Theoretical calculation proposes using heat to power QCLs

Heat generated in a quantum cascade laser (QCL) – something that normally causes the lasing to turn off – could actually be used to power the device instead, according to a theoretical calculation developed at the University of Innsbruck.

Helmut Ritsch of the university’s Institute for Theoretical Physics and doctoral candidate Kathrin Sandner came up with the novel idea while looking for ways to reduce heat in quantum cascade lasers. They proposed that the heating effect in such lasers not only could be avoided but also could be reversed and used to the laser’s advantage by modifying the semiconductor layers’ thickness.

Light amplification in a QCL is achieved through a repeated pattern of specifically designed semiconductor layers of diverse doping through which electric current is running. Between these layers, electrons collide with other particles to heat the laser. Too much heat extinguishes the laser light in QCLs, which work only as long as they are strongly cooled.

A schematic picture of a quantum cascade laser. The layers of different semiconductor materials constitute the band structure shown in the inset. Heat generated in such a laser – something that normally causes the lasing to turn off – could actually be used to power the device instead, according to a theoretical calculation developed at the University of Innsbruck.

“A crucial part is to spatially separate the cold and warm areas in the laser,” Sandner said. Electrons are thermally excited in the warm area, and then tunnel into cooler areas, where photons are emitted. This produces a circuit where light particles are emitted and heat is absorbed from the system simultaneously.

It is between the consecutive emissions of light particles that a phonon is absorbed and the laser is cooled, Sandner said. When investigated further, the team discovered that phonons – a quantum mechanical description of a special type of vibrational motion – could adequately provide the energy needed for laser amplification.

“The emission/absorption of a phonon (and in particular a so-called LO [longitudinal] phonon) is connected to a tailored intrawell transition to enhance the coupling” between phonons and photons, Ritsch told Photonics Spectra. “These phonons then decay into acoustic phonons of longer wavelength, which finally generates heat.”

The concept is challenging to realize in an experiment, but if successful, Ritsch believes it will be a technological breakthrough. He is currently discussing prototype implementation with two experimental groups in the field.

“The first step will target laser operation with reduced heat production before heat-driven lasing is tried,” Ritsch said. “The precise dynamics of heat flow for large temperature gradients is not very well known so far and could bear some surprises.”

The physical principle behind the idea could be applied to existing QCLs to provide internal cooling. The calculation could also be applied to other systems.

“From a theoretical point of view, we will look at other solid-state laser models,” he said. These will include fiber lasers and more general setups where electric driving and heat absorption steps are combined.

“From a more distant black-box perspective, the device could be also reinterpreted as a combination of a photoelectric cell combined with a QCL in a single device,” Ritsch said. “In such an implementation, blackbody photons would directly excite electrons in the material without being converted to phonons first.”

The theory was published in Physical Review Letters (doi: 10.1103/PhysRevLett.109.193601).