Observation of laser-induced fluorescent cooling of a solid

THE possibility that an object might cool through its interaction with radiation was suggested as early as 1929 by Pringsheim¹. After Landau² established the basic thermodynamic consistency of such a process, certain aspects of fluorescent cooling were vigorously pursued^3á¤-11. In particular, laser 'Doppler' cooling of gas-phase atoms and ions has today grown into a robust research area^12á¤-15. In contrast, attempts to cool solids with light have met with limited success; non-radiative heating effects tend to dominate, and fluorescent cooling has at best resulted in a reduction in overall heating rates⁶. Here we report the experimental realization of net cooling of a solid with radiation. The cooling efficiencies achieved (up to 2%) are more than 10⁴ times those observed in Doppler cooling of gases. By pumping a fluorescent cooling element with a high-efficiency diode laser, it may be possible to construct a compact, solid-state optical cryocooler, thereby allowing widespread deployment of cryogenic electronics and detectors in space and elsewhere¹⁶.