With precarious particles called polaritons that straddle the worlds of light and matter, University of Michigan researchers have demonstrated a new, practical and potentially more efficient way to make a coherent laser-like beam.
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."