Light-induced enhancement of the third-order nonlinear optical susceptibility of a heterodimer consisting of two different porphyrin monomers, a zinctetra(4-sulfonatophenyl)porphyrin and a zinctetra(4-N-methyl-pyridyl)porphyrin, was observed when the sample was optically pumped by a 30 ps, 532 nm pump pulse. The temporal behavior of the enhancement was studied. It was found that the enhancement was due predominantly to the population of the charge-separated state with a large electronic third-order nonlinear optical susceptibility © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]