Need to cool down? Maybe you should warm up. It works for semiconductor membrane materials, paradoxically. By combining two worlds – quantum physics and nanophysics – researchers have discovered a laser cooling method for semiconductor membranes that could lead to more efficient cooling systems for ultrasensitive sensors and quantum computers. Developed by researchers at Niels Bohr Institute, the technique works by actually heating the material. Using lasers, the scientists cooled semiconductor membrane fluctuations to —269 °C. Researchers have shown that a laser can precisely control the temperature of semiconductor membranes. Courtesy of 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. The researchers allowed a laser light to interact with a 160-nm-thick semiconductor membrane with a surface area of 1 mm2 in a vacuum chamber in such a way that the light hitting 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. The light absorbed then caused the release of free electrons, which decayed, heating the membrane and resulting in thermal expansion, Usami explained. In this way, the distance between the mirror and the membrane fluctuated continuously. Laser cooling 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 light is reflected back again via a mirror so that the light 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. “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,” he said. The research group has practiced experimental laser cooling of atoms for several years to understand optomechanics, said Eugene Polzik, head of Quantop. This new field has the potential to pave the way for more efficient cooling components in quantum computers, he added. “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 results were published online Jan. 22 in Nature Physics (doi: 10.1038/nphys2196).