Abstract

The premise that anti-Stokes fluorescence of a solid material can form the basis of a refrigeration cycle is an old idea first proposed in 1929.¹ Consider a system of isolated impurities, each with a simplified three-level energy structure, in order to understand the physical principles: Label the ground state "1" and suppose that the two excited states, labeled "2" and "3", are split in energy by a few kT. Now further suppose that the energy gap between the excited states and the ground state is large enough so that nonradiative relaxation across this energy gap is negligibly slow. On the other hand, nonradiative thermal equilibration between the two excited states occurs rapidly compared with radiative relaxation between these levels and the ground state. Thus, if a laser is tuned to the 1→2 transition, fluorescence from the 2→1 and 3→1 transitions will occur with a mean photon energy higher than that of the input photons. Cooling results as long as the fluorescence quantum efficiency is close to unity.

© 1995 Optical Society of America