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Beyond supercool: room-temperature THz lasers

Posted: 16 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:laser? temperature? optical? devices?

The world�s first room-temperature THz laser harnesses the optical equivalent of heterodyning to bridge the THz gap, the region between microwave wavelengths (in cm) and optical wavelengths (in ?m) in which most current semiconductor lasers fail to operate.

Today, the only semiconductor lasers that operate at THz frequencies (1THz to 10THz) in millimeter wavelengths are supercooled quantum cascade lasers (QCLs). Recently, Harvard professor Federico Capasso has demonstrated a heterodyning method, cast in nonlinear materials, that mixes two optical frequencies whose difference is the desired THz frequency, resulting in a room-temperature THz laser.

Interesting property
"This class of nonlinear optical materials has an interesting property. 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 Capasso, who co-invented the QCL while working at Bell Labs Inc. in 1994. "As a result, one may observe light not only at the pump frequencies, but also at the difference frequency. The process is similar to the heterodyne principle widely used in radio," he added.

Using easy-to-generate optical wavelengths at room temperature and that differ exactly by desired THz frequency, Capasso and his research associate Mikhail Belkin achieved a room-temperature THz laser. The demonstration used 33.7THz (8.9?m wavelength) and (28.5THz 10.5?m wavelength) lasers, yielding a difference frequency of 5.2THz.

"Basically, electrons are driven to oscillate all in phase at this frequency, thus producing coherent THz 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 is also generated at room temperature. In this way, we have circumvented the limitation of THz QCLs, which operate so far only at cryogenic temperatures."

Capasso and Belkin's demonstration used 33.7THz and 28.5THz lasers yielding a difference frequency of 5.2THz.

Conventional lasers energize electrons, which then emit a single photon by jumping from the semiconductor�s conduction band to its valence band. QCLs, on the other hand, arrange a "stair-step" of quantum wells, each at a progressively lower energy level, allowing electrons to cascade down an energy staircase and emit a photon at every step. But today�s QCLs lose their ability to work in the THz gap without supercooling. The Harvard team�s twin QCLs yield a mixed output in the THz gap at room temperature.

THz scanners act like x-rays but at power levels that are completely safe for people. Using a THz scanner, airports can detect hidden weapons under clothing and toxic materials in luggage. THz lasers can also remotely identify hazardous gases in the air and offer a solution in distinguishing improvised explosive devices at a distance.

Using DFG
The heterodyning principle is popular 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 its own internal natural resonant frequency. Nonlinear materials such as vacuum tubes and transistors, however, can be resonated at the sum and difference frequencies of the two inputs, enabling radios to move signals between bands or encode and decode them.

Others have demonstrated the feasibility of THz lasers using DFG, but bulky external pump lasers were used just to prove the principle. The group accomplished the task with semiconductor materials that might eventually be mass produced for inexpensive room-temperature devices.

"Our device does everything in one small semiconductor crystal with the advantages of compactness, portability and low-power use," said Capasso. "The material of the device is designed so that when bias current is applied to it, not only are two laser beams generated that emit at two mid-infrared frequencies, but coherent radiation is generated at the difference frequency."

The mechanism by which nonlinear devices perform operations such as mixing (in terms of generating sum and difference frequencies) depends on the materials used. The QCL is fabricated using molecular-beam epitaxy, one layer of atoms at a time, from alternating layers of gallium and aluminum. Each layer is slightly thinner than the one ahead of it.

The researchers plan to optimize their design in an attempt to increase the output power to mW from its current nW level. To achieve this is to first add low-cost thermoelectric coolers to the laser�s substrate, since the cooler the laser runs, the higher is the output power. Second, the group plans to switch from edge emission to surface emission for its semiconductor material.

"Our approach will be increase more the surface area used for emission," said Capasso. "Increased surface emission will be achieved by fabricating a suitable grating to scatter vertically the THz radiation generated in the device�s active region," he added.

R. Colin Johnson
EE Times





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