FREQUENCY-DEPENDENCE OF 3RD-ORDER NONLINEAR SUSCEPTIBILITIES IN POLYARYLENEVINYLENE THIN-FILMS

The frequency dependence of the third-order nonlinear susceptibility, chi(3)( - 3omega; omega, omega, omega) spectra, of polyarylenevinylenes, poly(2,5-dimethoxy-1,4-phenylenevinylene) (PDMPV) and poly(2,5-thienylenevinylene) (PTV) was measured by third-harmonic generation (THG) using fundamental wavelengths ranging from 1475 to 2100 nm. THG measurements were performed for both unstretched PDMPV and PTV films with varied extents of conversion from the precursor polymer and also for uniaxially stretched PTV films. The chi(3) values dramatically increased with increasing conversion. The largest chi(3) values reached 1.6 X 10(-10) e.s.u (PDMPV) and 4.5 X 10(-10) e.s.u. (PTV) at the position of the three-photon resonance with the lowest pi-pi* transition band. Two chi(3) peaks at the fundamentals of 1600 and 1950 nm were found in the unstretched PTV film. It was also found that the overall profile of the chi(3) spectrum drastically changed through uniaxial stretching. In the stretched PTV film, the resonant peak at 1950 nm significantly enlarged, whereas the peak at 1600 nm became obscure. The change of the spectral profiles between the unstretched and the stretched PTV films was ascribed to different average pi-conjugation lengths.