Excited State Dynamics of the Third Order Nonlinear Optical Susceptibility in Conjugated Linear Chains

We have earlier presented theoretical and experimental results demonstrating that the nonresonant third order optical susceptibility χ(3)(−ω;ω,ω,−ω) can be enhanced for nonlinear optical processes originating from real population of electronic excited states in conjugated linear chains[1,2]. Compared with the ground state, χ(3)(−ω;ω,ω,−ω) of π-conjugated linear chains can be enhanced by orders of magnitude when the first S1 or second S2 π-electron excited state is optically pumped and then populated for times long enough to perform nonresonant measurements of χ(3)(−ω;ω,ω,−ω) at frequencies different from the resonant pump frequency. In this paper, a dynamical model is developed for the time evolution of the nonresonant microscoopic degenerate four wave mixing (DFWM) susceptibility γ(-ω;ω,ω,-ω) for the excited state enhancement mechanism. The model is applied to the case of the six site chain, diphenylhexatriene (DPH).