers using DFG, but bulky external "pump" lasers were used just to prove the principle. The Harvard group accomplished the task with semiconductor materials that, if all goes well, eventually could be mass produced for inexpensive room-temperature devices.

"Our device does everything in one small semiconductor crystal with no need for bulky external lasers for pumping; hence, the advantages of compactness, portability and low power consumption," said Capasso. "In essence, the material of the device is designed and grown so that when a bias current is applied to it, not only are laser beams emitting at two different mid-infrared frequencies generated, but also coherent radiation at the difference frequency corresponding, in our case, to 5THz".

The mechanism by which nonlinear devices perform operations like mixing!generating sum and difference frequencies!depends on the materials used. The quantum cascade laser is fabricated using molecular-beam epitaxy, a layer of atoms at a time, from alternating layers of gallium and aluminum. Each layer is slightly thinner than the one before it.

Next, the Harvard researchers plan to optimize their design in an attempt to increase the output power to milliwatts, from its nanowatt levels today. One way is to add low-cost thermoelectric coolers to the laser's substrate!since the cooler the laser runs, the higher its output power. Secondly, the group plans to switch from edge emission to surface emission for their semiconductor material.

"Our approach will be to greatly increase the surface area used for emission," said Capasso. "Surface emission will be achieved by fabricating a suitable grating to scatter vertically the terahertz radiation generated in the device's active region."

Belkin and Capasso performed the work in cooperation with researchers Feng Xie and Alexey Belyanin, at Texas A&M University (College Station), and researchers Milan Fischer, Andreas Wittmann, and Jrme Faist, at ETH (Zurich, Switzerland). Funding was provided by the Air Force Office of Scientific Research, the National Science Foundation and two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.

- R. Colin Johnson
EE Times





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