Review of scientific instruments vol:69 issue:6 pages:2233-2241
A novel technique for suppression of the multi-photon fluorescence contribution in second-order nonlinear optical hyper-Rayleigh scattering experiments is described. The technique takes advantages of the demodulation and the phase shift in the frequency domain of the time-delayed (multi-photon) fluorescence in the time domain. We demonstrate the effectiveness of demodulation at high modulation frequencies of the fundamental laser beam by determining the molecular second-order nonlinear polarizability for a reference molecule under fluorescent conditions. The value that was obtained for crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore compares very well with the values that were previously obtained. The possibility of complete suppression of all fluorescence, based on phase-sensitive measurements in quadrature with the fluorescence, is also discussed. (C) 1998 American Institute of Physics. [S0034-6748(98)04705-4].