Radiation efficiency of partially coherent electromagnetic beams

We present a general definition of the radiation efficiency of stationary e lectromagnetic fields and prove that it is bounded between zero and unity f or beams of any state of coherence and polarization. The radiation efficien cy may be interpreted as a measure of how directed the radiated fields are, and therefore it can be used to assess the allowed spatial coherence and i ntensity variations across a beam. We consider a class of partially coheren t electromagnetic fields that were recently introduced in the literature an d evaluate the radiation efficiencies for two particular examples, namely, the azimuthally polarized symmetric beams and the dipolar beams that are ne arly linearly polarized in the central region. The results show that the ra diation efficiency is fairly insensitive to the state of polarization and t hat it differs appreciably from unity for only small values of source and c orrelation widths. (C) 2001 Optical Society of America.