THE possibility that an object might cool through its interaction with
radiation vas suggested as early as 1929 by Pringsheim(1) After Landa
u(2) established the basic thermodynamic consistency of such a process
, certain aspects of fluorescent cooling were vigorously pursued(3-11)
. In particular, laser 'Doppler' cooling of gas-phase atoms and ions h
as today grown into a robust research area In contrast, attempts to co
ol solids with light have met with limited success; non-radiative heat
ing effects tend to dominate, and fluorescent cooling has at best resu
lted in a reduction in overall heating rates(6). Here we report the ex
perimental realization of net cooling of a solid with radiation. The c
ooling efficiencies achieved (up to 2%) are more than 10(4) times thos
e observed in Doppler cooling of gases. By pumping a fluorescent cooli
ng element with a high-efficiency diode laser, it may be possible to c
onstruct a compact, solid-state optical cryocooler, thereby allowing w
idespread deployment of cryogenic electronics and detectors in space a
nd elsewhere(16).