High-frequency demodulation of multi-photon fluorescence in hyper-Rayleigh scattering

Femtosecond hyper-Rayleigh scattering (HRS) is used for the suppression of multi-photon fluorescence contributions to the apparent HRS signal. The intrinsic high harmonic content of the femtosecond pulse is used as a high frequency amplitude modulation source. Due to the nonzero fluorescence lifetime, a high amplitude modulation frequency will result in an amplitude demodulated and phase shifted fluorescence signal. At very high modulation frequencies, the fluorescence signal becomes completely demodulated and only the inherent HRS signal will remain. Experimental verification has been obtained by the fluorescence suppression for a well known centrosymmetric fluorophore, 9,10-diphenylanthracene, added to a solution of a well characterized nonlinear optical (NLO) chromophore, crystal violet (CV+). A comparison has also been made between the first hyperpolarizability value beta of a nonfluorescent ionic hemicyanine dye (DASPC(22)(+)) and the beta value of the fluorescent inclusion complex of the dye incorporated in an amylose matrix. The inherent fluorescence-free beta value for the complex appeared to be twice [(200 +/- 5)x10(-30) esu] the value for the dye itself [(100 +/- 10)x10(-30) esu].