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

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Abstract
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.