Radiation , along with conduction and convection, is a form of heat transfer. But in some circumstances, radiation can also induce cooling: Laser light can cool dilute gases of atoms whose thermal energy takes the form of relative translational motion. It can also cool some specially prepared solids whose thermal energy is contained in lattice vibrations.
The basic scheme for optical refrigeration of a solid is shown in figure 1(a). A laser excites a transition from an upper level of one state to a lower level of another, and a higher-energy photon is emitted, with phonons making up the energy difference. If the pump light is generated by a semiconductor diode laser, the cooling system as a whole has no moving parts or fluids—a particular advantage for spaceborne applications.
So far, every demonstration of optical refrigeration has used a transparent material doped with a rare-earth element—usually ytterbium, but some...
