Dynamics of third-order nonlinear processes in organic molecules and polymers

Systematic degenerate four-wave mixing studies of several classes of conjugated organic molecules and polymers with femtosecond resolution are reported. The objective is to understand the factors governing the relationship between the chemical structure and optical nonlinearities. An interesting class of organic molecules with high χ⁽³⁾ values that have been studied are macrocycles, including phthalocyanines and perylene derivatives. The time-resolved phase conjugate response of these organics exhibits power-dependent decays that furnish information on the dynamics of excitations responsible for the resonant χ⁽³⁾ effects. Conjugate polymers, e.g., polythiophene, polydiacetylenes, and poly(p-phenylenevinylene) (PPV), exhibit large χ⁽³⁾ values in both the resonant and nonresonant regimes. PPV shows the fastest response of all the π-conjugated polymers studied. The conjugate signal decay is not resolvable even with the use of laser pulses of ~100-fs pulse width, suggesting nonresonant behavior.