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 S₁ electronic excited state is experimentally observed for quasi-two-dimensional conjugated disklike structures. The isotropically averaged S₁ excited-state microscopic susceptibility 〈γ₁S(-3{ω};{ω},{ω},{ω})〉 is -1640±{}100×{}10^-36 esu, and the corresponding macroscopic χS1⁽³⁾(-3{ω};{ω},{ω},{ω}) exhibits temporal decay and pump-power saturation behavior associated with the expected decay and saturation of the S₁ 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.