Optical cavity cooling: a quantic leap for optical refrigeration?

A team of researchers at the Niels Bohr Institute have combined laser cooling technology with nano-physics, and have discovered a revolutionary way of bringing a semi-conductor nano-membrane’s temperature down to -269°C.

The research initially focused on the electronic and optical properties of a crystal membrane made of gallium arsenide (GaAs), a semiconductor. After a year, and against all odds, the team managed to produce a nano-membrane 160 nanometers thick with an area of over 1 mm², a comparatively enormous size at such a small scale. The extreme thinness of the membrane allows for it to be affected by a tiny effect, such as the mechanical effect of photons. In the experiment, a laser light was shone onto the nano-membrane in a vacuum chamber equipped with mirrors, forming the optical cavity. Some of this light was reflected onto a mirror and then reflected back by the mirror. This reflecting back and forth of photons forms an optical resonator. Some of the light is absorbed by the membrane and releases free electrons which decay. This heats the membrane and causes it to expand and fluctuate, thereby constantly altering the distance between the membrane and the mirror.

According to Koji Usami, one of the researchers in the team, “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” allowing for the possibility to cool the temperature of the membrane fluctuations, “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.” This is achieved by isolating and controlling specific modes in the membrane, a bit like specific colours can be isolated in the whole spectrum, and then bringing them to ultra-low temperatures, while the membrane as a whole remains slightly warmer (around +1°C) than the ambient. The cooling itself is related to the “electron-hole generation” caused by the photons in the semi-conductive material forming the membrane, and can be controlled thanks to laser light.
Ultimately, the potential of optomechanics could pave the way for cooling components in quantum computers and lead to the development of new sensors for electric current and mechanical forces and even replace cryogenic cooling in some cases, according to Prof. Eugene Polzik, also working on the project.