The third-order nonlinear optical susceptibility ͑3͒ ͑Ϫ3;,,͒ of thin films of poly͑p-phenylenevinylene͒ ͑PPV͒ and several corresponding oligomers ͑OPV-n͒ has been investigated by third-harmonic generation using variable laser wavelengths from 900 to 1520 nm. The oligomers show a single three-photon resonance of ͑3͒ ͑Ϫ3;,,͒ which is closely related to the linear absorption spectrum. We can identify, however, two maxima in the ͑3͒ spectrum of PPV. They are assigned to three-photon resonances with the maximum of the exciton absorption and with the threshold of the continuum of states, which can be located at 3.2Ϯ0.1 eV. This corresponds to an exciton binding energy of 0.7Ϯ0.1 eV. We observe a general scaling behavior for PPV, OPV-n, and other one-dimensional conjugated -electron systems in their neutral form. Their ͑3͒ values, evaluated at comparable resonant or low-resonant conditions, follow an empirical scaling relationship(3) /␣ max ϳ max x , where ␣ max and max denote the absorption coefficient and wavelength of the low-energy absorption maximum. We obtain an exponent xϭ10Ϯ1 which is much larger than expected from an earlier theory. Possible reasons for the difference between theory and experimental results are discussed.

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+ Present address: BASF AG, D-67056 Ludwigshafen, FRGThe synthesis and the optical properties of the novel polymer polyphenothiazinobisthiazole (PPT) are described. The design of this polymer was planned taking into consideration several aspects that were anticipated to lead to a material with an enhanced nonlinear optical susceptibility Monomer 16 was prepared in a five-step synthesis starting from N-dodecyl-3,7-diaminophenothiazin (11). 16 was reacted with terephthaloyl chloride to yield the precursor polyamide 17.This precursor polyamide was subjected to a thermal cyclization at 300 OC under vacuum leading to the title polymer PPT (18).PPT shows UVIvis absorption maxima at 1 =282 nm,305 nm and 463 nm.The macroscopic nonlinear optical suscepti-bility~'~)(-3w; w, w, w ) of a thin film ofPPTwas determined with a third harmonic generation (THG) technique at a fundamental laser wavelength 1,. = 1064 nm. The results show that PPT exhibits nonlinear optical properties comparable to those of poly-p-phenylenevinylene (PPV).

Poly(p-phenylenevinylene) (PPV) is a typical representative of conjugated polymers, which have a highly delocalized, one-dimensional conjugated π-electron system. These polymers have found much interest, because most of them can be processed rather easily to thin film waveguides and because of their large third-order nonlinearities [1-3]. Thin films of PPV are prepared by spin-coating of a soluble precursor polymer on fused silica substrates and thermal annealing under vacuum as described recently [4]. We have used thin films of PPV to develop and demonstrate a sensitive characterization method of the intensity dependent refractive index n = n0 + n2I and absorption coefficient α = α0 + α2I. The utility of any material for applications in photonics depends primarily on these quantities.