Laser cooling of semiconductors
Scientists have discovered a unique and counterintuitive method of cooling semiconductors by heating a semiconductor membrane using a laser.
In experiments by researchers at the Niels Bohr Institute in Copenhagen, fluctuations of a gallium arsenide (GaAs) membrane were cooled to -269ºC (4K). Existing thermoelectric coolers are quite adequate for current applications but this new method holds promise for quantum computing processors of the future, where temperatures close to absolute zero will be needed to eliminate thermal interference from electrons.
With an area of 1mm2 and 160 nm thickness, the membrane is huge for this realm; its dimensions were dictated by factors such as laser beam spot size and optical reflectivity. Despite its size, team member Professor Søren Stobbe says standard semiconductor etching techniques were used in its fabrication, although they had to be tweaked to give an extremely smooth membrane surface, a high aspect ratio and very high mechanical Q-factor.
The laser is a typical continuous-wave solid state unit, delivering 50W of input power. When the laser light hits the membrane, some is reflected, some absorbed and the rest transmitted. The transmitted light is reflected back to the membrane by a mirror, then reflected again by the membrane, forming an optical cavity or resonator inside where laser photons are trapped.
The membrane is suspended on a frame angled slightly towards the mirror, and the membrane’s thermal Brownian motion (whose amplitude is set by the membrane’s temperature) constantly alters the width of the cavity. At the same time, light absorbed by the membrane generates electrons in its conduction band and holes in its valence band; these both lead to mechanical expansion of the membrane, which in turn deforms it towards the mirror and changes the membrane-mirror distance.
This deformation degrades the cavity’s resonance with the laser, however, so the light is trapped less well. Electron-hole generation is therefore reduced, counteracting the deformation and improving the resonance. These feedback mechanisms can be harnessed to dampen the membrane’s thermal motion, and it’s this that lowers the temperature – even though the membrane itself is actually heated slightly, by about 0.1ºC.
‘This “cooling by heating” would be the most vexing issue we would need to address for a practical application,’ says fellow team member Professor Koji Usami, ‘since reducing the extraneous heating effect due to the non-radiative decay of the electrons and holes would be of the utmost importance.’
In practice, he adds, a semiconductor would be embedded in a free-standing membrane that would be cooled using a laser from another semiconductor, but concedes that this is ‘a couple of decades’ away.
Commenting on the work, Prof essor Stephen Sweeney, Head of Photonics at the University of Surrey, UK, says, ‘I think this is a very elegant piece of research and a positive move forward. I am more dubious about its practical application though; as the authors concede there are some fairly key steps required to take this through to application. The key issue will relate to material quality and the use of structures such as quantum wells.
‘This is the first step on a long path, but it's an exciting area of research.’