An economical method of creating laser-like beams

The researchers have succeeded in creating what they believe is the first ever polariton laser that operates on an electric current, as opposed to lasers that operate on light, and one that is capable of operating at room temperature and not just at sub-zero temperatures

With the help of unstable particles called polaritons that mediate between the world of light and the world of matter, researchers were able to demonstrate a new, practical and more efficient method for creating a laser-like coherent beam.

The researchers have succeeded in creating what they believe is the first ever polariton laser that operates on an electric current, as opposed to lasers that operate on light, and one that is capable of operating at room temperature and not just at sub-zero temperatures. These features make the device the most practical device today among the handful of polariton-based lasers developed so far. This discovery marks a milestone not seen since the invention of the most common type of laser – the semiconductor diode – in the early XNUMXs, the researchers say. Although the first lasers were developed back in the XNUMXs, the technology did not make waves until the development of the semiconductor version that operated on the basis of electricity and not on the basis of light.
This discovery may advance efforts to insert laser components into computer circuits instead of metal connections, which will result in the creation of smaller and more powerful electronic components. The researchers did not develop the device with a specific application in mind. They commented that when conventional lasers were first introduced, no one believed how common and versatile they would become in the future. Today they are used in the field of optical fiber communication that enable the existence of the Internet and cable television. Lasers are also used in DVD players, for eye surgery, detectors for robots and technologies in the fields of security.

The polariton particle (Wikipedia) is partly light and partly matter. Lasers based on polariton utilize these particles to emit light. The researchers predict that they will be more energy efficient than conventional lasers. The new prototype requires 1000 times less electricity than a parallel laser composed of the same materials but operating in the usual way.

"Our discovery is really amazing," said Pallab Bhattacharya, a professor of electrical engineering and computer science at the University of Michigan. "During the last 50 years we have relied only on lasers for the production of coherent radiation and now we have another device based on a completely new idea."

The new system is not really a laser. The term laser was originally an acronym for Light Amplification by Stimulated Emission. Lasers based on polariton do not force emission of radiation, but they force scattering of polaritons.

In a conventional laser, light, or more often an electric current, is pumped into a material designed to amplify the signal. Before the pumping begins, most of the electrons in the designated material are in the lowest energy level, which is known as the ground state. As soon as light or electric current hits these electrons, the electrons absorb the extra energy and rise to a higher energy level. At a certain point, the number of high-energy electrons is higher than the number of electrons in the ground state, and the device achieves what is known as a "population reversal". At this point, any amount of electric current or light introduced into the material results in the opposite result - the electrons return to the ground state while emitting radiation in the process.
Lasers based on polariton are not based on such population inversions, so they do not require a high initial amount of energy to excite the electrons and return them to the ground state afterwards. "The starting current can be extremely low, a very attractive feature for future developments," says the lead researcher. The researchers combined the appropriate material - the transparent semiconductor gallium nitride - along with a special design in order to preserve the controlled conditions that encourage the polaritons to form and emit light.

How does it work? A polariton is a combination of a photon (a particle of light) together with an exciton (an electron-hole pair). The electron is negatively charged and the hole is actually the absence of the electron, but it behaves as if it were positively charged. Excitons will merge with light particles only when the exact conditions are met. Too much light or electric current will cause the excitons to decay too quickly. However, just enough light or electric current will create polaritons that will oscillate in the system until it reaches equilibrium at the lowest energy level in what the lead researcher describes as a coherence buffer. There, the polaritons decay and in the process release a beam of monochromatic light. The beam that the researchers received was ultraviolet radiation and had a very low energy - less than a million watts. For comparison, the laser used in CD players is a thousandth of a watt.

The design used by the researchers helped them receive the beam with an electric current instead of with light. Introducing the electric current into the system required placing electrodes between the gallium nitride and several layers of mirrors that make the electric signal usable. Approaches by other researchers place the electrodes outside the mirrors. The new design includes placing the mirrors on both sides of the gallium nitride and placing the electrodes above and below the gallium nitride.
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