Excited-state nonlinear optics and enhancement mechanisms for nonresonant processes

A general enhancement mechanism is reviewed for nonlinear optical processes originating from real population of electronic excited states. The nonresonant third order optical susceptibility y1(-w4;wi,w2,w3) of quasi-one dimensional conjugated linear chains can be enhanced by orders of magnitude, or even change sign, when the first (S1), or second (52), electronic excited state is optically pumped and then populated for times suitably long to perform nonresonant measurements of y1(-w4; w1 , w2,w3) at frequencies different from the resonant pump frequency.