Researchers at the University of Michigan (U-M; Ann Arbor, MI) have created what they say is the first room-temperature electrically pumped polariton laser (other electrically pumped polariton lasers typically operate at cryogenic temperatures).
The device is gallium arsenide (GaN)-based. The work could advance intrachip and interchip optical interconnects (the lasers can potentially be integrated into semiconductor-based photonic chips), and may also have applications in medical devices and treatments.
A polariton is a quasiparticle consisting of a combination of a photon and an exciton (which itself is an electron-hole pair). Polariton lasers harness these particles to emit light; they are predicted to be more energy efficient than traditional lasers. The new prototype requires 1000 times less electricity to operate than its conventional counterpart made of the same material. The beam emitted by the device is ultraviolet and very low power (less than a microwatt).
"For the past 50 years, we have relied on lasers to make coherent light and now we have something else based on a totally new principle," says Pallab Bhattacharya, a professor of engineering at U-M.
No population inversion
Bhattacharya's device isn't technically a laser. Polariton lasers don't stimulate radiation emission; instead, they stimulate scattering of polaritons.
In addition, polariton lasers don't rely on population inversions, so they don't need a lot of start-up energy to excite electrons and then knock them back down. "The threshold current can be very small, which is an extremely attractive feature," Bhattacharya says.
Excitons will only fuse with photons to form polaritons under just the right conditions; too much light or electrical current will cause the excitons to break down too early. But with just enough, polaritons will form and then eventually come to rest at their lowest energy level in what Bhattacharya describes as a coherent pool. There, the polaritons decay and in the process, release monochromatic light.
The paper, "Room Temperature Electrically Injected Polariton Laser," will be published online in Physical Review Letters on June 10, 2014. The work was funded by the National Science Foundation.
