Enhancing the accuracy and precision in hyper-Rayleigh scattering: frequency- and angle-resolved femtosecond nonlinear scattering

Abstract

Hyper-Rayleigh scattering, or second-order nonlinear incoherent scattering, has become a standard technique for the experimental determination of the molecular second-order nonlinear optical polarizability, or first hyperpolarizability, of nonlinear optical chromophores in solution. We review the different contributions to the hyper-Rayleigh signal and discuss the impact on the accuracy of the resulting value for the first hyperpolarizability. Especially incoherent multi-photon fluorescence and coherent second-harmonic generation deserve our attention. Temporal resolution of the response enables the distinction between the immediate scattering and time-delayed fluorescence. This has been implemented in the Fourier domain, where the fluorescence contribution exhibits a demodulation (a reduction in amplitude) versus the scattering for increasing modulation frequency of the fundamental laser light. By adding the experimental determination of the phase shift as a function of modulation frequency, ana analyzing the demodulation and phase shift simultaneously, the accuracy and the precision of the measurement was increased substantially. In addition, hyper-Rayleigh scattering as a function of incidence angle clearly shows any coherent contribution to the signal. Selected examples show that the analysis as a function of angle and time results in a value that is ultimately completely free of systematic error. A comparative study shows the importance of the improved precision.