Laser Cooling of Semiconductors

Surface Plasmon Assisted Laser Cooling of Solids

Jacob Khurgin

PRL 98, 177401 (2007)

URL: http://link.aps.org/abstract/PRL/v98/e177401


Laser cooling of atoms is based on the Doppler effect. Laser frequencies are tuned below an atomic transition so that atoms moving toward the laser source will be more likely to absorb a photon than those moving away. The atom will then emit a photon in a random direction as it relaxes back down to its ground state, but the radiated photon will be of higher energy than the absorbed photon. As a result, the atom has radiated away some of its kinetic energy, so it has been cooled by a laser.

In solids the basic idea is similar. The solid absorbs one frequency of light and emits at a higher frequency, losing energy as a result. The mechanism is different, and it called anti-Stokes photoluminescence. Light is absorbed at a specfic frequency, then this excited state comes to thermal equilibrium with the system. Later, light is emitted at a higher frequency. The shift in frequency would be on the order of kT, where T is the temperature of the solid, and the system would gradually cool down.

Khurgin points out that there are several difficulties in the case of semiconductors. There are a lot of nonradiative decay channels, and the high index of refraction leads to low efficiencies --- i.e., even if you generate a photon of the right frequency, it's hard for it to get out of the semiconductor. One way to improve the efficiency is to make the density of absorbing states small and the density of emitting states large. When a photon is absorbed, the energy is more likely to be transferred to a higher-energy emitting state than to remain in the absorbing state for a time, then be re-emitted at the same frequency.

Khurgin's approach to the problem is to exploit surface plasmon polaritons. These occur at the interface between a dielectric and a metal at frequencies where the dielectric constants of the two media are equal in magnitude but opposite in sign. The density of plasmon states has a sharp resonance, which leads to an increase of spontaneous emission at the resonant frequency. (Apparently, Purcell worked this out back in 1946.)

Khurgin notes that the plasmon modes still have to couple to radiative modes before they give up their energy, so it might seem that nothing has been gained. However, he goes on to demonstrate that the plasmons can couple to the phonons of the metal. The metal will heat up, but the goal was never to cool the metal and the dielectric together --- only to cool the dielectric medium.

Based on this observation, Khurgin proposes placing a layer of silver on top of a gallium arsenide layer with a gap between them of a couple nanometers. The gap is a thermal insulator between the silver and gallium arsenide. The only coupling between the two systems are the plasmons. A laser will produce excitations in the gallium arsenide layer, and many of these will relax into the many available plasmon modes. The plasmon modes will couple to the phonon modes in silver, but not in gallium arsenide, so they will gradually transfer energy from the semiconductor to the metal. This four-step process leads to laser cooling of the semiconductor:

laser ---> semiconductor excitations ---> plasmons ---> phonons in metal

Khurgin estimates that the silver and gallium arsenide system could have a cooling efficiency of 2 percent or more.