Lasers have been around since the 1950s, but the technology didn’t really take off until about a decade later, when scientists invented the semiconductor diode. This is the most common type of laser we have now -- found in DVD players, sensors, and fiber-optic communication -- and it’s fueled by electricity, rather than light.
Now, researchers have found a practical way to make laser-like beams that use 250 times less power to operate than its conventional counterparts. And it works at room temperature, rather than well below zero.
In a typical laser, light or electrical current is pumped into a material (called the gain medium) that’s designed to amplify the signal. Before the pumping starts, electrons in the gain material are in their least energetic state (the ground state). Once the light or current hits them, the electrons absorb the energy and move to a higher-energy state. When there are more high-energy electrons than low-energy ones -- and “population inversion” is achieved -- any light or current that goes in has the opposite effect: It kicks them down to the ground state, releasing pent-up light in the process.
Now, a team from University of Michigan, Ann Arbor, has created the first polariton laser fueled by electrical current. Polaritons are precarious little particles are part light, part matter. They combine a photon (a light particle) and an exciton (an electron-hole pair). The electron is negatively charged, and while the hole is technically the absence of an electron, it behaves as if it was positively charged. Excitons only fuse with photons under just the right conditions: Too much light or electrical current will cause the excitons to break down. With just enough though, polaritons will form and bounce around until they come to rest at their lowest energy level. Once there, the polaritons decay and release a beam of light.
For this polariton laser, the hard, transparent semiconductor gallium nitride is paired with a design that maintains ideal conditions to encourage polaritons to form and emit light. Getting the electrical current into the system requires sandwiching the gallium nitride with electrodes and several layers of mirrors to render the electrical signal useable. Some designs put the electrodes outside the mirrors, but that makes it difficult to get strong enough signals. So the U-M team deconstructed the sandwich, putting the mirrors on the sides of the gallium nitride and left the electrodes on the top and bottom (pictured above).
The beam they demonstrated was ultraviolet and very low power: less than a millionth of a watt. The laser in a CD player, for context, is about one-thousandth of a watt. "This is big," U-M’s Pallab Bhattacharya explains in a news release. "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."
Polariton lasers aren’t technically lasers since that acronym stands for Light Amplification by Stimulated Emission of Radiation. These laser-like beams don’t stimulate radiation emission; they stimulate the scattering of polaritons. But since they don’t rely on population inversions like lasers, they don't need a lot of start-up energy to excite electrons. "The threshold current can be very small," Bhattacharya says, "which is an extremely attractive feature."
The work will be published in Physical Review Letters next week. [Link to come.]
[Via University of Michigan]
Image: Thomas Frost, U-M