Third order non-linear optical properties of poly-p-phenylene benzobisthiazole and its novel composite with Zytel processed via methane sulphonic acid solution extrusion

It was found that improved optical quality films of poly-p-phenylene benzobisthiazole (PBZT) and of a novel composite of PBZT and nylon (Zytel 330) can be produced by extruding below critical concentration solutions in methane sulphonic acid. The linear refractive indices and their dispersion behaviour in these films have been determined by analyses of the interference fringes observed in the optical transmission spectra. A relatively large value of the linear refractive index is obtained for PBZT. The third-order non-linear optical susceptibility, χ⁽³⁾, is determined for the films of PBZT and PBZT/Zytel composite using subpicosecond degenerate four-wave mixing. The χ⁽³⁾ value observed for this PBZT film is almost an order of magnitude larger than that previously reported for PBZT films of a rather inferior quality processed through polyphosphoric acid solution. This result emphasizes the importance of polymer processing to produce better optical quality films for evaluation of intrinsic χ⁽³⁾ values of polymers. The response time of the non-linearity is limited by the laser pulse width, indicating that the non-linear response is derived from the coherent electronic χ⁽³⁾. The χ⁽³⁾ value of the PBZT/Zytel composite is lower than that for pure PBZT because of the reduced number density. However, determination of the optical loss, α, in these films yields a much lower value of α for the PBZT/Zytel composite film, resulting in an improved value of the often used device figure of merit χ⁽³⁾ α. This result suggests that composite structures may be useful for optimizing the χ⁽³⁾ α ratio.