THz QCL Hits Light Output Milestone

Using special symmetrical laser structures, a group in Vienna says it has created a terahertz quantum cascade laser (QCL) with four times higher light output than other such existing devices, crossing a major hurdle to the widespread use of such lasers.

Two quantum cascade lasers are connected via a wafer-bonding process. ©Vienna University of Technology.

Because terahertz waves — the part of the electromagnetic spectrum between microwaves and infrared waves — are opaque to visible light and can penetrate many materials, they are ideal for the detection of explosives. Additional applications of terahertz waves include security screenings, materials analysis and ultrafast wireless data transfer, and, because terahertz waves have less energy than x-rays and a smaller wavelength than microwaves, they are seen as a less dangerous option than x-rays and a higher resolution alternative to microwaves for diagnostic imaging.

A deployable terahertz quantum cascade laser. ©Vienna University of Technology.

To increase the power of a terahertz QCL, a team at Vienna University of Technology used a large number of semiconductor layers because the more layers the laser has, the more photons are generated. But there are technological limits to how many layers a laser can have.

The researchers, led by professor Karl Rainer at the university's Institute of Photonics, used a direct wafer-bonding technique to stack two separate QCLs and increase the active region and the waveguide thickness. They found that doubling the semiconductor layers can quadruple the laser's optical output power.

"In this way, we enhance the generated optical power in the cavity and the mode confinement," the researchers report in a paper on the work. "We achieved 470-mW peak output power in pulsed mode from a single facet at a heat sink temperature of 5 K and a maximum operation temperature of 122 K. Furthermore, the devices show a broadband emission spectrum over a range of 420 GHz, centered around 3.9 THz."

Christoph German, Martin Brandstetter and Michael Krall in the cleanroom of the Center for Micro and Nanostructures (ZMNS) at TU Vienna. ©Vienna University of Technology.

The paper, "High power terahertz quantum cascade lasers with wafer-bonded symmetric active regions," by Martin Brandstetter et al, appears in Applied Physics Letters http://dx.doi.org/10.1063/1.4826943.

The previous record for terahertz quantum cascade lasers was achieved at almost 250 mW at MIT, the Vienna researchers said, while theirs has reached 1 W. Overcoming the watt limit removes a major hurdle to the practical use of terahertz quantum cascade lasers, they said.

For more information, visit: www.tuaustria.at