Square law behavior of photocathodes at high light intensities and high frequencies

Beats between modes of a ruby laser at frequencies more than twice the mode separation observed by purely optical techniques have been detected by photodiodes. It is demonstrated that the photodiode system was operating linearly when detecting these beats, thus removing the possibility of harmonic generation in the detection process. The observed beats are believed to arise from subthreshold or enhanced spontaneous emission modes that exist outside the oscillating linewidth.