Experimental observation of enhanced nonresonant nonlinear optical responses from optically pumped electronic excited states

Enhancement of greater than 2 orders of magnitude of nonresonant third-harmonic generation from an optically pumped ${\mathit{S}}_1$ electronic excited state is experimentally observed for quasi-two-dimensional conjugated disklike structures. The isotropically averaged ${\mathit{S}}_1$ excited-state microscopic susceptibility 〈${\gamma }_1^{\mathit{S}}$(-3ω;ω,ω,ω)〉 is -1640±100×${10}^{\mathrm{-}36}$ esu, and the corresponding macroscopic ${\mathrm{\chi }}_{\mathit{S}1}^{\mathrm{}\left(3\right)\mathrm{}}$(-3ω;ω,ω,ω) exhibits temporal decay and pump-power saturation behavior associated with the expected decay and saturation of the ${\mathit{S}}_1$ state. The enhancement mechanism through optical pumping followed by a nonresonant nonlinear optical probe is generalizable to both second- and third-order optical processes in nonlinear optical media.