Spectral content and dispersion of hyper-Rayleigh scattering

We have used a high-intensity tunable picosecond infrared laser source to measure the spectral content of light scattered near the second harmonic for two well-known organic dyes, p-Nitroaniline and Disperse Red 1, at a number of wavelengths in the near infrared. We found that in Disperse red 1 a broad two-photon fluorescence competes with the sharp hyper-Rayleigh peak. Further, dephasing of the virtual excited state leads to elimination of hyper-Rayleigh scattering in favor of fluorescence if the harmonic is in the linear absorption tail. We determined the dispersion of the magnitude of the first hyperpolarizability for both dyes, using a simple referencing technique, and found it to be consistent with that of a two-level dispersion model.