Susceptibility multipolar

The inclusion of the environment effects for non-linear optical (NLO) properties is presented within the PCM (Cammi Mennucci) and the multipolar expansion (Agren Mikkelsen) solvation models. In the first contribution the attention is focused on the connection between microscopic effective properties and macroscopic NLO susceptibilities, whereas in the latter contribution the analysis is extended to treat heterogeneous dielectric media. 

Another scheme to calculate and interpret macroscopic nonlinear optical responses was formulated by Mukamel and co-workers [112 114] and incorporated intermolecular interactions as well as correlation between matter and the radiation field in a consistent way by using a multipolar Hamiltonian. Contrary to the local field approximation, the macroscopic susceptibilities cannot be expressed as simple functionals of the single-molecule polarizabilities, but retarded intermolecular interactions (polariton effects) can be included. 

The zero-field and field-induced optical second harmonic generation (SHG) was investigated for the nematic and smectic A phases of various liquid crystals. The components of the cubic non-linear susceptibility tensor were measured for substances with different molecular structure. The phase-matched SHG was observed for all the compounds investigated. The directions of the phase synchronisms as well as the corresponding non-linear susceptibilities were determined for the ee-o and oe-o interactions. The zero-field phase-matched SHG was observed for the oe-o interaction. It was accounted for by a multipolar mechanism. 

The zero-field oe-o type SHG, in our opinion, may be accounted for by a multipolar (the quadrupolar) mechanism known for the solid crystals. The effective second-order susceptibilities calculated from the experimental data on the oe-o synchronism are shown in the Table. Their order of magnitude is typical of the quadrupolar SHG mechanism. So, the phenomenon of the zero-field second harmonic generation in nematic and... 

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