FA5 9:15am zyxwvutsrqp - zyxwvutsr 9:30am Femtosecond incoherent second-order nonlinear light scattering: opportunities for molecular and device characterization Koen Clays, Geert Olbrechts, and Andr6 Persoons, Laboratory of Chemical and Biological Dynamics, Center for Research in Molecular Electronics and Photonics, Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium tel.: 321-6327508, fax: 321-6327982 e-mail: koen@lcbdiris.fys.kuleuven.ac.be zyxwvuts SUMMARY: Incoherent second-order nonlinear optical scattering, also known as hyper- Rayleigh scattering (HRS)’, has become widely accepted as an experimental technique for the determination of the molecular second-order nonlinear optical polarizability (first hyperpolarizability). The generic laser type used for this experiment is a Q-switched nanosecond pulse laser,2 due to the low quantum efficiency of the process. Hyper- Rayleigh scattering is not only simpler than the previously developed Electric-Field- Induced Second-Harmonic Generation (EFISHG) technique (no need for knowledge of dipole moment, local field correction factors, third-order nonlinear polarizability), it is also complementary to it, providing values for individual elements of the third-rank second-order hyperpolarizability t e n ~ o r . ~ ’ ~ Moreover, it is the only experimental technique applicable to ionic or apolar species. Hence, HRS has provided a wealth of information on the tensor properties of the hyperpolarizabilityof molecules of all types of symmetry, it has proven indispensable for the experimentalverification of the octopolar paradigm in second-order nonlinear optic^,^ and it has been instrumental in relating the bulk susceptibility of ionic materials to the molecular hyperpolarizability of the constituting ions. In this field of molecular characterization by incoherent second-order nonlinear light scattering, the multiphoton fluorescence contribution to the hyper-Rayleigh scattering signal has become recognized as an experimental p r ~ b l e m . ~ A number of very large hyperpolarizabilitieshave been suspected to be caused by fluorescence contribution to the incoherently scattered second-harmonic intensity. A very elegant solution to this problem had been proposed based on the temporal difference between the immediate scattering and the time-delayed fluorescence.8 Only the photons detected in an early and narrow time window are regarded as due to scattering. We now demonstrate a novel technique in the frequency-domain for the suppression of the multi-photon fluorescence contribution in hyper-Rayleigh scattering experiments. The technique takes advantages of the demodulation and the phase-shift in the frequency-domain of the time-delayed o-78o3-4950-4/98/$10.00 1998 IEEE zyxwvuts 6 zyxwvutsrq 438