Resonant third-order nonlinear optical properties of poly(3-dodecylthiophene)

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Abstract
We have investigated the resonant nonlinear optical properties of both chemically prepared and electrochemically prepared poly(3-dodecylthiophene) samples using femtosecond degenerate four-wave mixing. The measured χ(3) values for the two polymers are 3×10−10 and 5×10−10 esu at 602 nm. The value of χ(3) at 705 nm is an order of magnitude less. The nonlinearity shows a subpicosecond response. The power dependence of the conjugate reflectivity shows saturation at high input powers. The origin of this saturation is discussed in terms of the saturable absorber behavior. It is concluded that the saturation is consistent with the predictions of simple kinetic models. The origin of the phase conjugate response is discussed in terms of two models: (i) phase space filling model involving unrelaxed excitons and (ii) rapid conformational deformation leading to shift of oscillator strength to photogenerated polaronic states. The analysis of our results favors the mechanism involving unrelaxed excitons as the source of the observed third-order optical nonlinearity.