Efficient room temperature polariton laser could advance intrachip and interchip optical interconnects

University of Michigan researchers have demonstrated a new, practical and potentially more efficient way to make a coherent laser-like beam using polaritons. This is the first room-temperature electrically pumped polariton laser (other electrically pumped polariton lasers typically operate at cryogenic temperatures).

This is most real-world ready polariton lasers ever developed. It represents a milestone like none the field has seen since the invention of the most common type of laser – the semiconductor diode – in the early 1960s, the researchers say. While the first lasers were made in the 1950s, it wasn’t until the semiconductor version, fueled by electricity rather than light, that the technology took off.

This work could advance efforts to put lasers on computer circuits to replace wire connections, leading to smaller and more powerful electronics. It may also have applications in medical devices and treatments and more.

The researchers didn’t develop it with a specific use in mind. They point out that when conventional lasers were introduced, no one envisioned how ubiquitous they would become. Today they’re used in the fiber-optic communication that makes the Internet and cable television possible. They are also in DVD players, eye surgery tools, robotics sensors and defense technologies, for example.

A polariton is part light and part matter. Polariton lasers harness these particles to emit light. They are predicted to be more energy efficient than traditional lasers. The new prototype requires 250 times less electricity to operate than its conventional counterpart made of the same material.

Laser Focus World – 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).

The paper, “Room Temperature Electrically Injected Polariton Laser,” will be published online in Physical Review Letters on June 10, 2014.

Bhattacharya’s system isn’t technically a laser. The term was initially an acronym for Light Amplification by Stimulated Emission of Radiation. Polariton lasers don’t stimulate radiation emission. They stimulate scattering of polaritons.

In a typical laser, light–or more often electrical current– is pumped into a material called a gain medium that’s designed to amplify the signal. Before the pumping begins, most of the electrons in the gain medium are in their least energetic state, also known as the ground state. Once the light or current hits them, the electrons absorb that energy and move to a higher-energy state. At some point, more electrons are high-energy than are low-energy and the device is said to have achieved a “population inversion.” Now any light or current that goes in has the opposite effect on the excited electrons. It kicks them down to the ground state and releases pent-up light in the process.

Polariton lasers don’t rely on these 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 said.

He and his team paired the right material – the hard, transparent semiconductor gallium nitride – with a unique design to maintain the controlled circumstances that encourage polaritons to form and then emit light.

A polariton is a combination of a photon or light particle and an exciton – an electron-hole pair. The electron is negatively charged and the hole is technically the absence of an electron, but it behaves as if it were positively charged. Excitons will only fuse with light particles 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 bounce around the system until they 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 a beam of single-colored light.

The beam they demonstrated was ultraviolet and very low power – less than a millionth of a watt. For context, the laser in a CD player is about one-thousandth of a watt.

“We’re thrilled,” said Thomas Frost, a doctoral student in electrical and computer engineering. “This is the first really practical polariton laser that could be used on chip for real applications.”

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