Lasers usually heat things up. But in some crystals, light can carry heat away, like a teapot cooling under a bright lamp. In diamond, this job falls to defects: spots where a nitrogen atom replaces carbon, with a vacancy nearby. Spectroscopy showed that when illuminated by a laser with a wavelength just below resonance, the defect emits light with more energy than it absorbed. The difference comes from thermal vibrations of the lattice — the crystal cools.
But there's a catch. The same laser causes the defect to change charge: it goes from a negatively charged state to a neutral one. And a neutral defect no longer cools.
Time-resolved photoluminescence measurements showed: the stronger the illumination, the more neutral defects and the worse the cooling. The scientists built a simple model that accounts for pumping rates and charge-switching rates. It predicts that cooling is self-limiting: with too powerful a laser, the defect stops cooling. But with the right conditions, net cooling can still be achieved. Estimates suggest that a single defect cools as efficiently as a quantum dot or a rare-earth ion. So diamond defects are a promising material for precise cooling of microchips and quantum devices.
Diamond is one of the best heat conductors among solids. That's why it's used to draw heat away from powerful lasers and processors.