Macroscopic and microscopic second-harmonic response from subphthalocyanine thin films

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Abstract
The second-harmonic generation (SHG) yield of both evaporated and spin-coated films of some tri-substituted subphthalocyanines has been measured for three linear polarizations of the fundamental beam. Partial molecular ordering in the spin-coated films was achieved by the corona poling technique. For the evaporated films the ordering is obtained through the evaporation process itself. By proper fitting of the SHG yield data and assuming C ∞v symmetry the three nonzero independent components, χ 31 (2) , χ 15 (2) and χ 33 (2) , of the susceptibility tensor have been determined for the two types of films. Kleinman symmetry condition (χ 31 (2) =χ 15 (2) ) is not obeyed. Values close to 10 −9 esu have been obtained for the highest χ 31 (2) component of the spin-coated films containing trioctylsulfonylsubphthalocyanine molecules. For the evaporated films the susceptibility values are somewhat higher, indicating a significant degree of molecular ordering. For both types of films χ 33 (2) ≪χ 31 (2) , χ 15 (2) , at variance with the case of linear molecules. From a statistical analysis of the macroscopic SHG emission the nonzero components of the microscopic (molecular) hyperpolarizability tensor, β 15 , β 31 and β 33 , have also been estimated. The results indicate that β 33 ≈0, and that β 31 and β 15 are dominant in accordance with the pyramid-shaped molecular geometry. The additional inplane components β yxx =−β yyy =β xxy =−β 22 appearing in the C 3v molecular symmetry cannot be determined from the macroscopic data.