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  • Groups demonstrate electrically driven polariton lasers

Photonics Spectra
Aug 2013

WÜRZBURG, Germany, and ANN ARBOR, Mich. – An electrically driven polariton laser – one fueled by electricity instead of light – was demonstrated recently by two separate groups.

Optically driven polariton (a mix of photons and excitons) lasers have been demonstrated before, but to be of practical use, they must operate electrically. Such a device was proposed in 1996, but until now has remained a scientific curiosity.

Diagram of the electrically driven polariton laser developed by scientists from Stanford University, the University of Würzburg and the National Institute of Informatics of Tokyo. The polariton laser could significantly improve the efficiency of such devices by requiring 100 times less power than conventional lasers, its creators say.

Two independent groups – an international team including Sven Höfling and colleagues at the University of Würzburg and the other led by Pallab Bhattacharya at the University of Michigan – recently demonstrated such a device, and both teams published papers during the same week in May. Höfling’s appeared in Nature (doi: 10.1038/nature12036), while Bhattacharya’s was published in Physical Review Letters (doi: 10.1103/PhysRevLett.110.206403).

Conventional lasers operate via stimulated emission, generating coherent light from the gain medium inside the cavity. Polariton lasers, in contrast, rely on coherent stimulated scattering processes that cause condensation of quantum bose particles.

As early as 2007, Höfling had the idea of developing an electrically driven polariton laser; the study group began experimenting in 2008. Working in collaboration with colleagues at Stanford University, the researchers obtained their first results but discovered a problem: “It is extremely difficult to determine whether you have created a polariton laser or just a normal laser. The characteristics of the emitted light are generally quite indistinguishable,” Höfling said.

To establish clear proof of polariton operation, an international team with partners in the US, Japan, Russia, Singapore and Iceland supplemented the first experiments with another component that clearly showed that polaritons were present.

Because the new device is not driven by stimulated emission, calling it a polariton “laser” can be a misnomer, said Na Young Kim of Stanford.

Physicist Na Young Kim, at the optical bench, is a member of the international team that has demonstrated a revolutionary electrically driven polariton laser that could significantly improve the efficiency of lasers.

“The polariton community has been struggling to find the proper word to describe the device,” she told Photonics Spectra. “At the moment, ‘polariton laser’ or ‘polariton condensates’ are used exchangeably. I personally think that ‘polariton coherent matter waves’ would be rather clear and more or less accurate.”

Even though her group’s laser is electrically driven, it operates at a temperature of 4 K and requires constant cooling by liquid helium. Still, it requires two to five times less energy than a comparable conventional laser; in the future, a device that operates at room temperature could require 10 to 100 times less energy.

The biggest hurdle to room-temperature operation is finding the right material and fabrication techniques, Kim said.

“Our current device is based on gallium arsenide semiconductors,” she noted. “However, the exciton binding energy (10 meV) is smaller than the room-temperature thermal energy, which can dissociate excitons into electron-hole pairs to lose the strong coupling between quantum well excitons and cavity photons.”

For that reason, she said, many groups are exploring wide-bandgap inorganic semiconductors, such as gallium nitride and zinc oxide, and organic materials, but the search continues for a practical fabrication technique to realize polariton condensation in gallium arsenide semiconductors.

In Michigan, Bhattacharya’s team generated polaritons by using electricity to excite samples of gallium arsenide in a microcavity under certain conditions. The polaritons quickly decayed by transferring their energy to photons.

“Our success is based on two novel features,” Bhattacharya said. “First, we deployed additional electron-polariton scattering to enhance the relaxation of polaritons to form the coherent ground state. Second, we applied a magnetic field so that more carriers can be injected with the bias current without losing the required conditions for polariton lasing.”

His team also is working on a room-temperature version.

As for the instrument’s potential applications, Kim said, “We envision that polariton lasers would be engineered for power-efficient, compact coherent light sources and ultrafast optical switches, which are integrable and scalable.” Such applications could include quantum information processing as well as optoelectronic devices.

“GaAs (or InGaAs) polariton lasers are typically operating around 770 to 820 nm, and gallium nitride, zinc oxide polariton lasers operate in the visible wavelength,” she said. “We have been considering other semiconductors for 1.55 µm,” which would be compatible with fiber optic communications.

A moving, electrically neutral, excited condition of holes and electrons in a crystal. One example is a weakly bound electron-hole pair. When such a pair recombines, with the electron "falling" into the hole, the energy yielded is the bandgap decreased by the binding energy of the pair.
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