A comparative study of third-order electronic and thermally induced optical nonlinearity in a polythiophene thin film

초록

Third-order optical nonlinearities of poly (3-dodecylthiophene) thin films were investigated using pulsed and continuous-wave (cw) lasers. The nonlinearity measured by the z-scan method at 532 nm (second harmonic output of the Nd:YAG laser) with nanosecond laser pulses yielded the value chi((3))similar to 10(-7) esu, similar to the previous experimental results. The value of the off-diagonal element chi((3)) determined from the nanosecond degenerate four-wave mixing method indicated that the nonlinearity with the pulsed laser input was mainly electronic in origin. On the other hand, the nonlinearity measured using the z-scan method with a cw He-Ne laser at 633 nm was more than 10(-4) esu, too large to be accounted for by an electronic or saturation effect. Local heating in the polymer film by light absorption is thought to be a cause of the large nonlinearity. This temperature increase at the beam center by the He-Ne laser beam was estimated from measurements of the UV-visible absorption spectra and the refractive indices of the polymer film at various temperatures.

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