High-frequency demodulation of multiphoton fluorescence for hyper-Rayleigh scattering in solution

We demonstrate a novel technique in the frequency-domain for the suppression of the multiphoton 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 (multiphoton) fluorescence in the time-domain. For this technique, we use the improved temporal resolution offered by the femtosecond pulse versus the nanosecond pulse. We demonstrate the effectiveness of the demodulation of the fluorescence 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 the /spl beta/ (second-order nonlinear optical polarizability) of crystal-violet in methanol with 9,10-diphenylanthracene added as a centrosymmetric fluorophore, compares very well with the values that were previously obtained. The frequency-dependence of the retrieved value shows both the overestimation at DC and low modulation frequencies and the convergence towards the fluorescence-free value at high modulation frequencies.