Third-order nonlinear optical response from polythiophene-based thin films

Optical quality films of the amphiphillic, regioregular polymer PTHPUDT were fabricated via the Langmuir-Blodgett technique, and their third-order nonlinear optical response was investigated using z-scan methods. Analysis of the resulting z-scan data yielded values of chi((3))(R) = - 1.2 x 10(-9) esu and chi((3))(I) - 3.5 x 10(-11) esu, consistent with previous studies on polythiophene-based polymers systems. However, even with relatively low intensities incident on pristine PTHPUDT polymer films, the required power to achieve a nonlinear response in the film is sufficient to produce photoinduced changes in the original polymer. These photoinduced changes are consistent with the onset of photooxidation, photobleaching, and chain scission processes. At higher incident intensities, prolonged exposure of the film results in the removal or partial removal of the polymer from the irradiated region. This leads to linear optical effects that manifest themselves as z-scan traces that show pronounced asymmetry and intensity-independent peak-to-valley transmittance. Although our observations pertain specifically to PTHPUDT, our results may extend to the broader class of polythiophene-based materials more generally. These observations put into question whether the nonlinear optical susceptibility values of polythiophene-based materials currently being reported in the literature result from pristine polymers or from their photoconverted products.