New advances in molecular engineering for quadratic nonlinear optics

Abstract

Optimization of organic molecules for quadratic nonlinear optics is investigated in the perspective of nonlinearity/transparency trade-off, using intramolecular charge-transfer compounds with limited conjugation, such as oligomers of polysilane or polyphenyl. A large increase of the transparency in the blue and near UV spectral range is obtained for silane derivatives, but at the cost of a limitation of the nonlinear optical response at the molecular scale. On the contrary, polyphenyls appear to be ‘optimized’ materials, their nonlinear optical performances, both at the microscopic and macroscopic scale, being much better than those of paranitroanilines, with a significant improvement of the transparency. On the other hand, organometallic complexes containing ferrocene moieties conjugated to Ni, Pd or Pt metals through an organic polarizable unit display interesting nonlinear optical properties: the role of complexation appears to be quite important for ferrocenyl-containing ligands. A metal-to-metal intramolecular charge transfer is clearly evidenced. The role of the nature of the complexing metal and the influence of the nature of the ligand are also investigated. The moderate static hyperpolarizability values (of the order of 20 × 10 −30 e.s.u. may be partially accounted for by the defavourable projection factor between the ground state dipole moment and the vector part of the molecular hyperpolarizability.