Linear and nonlinear optical spectra of polyazomethines fabricated by chemical vapor deposition

We report on the results of third-harmonic generation (THG) measurements on vapor-deposited thin films of two polyazomethines, poly(nitrilo-1,4-phenylenenitrilomethylidyne-1,4-phenylenemethylidyne) (PNPP) and poly(nitrilo-2,5-azinylnitrilomethylidyne-1,4-phenylenemethylidyne) (PNAP) covering pump energies from 0.6 to 1.4 eV. The THG data show the presence of a strong three-photon resonance centered near 0.9 eV for both samples; additional weaker structure is observed for PNPP near 1.2 eV. The peak values of the third-order susceptibility are 2.8 × 10−11 e.s.u. for PNPP and 1.8 × 10−11 e.s.u. for PNAP; off-resonance values are nearly an order of magnitude smaller. We have also obtained the χ(3)(3ω) spectrum of an oriented PNPP film grown on an obliquely evaporated SiO2 pattern. An enhancement by nearly a factor of three in the peak nonlinear optical susceptibility is observed relative to that of the unoriented PNPP film, demonstrating that a moderate degree of molecular orientation was achieved.