Physicists in Singapore are the first to create a refrigerator that cools a piece of semiconductor using light, using their technique to cool a room-temperature sample of cadmium sulphide by some 40 K. Although a similar technique has previously been used to chill glasses doped with rare-earth elements, this latest work could lead to practical optical refrigeration devices for use in satellites, or even "self-cooling" lasers.

First developed in the 1980s, laser cooling has opened up the new and incredibly fruitful study of ultracold atomic gases. The technique involves firing counter-propagating laser beams at an atomic gas, with the atoms absorbing and emitting photons in such a way that the net effect is to reduce the average motion of the atoms, and thus lower the temperature of the gas.

Removing phonons

The laser cooling of solids is somewhat different because heat is stored in a solid in the form of quantized lattice vibrations called phonons, which do not interact directly with light. In the case of rare-earth-doped glasses, energy is removed from the phonons when a single atom in the glass undergoes an "anti-Stokes" transition. This involves a photon being absorbed by an atom before emitting a higher-energy photon – with the extra energy coming from phonons.

From a technological point of view, it would be much more useful to be able to laser-cool a more conventional material, such as a semiconductor, than a doped glass. Last year, Eugene Polzik and colleagues at the University of Copenhagen managed to use a laser to cool an extremely thin sheet of semiconductor that was stretched like a drumhead. Rather than being a general refrigeration method, however, the optomechanical technique focused on damping out a specific subset of drum-like phonon modes in the sheet.

Chilly nanobelts

What Qihua Xiong and colleagues at Nanyang Technological University in Singapore have now demonstrated is a more general technique that uses lasers to cool an extremely thin ribbon (or "nanobelt") of the semiconductor cadmium sulphide (CdS). The method also relies on an anti-Stokes process, but in this case the transition involves an absorbed photon being converted into an electron–hole pair. This "exciton" annihilates and the semiconductor emits a higher-energy photon – with the extra energy coming from the annihilation of phonons. As a result, the sample loses phonons and cools.

Xiong told that his team stumbled upon the effect by accident when doing laser-based Raman-spectroscopy experiments on the CdS nanobelts – materials that have a particularly strong anti-Stokes photoluminescence. The nanobelts were about 3 μm wide and about 100 nm thick, and were draped across a silicon-oxide substrate that was peppered with holes that were about 4 μm across. Measurements were made on the portions of the nanobelts that were suspended over the holes.

Strong coupling

The experiment involved firing "pump" laser pulses at the nanobelt to create excitons, with the laser energy adjusted so that the exciton energy plus the energy of several phonons equals the energy of a photon emitted in an anti-Stokes transition. Each photon emitted in this way therefore takes a significant amount of heat energy. Indeed, Xiong says that more than 100 meV of energy is removed per pump photon – the very high efficiency being because excitons and phonons in CdS nanobelts couple very strongly.

The team began the cooling process with the nanobelt at room temperature (290 K) and then reduced the temperature to about 250 K in about 40 min. This corresponds to a cooling power of 180 μW. The temperature of the sample was measured using a technique called pump-probe luminescence thermometry, which involves firing a second "probe" laser pulse at the sample.

Sensors in space

According to Xiong, the cooling technique could be used to cool tiny devices. As well as being relatively straightforward to miniaturize, laser cooling does not involve mechanical refrigeration – which can introduce unwanted vibration – or cryogenic liquids. One application that Xiong says is "particularly appealing" is the cooling of sensors used on satellites and other space missions. He also says that the technique could be used to cool a laser by using some of its own light.

While Xiong says that there are several challenges that must be overcome to make the technique work on larger samples of semiconductor, it is, in principle, possible. Polzik described this latest cooling technique as "a very interesting result" adding that, in principle, the technique could be used to remove heat from semiconductor devices.

The cooling method is described in Nature.