Lasers heat things up, right? – unless you happen to hit upon the right resonance, in which case it seems you can use lasers to cool things down.
Koji Usami carries out the experiments
at the Quantop laboratories at the Niels
Bohr Institute. Credit: Niels Bohr Institute
In an announcement that could be filed under either “counter-intuitive” or simply “wow”, scientists at Copenhagen University’s Niels Bohr institute have used a laser to cool a semiconductor membrane to -269°C.
The research, published in Nature Physics, extends a known quantum phenomenon to the macroscopic world. Focussed lasers have been used since the 1980s to cool atoms. In the current research, the group from the Niels Bohr Institute’s Quantop group wanted to try a similar trick in the macro world.
“It would mean entirely new possibilities for what is called optomechanics, the interaction between optical radiation … and a mechanical motion,” said Quantop’s head professor Eugene Polzik.
Here’s how it the cooling phenomenon happens for single atoms: if the atom is moving towards the laser, the photon striking the atom will slow it down; enough impacts will reduce its momentum to near-zero (at which point it would have a temperature of absolute zero).
There’s a problem, though: it only works for atoms traveling towards the laser. If the atom is traveling in the same direction as the laser, it will gain momentum instead (and heat up). Various techniques have been developed over the years to counter this; for the purposes of this discussion, I will stick with the simplest.
If the laser is tuned to just below the resonant frequency of the atom, the “head-on” photon is Doppler-shifted towards the resonant frequency. As a result, photons in that direction are absorbed just a little more strongly than photons headed in the same direction as the atom.
Making all this work for a complex material rather than a single atom is much harder – and that’s the breakthrough being claimed by the Niels Bohr researchers. Their choice of material is gallium arsenide, and since 2009, they have been working to create a semiconducting membrane with the right dimensions to use in their experiments.
The resulting membrane has an area of more than 1 square millimeter, but is only 160 nanometers thick. This, it seems, has the right resonant properties to be subjected to laser cooling, but at a macro rather than single-atom level.
Associate professor Koji Usami explains the experiement: “we let the membrane interact with the laser light in such a way that its mechanical movements affected the light that hit it.
“We carefully examined the physics and discovered that a certain oscillation mode of the membrane cooled from room temperature down to minus 269 degrees C, which was a result of the complex and fascinating interplay between the movement of the membrane, the properties of the semiconductor and the optical resonances”.
Usami says the technique might be useful for cooling components in quantum computers, and could also be used to create new electrical or mechanical sensors. If the technique can be applied more widely, it could also replace cryogenic cooling in a variety of applications. ®