Hyper-Rayleigh scattering for symmetry fluctuation spectroscopy

Incoherent second-order nonlinear scattering, or hyper-Rayleigh scattering (HRS) has been evaluated as a new experimental tool for symmetry fluctuation spectroscopy. As such, it is the second-order nonlinear analogue of the linear dynamic light scattering (DLS) of quasi-elastic light scattering (QELS) technique for density fluctuation spectroscopy. High peak power is needed in a short pulse for the second-order nonlinear optical effect to be observed. The periodical structure of the impinging light itself does not impede the measurement for intensity correlation times. longer than the pulse-to-pulse period. However, the inherently large spectral bandwidth of femtosecond pulses considerably reduces the amplitude of the autocorrelation function. Reducing the coherence volume to increase this amplitude results in a lower count rate. The low efficiency of the second-order nonlinear light scattering, possible relaxation oscillation in solid-state femtosecond lasers and its quadratic amplification in second-order nonlinear scattering have pronounced influence on the autocorrelation function. Finally, typical relaxation times expected for the dynamics associated with large fluctuations in second-order nonlinearity put a severe limit on the applicability of quasi-elastic nonlinear light scattering for the study of chemical reaction dynamics.