Fourier analysis of the femtosecond hyper-Rayleigh scattering signal from ionic fluorescent hemicyanine dyes

A series of five fluorescent and ionic dimethylamino stilbazolium homologue s with increasing conjugation length (from ethenyl to decapentaenyl) is inv estigated by high-frequency, amplitude-modulated femtosecond hyper-Rayleigh scattering at 1300 nm. A hyperpolarizability value that is not overestimat ed by the presence of a multiphoton fluorescence contribution is obtained f rom the Fourier analysis of the hyper-Rayleigh scattering signal. The demod ulation curve (decrease of Fourier amplitude versus modulation frequency) i s characterized by both the hyperpolarizability value and the fluorescence decay parameters. The fluorescence decay parameters are accurately determin ed independently by single-photon counting. A detailed analysis of the fluo rescence decay parameters from the hyper-Rayleigh scattering demodulation c urve and of their relation to the fluorescence decay parameters obtained fr om single-photon counting experiments is presented. The inherent hyperpolar izability value for these chromophores shows a maximum of (2045 +/- 35) x 1 0(-30) esu or (760 +/- 13) x 10(-50) C-3 m(3) J(-2) for the hexatrienyl con jugation length. A comparison with theoretical calculations suggests the im portance of trans-cis isomerization in the excited state. (C) 2000 Optical Society of America [S0740-3224(00)00102-8].