Third order nonlinear optical interactions in thin films of poly-p-phenylenebenzobisthiazole polymer investigated by picosecond and subpicosecond degenerate four wave mixing

The third order nonlinear optical susceptibility has been measured by degenerate four wave mixing in a 33-μm-thick biaxial film of a conjugated aromatic heterocyclic polymer, poly-p-phenylenebenzobisthiazole, commonly known as PBT, which has a very high mechanical strength due to its rigid rod conformation as well as environmental stability and a high laser damage threshold. For the first time, the response of the optical nonlinearity in the π-electron conjugated system has been experimentally verified to be in subpicoseconds. The value of χ⁽³⁾ is found to be about an order of magnitude larger than that of CS₂. The measurement at two different wavelengths suggests that they are nonresonant χ⁽³⁾ values. The measured anisotropy of χ⁽³⁾ as a function of angular orientation at two different sets of laser polarization is explained by using the tensor properties of χ⁽³⁾ in an anisotropic medium.