Disorder induced enhancement of the third order optical nonlinearity in a conjugated polymer

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Abstract
We report on the relationship between material disorder and nonlinear optical response for a conjugated polymer. Inverse Raman scattering, the technique used here, is sensitive to the transition cross sections of both the purely electronic (S v=0 1↔S v=0 0) and vibronic (S v=0 1↔S v=1 0) transitions accessed, and both of these quantities are related to the extent of disorder in the polymer backbone. We have verified these morphology dependencies using crystalline and amorphous samples of the polydiacetylene poly(4BCMU). The magnitude of the nonlinear response per backbone repeat unit in the amorphousmaterial is comparable to that of the crystalline form because of the large potential well displacement between the ground and excited electronic statescharacteristic of the disordered material. This increased potential well displacement compensates for the nonlinear response lost to the reduced macroscopic polymer alignment associated with the two‐dimensionally isotropic spin cast film.