What if you could use the heat generated in a quantum cascade laser — something that normally causes the lasing to turn off — to power the device instead? Two physicists from the University of Innsbruck have suggested just that. While looking for ways to reduce heat in these lasers, they came up with a novel idea: Let heat power the laser. They propose a theory that the heating effect in quantum cascade lasers (QCLs) could not only be avoided but, in fact, reversed and used to the laser’s advantage by cleverly modifying the semiconductor layers’ thickness. This is a schematic picture of a quantum cascade laser. The layers of different semiconductor material constitute the bandstructure shown in the inset. Courtesy of Christoph Deutsch. Lasers have been around for more than 50 years, but not all wavelengths have been equally researched. For the far-infrared and terahertz regimes, QCLs are the most important source of coherent radiation. Light amplification in such a cascade laser is achieved through a repeated pattern of specifically designed semiconductor layers of diverse doping through which electric current is running. “The electrons are transferred through this structure in a specific series of tunneling processes and quantum leaps, emitting coherent light particles,” said professor Helmut Ritsch of the university’s Institute for Theoretical Physics. “Between these layers, the electrons collide with other particles, which heat the laser.” Too much heat extinguishes the laser light in QCLs, which work only as long as they are strongly cooled. “A crucial part is to spatially separate the cold and warm areas in the laser,” said doctoral candidate Kathrin Sandner. “In such a temperature-gradient-driven laser, electrons are thermally excited in the warm area and then tunnel into the cooler area, where photons are emitted.” This produces a circuit where light particles are emitted and heat is absorbed from the system simultaneously. “Between the consecutive emissions of light particles, a phonon is absorbed, and the laser is cooled,” Sandner said. “When we develop this idea further, we see that the presence of phonons may be sufficient to provide the energy for laser amplification.” A phonon is a quantum mechanical description of a special type of vibrational motion; a laser powered in this way wouldn’t need electric current. “Of course, it is quite a challenge to implement this concept in an experiment,” Ritsch said. “But if we are successful, it will be a real technological innovation.” The physical principle behind the idea could be applied to existing QCLs to provide internal cooling. Experimental physicists are already examining the simplified concept. “Apart from the conceptual elegance of this idea, a completely new way may open up of using heat in microchips in a beneficial way instead of having to dissipate it by cooling,” Ritsch said. The theory was published in Physical Review Letters (doi: 10.1103/PhysRevLett.109.193601). For more information, visit: www.uibk.ac.at