Spatial and Temporal Orientational Correlations Between Nonlinear Optical Chromophores Probed by Femtosecond Hyper-Rayleigh Scattering

After designing and synthesizing a new candidate molecule for nonlinear optical (NLO) applications, the molecular hyperpolarizability of this molecule has to be experimentally determined. This measurement at the microscopic level can be performed in an isotropic solution using hyper-Raleigh scattering (HRS).1 For a second-order NLO application, the individual molecules have to be assembled in a macroscopically non-centrosymmetric fashion. The efficiency of a phase-matched second-order NLO process is highly dependent on the length over which the coherent nonlinear interaction can be maintained. In artificially ordered systems, such as poled polymers and Langmuir- Blodgett films, this length is much more determined by the length over which the local order is constant than by the coherence length, calculated from the bulk refractive indices at fundamental and second-harmonic wavelength. A measurement scheme to determine the degree of spatial correlation between the chromophores would therefore be instrumental in the development of reliable and reproducible devices.