By combining two worlds — quantum physics and nanophysics — researchers have discovered a laser cooling method for semiconductor membranes that could pave the way to developing more efficient cooling systems for ultrasensitive sensors and quantum computers. Developed by researchers at Niels Bohr Institute, the technique works paradoxically by heating the material. Using lasers, the scientists cooled semiconductor membrane fluctuations to –269 °C. Koji Usami conducts the experiments to cool semiconducting membranes in the Quantop laboratories at Niels Bohr Institute. The laser light that hits the semiconducting nanomembrane is controlled with a forest of mirrors. (Images: Niels Bohr Institute) “The paradox is that, even though the membrane as a whole is getting a little bit warmer, the membrane is cooled at a certain oscillation and the cooling can be controlled with laser light,” said Koji Usami, associate professor at the institute’s Center of Excellence Quantop. In the technique, the researchers produced a 160-nm-thick semiconductor membrane with a surface area of 1 mm2. They allowed a laser light to interact with the membrane in a vacuum chamber in such a way that the light that hit the membrane was affected by the membrane’s mechanical movements. The reflected light was reflected back once more using a mirror, bouncing the light back and forth within the space to create an optical resonator, Usami said. The experiments are carried out in this vacuum chamber. When the laser light hits the membrane, some light is reflected and some is absorbed, which leads to a small heating of the membrane. The reflected light is reflected back again via a mirror in the experiment so that the light bounces back and forth in this space and forms an optical resonator. Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances. The scientists can control the system to cool the temperature of the membrane fluctuations. The light absorbed then causes the release of free electrons, which decay, heating the membrane and resulting in thermal expansion, he explained. In this manner, the distance between the mirror and the membrane continuously fluctuates. “Changing the distance between the membrane and the mirror leads to a complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances, and you can control the system so as to cool the temperature of the membrane fluctuations,” Usami said. Koji Usami shows the holder with the semiconductor nanomembrane. The holder measures about 1 cm for each link, while the nanomembrane itself has a surface area of 1 × 1 mm and a thickness of 160 nm. Laser cooling of atoms has been practiced for several years in experiments in the quantum optical labs of the Quantop research group so as to understand optomechanics, a field that deals with the interaction between a mechanical motion and an optical radiation, said Eugene Polzik, head of Quantop. The potential for this new field could pave the way for more efficient cooling components in quantum computers, he said. “Efficient cooling of mechanical fluctuations of semiconducting nanomembranes by means of light could also lead to the development of new sensors for electric current and mechanical forces,” Polzik said. “Such cooling in some cases could replace expensive cryogenic cooling, which is used today and could result in extremely sensitive sensors that are only limited by quantum fluctuations.” The researchers’ results were published in Nature Physics. For more information, visit: www.nbi.dk