On the thermodynamics of the up-conversion and fluorescence of light

Abstract

Frequency doubling of light, made possible by the laser, has precipitated a host of anti-Stokes fluorescence activities. In this work, a criterion based on the principle of the increase of entropy in adiabatic processes is presented. This principle relates to the nature of the light rather than to the substances performing the harmonic generation. It is shown that changes in the spectral entropy can be used to discriminate which radiation is suitable for up-conversion. The spectral entropy of a blackbody has a maximum with respect to frequency at any given temperature. If the final state of a process is located at this maximum, then a critical minimum temperature, below which no up-conversion may be expected, can be found. A definition of the effective temperature of highly nonequilibrium sources such as the laser is presented. Fluorescence in the low-pressure mercury lamp is treated in the context of thermodynamics. Examples of practical light sources are discussed, and it is shown that up-conversion is, rather exclusively, the purview of the laser.