Abstract

The problem of multi-photon fluorescence in hyper-Rayleigh scattering has been realized some time ago. ^[1] A number of very large hyperpolarizabilities have been suspected to be caused by a fluorescence contribution to the incoherently scattered second-harmonic. Different measurement schemes have been devized to exclude this additional contribution to the signal, resulting in an overestimate for the value of the first hyperpolarizability β. The quadratic dependence is true for both hyper-Rayleigh and two-photon fluorescence. Hence, a discrimination based on intensity-dependence is not valid. Two-photon fluorescence can start from wavelengths shorter than the second-harmonic (anti-Stokes two-photon fluorescence^[2]) and always extends to wavelengths longer than the second-harmonic. Spectral discrimination is, hence, not possible neither. Fluorescence is spectrally not as narrow as is scattering. Fluorescence background subtraction or correction has been shown to be feasible, but time-consuming.^[3] A very elegant solution has been proposed on the single possible basis of the temporal difference between the immediate scattering and the time-delayed fluorescence.^[4] This solution uses essentially the same fast photon counting equipment as is used in single photon timing. Only the photons in an early and small window are regarded as due to scattering.

© 1997 Optical Society of America