Improved fitting equation for frequency-resolved femtosecond hyper-Rayleigh scattering experiments

One approach to the experimental determination of the molecular second-order nonlinear polarizability, or the first hyperpolarizability, of fluorescent species by hyper-Rayleigh scattering is based on high-frequency demodulation of the time-delayed fluorescence contribution to the immediate scattering signal [Rev. Sci. Instrum. 69, 2233 (1998)]. For typical fluorescence lifetimes of less than a nanosecond a detection bandwidth of more than 1 GHz is necessary. This bandwidth has not yet been realized. Measurements at successively higher modulation frequencies are performed instead. A fitting of the apparent hyperpolarizability as a function of the modulation frequency then reveals the inherent hyperpolarizability without the fluorescence contribution. An improved fitting function has been derived, resulting in the elimination of a small systematic error and in the reduction of the larger statistical uncertainty in the deduced value. Possible implications of the improved accuracy and precision are discussed. (C) 2000 Optical Society of America [S0740-3224(00)01110-3].