Nonlinear optical properties of rhodamine aggregates in solution at different pH studied by hyper-Rayleigh scattering technique

摘要

Abstract: The potential development of optoelectronic devices based on the NLO response of organic molecules has aroused much recent interest. The influence of the molecular interactions on their NLO properties is important to both basic and application study. The dye rhodamine B can form different protonation and aggregation states by changing the bulk pH, which is revealed by the UV-visible absorption spectra and the fluorescence emissions spectra. Here we also use the hyper-Rayleigh scattering (HRS) technique to probe more detailed information about the protonated forms and aggregates. Because rhodamine dye is an ionic compound, the classical technique electric-field-induced second-harmonic generation can not be used to get the first-order hyperpolarizability $beta@. Our further studies show that rhodamine has strong multi-photon fluorescence emission under the radiation of 1064nm and it overlaps the HRS signal at 532 nm. The concentration dependence of the HRS intensity shows that only in the very low concentration range the HRS signal increases linearly with the increasing concentration. In the higher range, the signal deviates from the line on the below side, which may arise for the linear absorption of the signal at 532 nm, nonlinear absorption of the incident laser beam, the intermolecular effect and distortion of the incident laser at high concentration. !13