Morphology, linear and nonlinear optical studies of poly[1,6-di(N-carbazolyl)-2,4 hexadiyne] thin films (pDCH)

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Abstract
A joint work on morphology, structure, and nonlinear optical properties of vacuum evaporated thin films of pDCH on different amorphous or monocrystalline substrates is presented. The thin film evaporation of some diacetylene monomers on the alkali‐halide single crystals leads to epitaxy with particularly good biorientation of the polymer chains, the b repetitive distance of the monomer matches well with two equivalent [110] and [1̄10] directions of the cubic KBr cystal. Polymerization is thermally induced. The resulting pDCH films are polycrystalline with low optical defects. Crystal sizes are in the range of 0.1 to 0.3 μm with some long needles of 2–3 μm length. Third harmonic generation (THG) and electric field induced second harmonic generation (EFISHG) measurements on disordered thin films show the two‐photon resonances at 1.35–1.45 μm wavelength range, similarly as in other polydiacetylenes, with resonant values 〈χ(3) (−3ω; ω,ω,ω)〉=(1±0.1)×10− 1 0 esu (THG) and 〈χ(3) (−2ω, ω,ω,0)〉=(6.4±0.4)×10− 1 1 esu (EFISHG). The biorientational order of the pDCH chains observed by electron microscopy on various alkali‐halide single crystals is confirmed on a macroscopical scale by third harmonic generation. As expected for a bioriented film, the angular dependence of THG intensity follows the theoretical law. So, for the first time, both morphological and optical techniques have been used in combination to characterize thin films and have shown a nearly perfect biorientation of the polymer crystals.