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  • Polariton Laser Is Electrically Injected
May 2013
STANFORD, Calif., and WÜRZBURG, Germany, May 16, 2013 — An international team demonstrated a revolutionary electrically driven polariton laser that could significantly improve the efficiency of such devices by requiring a hundred times less power than conventional lasers.

The system makes use of the unique physical properties of bosons that scientists have attempted to incorporate into lasers for decades and was created by a team led by Stanford's Yoshihisa Yamamoto, a professor of electrical engineering and of applied physics. Collaborators on the project were the National Institute of Informatics in Tokyo and a team from the University of Würzburg in Germany.

"In simplified terms, we send electrons and electron holes into a quantum well by applying an electric voltage. Due to their opposite charge, they are attracted to each other and form a so-called exciton together. The polaritons result from the strong light-matter coupling of these excitons in semiconductor microcavities. These decay after a short time, emitting photons in the process," said Sven Höfling, a research associate in the University of Würzburg's Department of Applied Physics. He conducted the relevant experiments with Christian Schneider and Arash Rahimi-Iman.

Diagram of the electrically driven polariton laser.
Diagram of the electrically driven polariton laser. Courtesy of Arash Rahimi-Iman, Department of Applied Physics, University of Würzburg

"We've solidified our physical understanding, and now it's time we think about how to put these lasers into practice," said physicist Na Young Kim, a member of the Stanford team. "This is an exciting era to imagine how this new physics can lead to novel engineering."

Electrically driven polariton lasers, she said, could one day be used in many places from consumer goods to quantum computers.

The polariton laser pairs electrons with so-called "holes" to form excitons. These excitons are bosons, and an unlimited number of them can inhabit any given energy level. Using bosons in lasers has been a scientific goal for decades, but Yamamoto's team is the first to successfully build an electrically driven laser using bosons. (The result was recently reproduced and confirmed by scientists at the University of Michigan, who published their work in the journal Physical Review Letters (doi: 10.1103/PhysRevLett.110.206403)).

This change drastically reduces the amount of power required to run the laser. The current iteration of the polariton laser requires two to five times less energy than a comparable conventional laser, but could require 100 times less energy in the future.

"The outcome would look similar to that of the traditional photon lasers, but the physical mechanisms inside are very different," Kim said.

The laser consists of an electron reservoir and a hole reservoir. When a current is applied, electrons and holes come together to form excitons in excited energy levels. When a photon hits an exciton, it forms a polariton and emits an identical photon.

The entire process is like a solar cell in reverse, Kim said.

"In a solar cell, you use light to form excitons and separate them into an electron and a hole electrically," she said. "We bring together an electron and a hole electrically to emit light."

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.
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. Courtesy of Stanford/L.A. Cicero.

One benefit of the electrically driven polariton laser is it only needs to be attached to a power supply to emit photons, allowing it to be easily integrated with existing semiconductor chips in the future.

The current polariton laser can run only at a chilly 4 degrees Kelvin (minus 452 degrees Fahrenheit) and requires constant cooling by liquid helium to prevent the excitons inside the gallium arsenide semiconductors from being pulled apart by thermal energy. The team hopes switching to a material that requires more energy to break apart excitons will allow them to build polariton lasers that work at room temperature, an important step toward widespread use.

"We're hoping we can replace conventional semiconductor lasers with these polariton lasers in the future," she said. "There are a lot of hurdles in the way, but we aim to bring novel devices built on sound physical understanding for cost-effectiveness and efficient power consumption."

The polariton laser is already being used at Stanford to develop quantum computers and quantum simulators. Kim believes similar lasers will be available to those outside the scientific community within the next five to 10 years.

The research was supported by the National Science Foundation, the DARPA QUEST program, the Japan Society for the Promotion of Science through its "Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST Program) and the State of Bavaria.

The work appears in the May 16 issue of Nature. (doi:10.1038/nature12036)

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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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