Chilling out with polaritons

11 May 2007

US researchers believe that polaritons could offer the key to laser cooling a semiconductor.

Shining light on an object might seem like a strange way of reducing its temperature, but laser cooling is routinely used to create samples of ultracold atoms. Now, however, a physicist in the US has devised a way to laser-cool a semiconductor.

Although Jacob Khurgin from Johns Hopkins University has not yet proved his technique experimentally, he reckons it could lead to more efficient ways of cooling infrared detectors and other electronic devices (Phys. Rev. Lett. 98 177401).

Solids can be cooled with light if they absorb a photon of light at one energy and then re-radiate a photon of higher energy. As long as slightly more energy goes out than in, the temperature of the material will drop. This effect, known as anti-Stokes photoluminescence, has been used since the mid-1990s to cool various glasses doped with ytterbium and other heavy-Earth elements.

However, trying to laser-cool a semiconductor is far harder because the absorbed photon creates an electron-hole pair that only occassionally recombines to create a higher-energy photon. Instead, recombination usually results in heat being transferred to the surrounding lattice. Even if reradiation occurs, there is a very good chance that the new photon will be reabsorbed by the semiconductor, further increasing the chances of heating.

Khurgin’s solution is to place a small block of metal, such as silver, about 10 nm away from the semiconductor. This is done to take advantage of the cooling effects of surface plasmon polaritons (SPPs), which exist on metallic surfaces. SPPs are quantum oscillations that arise from the interaction of light with the metal’s conduction electrons.

While SPPs are normally found on the surfaces of metals, Khurgin has calculated that if the metal and semiconductor surfaces are separated by a very narrow gap, the SPPs could be created within the semiconductor material by the recombination of electron-hole pairs (see figure). Khurgin has also calculated that nearly all these SPPs would exit the semiconductor and deposit 99.9% of their energy in the metal – thus cooling the semiconductor.

Khurgin says that if silver was used as the metal and gallium nitride as the semiconductor, each SPP should remove nearly three times as much energy as a photon. He predicts that the device could achieve cooling efficiencies of about 3%, which is enough for practical applications.

According to Khurgin, the technique could allow the semiconductors in electronic devices to be cooled directly, rather than employing an external refrigerator. This could be particularly important for those designing compact infrared detectors for use on Earth observation satellites or in portable night-vision systems.