Fluorescence-free hyperpolarizability values by near-infrared, femtosecond hyper-Rayleigh scattering

It is shown that femtosecond modulated hyper-Rayleigh scattering (HRS) can be used to suppress multi-photon fluorescence (MPF) contributions to the hyper-Rayleigh scattering signal. A high frequency modulation of the fundamental beam modulates both the immediate scattering and the time-delayed fluorescence. Due to the finite lifetime of the excited state, the fluorescence exhibits essentially complete demodulation (zero amplitude) while the scattering is unaffected. The fundamental wavelength of 1300 nm is accessible by the use of a Ti:Sapphire femtosecond laser together with an Optical Parametric Oscillator. Even at the latter non-resonant wavelength, fluorescence is observed for several chromophores. 4-(N,N-dimethylamino)-4′-nitrostilbene (DANS) and 4-methoxy-4′-nitrostilbene (MONS) were used to demonstrate the fluorescence-issue in HRS experiments. The ability to measure the fluorescence-free hyperpolarizability value β of an ionic compound was demonstrated on 4-[4-(N,N-methyl-octadecylamino)styryl]-1-methyl-pyridinium bromide (MO). Moreover, two examples are discussed in which the technique proved successful. First, the molecular second-order properties of some subphthalocyanine derivatives, i.e. chloro[2,3,9,10,16,17-hexa-alkylthiosubphthalocyaninato]-boron(III) homologues with as the alkyl substituent an octyl-, decyl-, dodecyl-, or hexadecyl-chain, were studied. For all homologues, identical β-values of (190±30)×10−30 esu were found. Second, the β-values of a series of five fluorescent and ionic N,N-dimethylamino stilbazolium (DAST) homologues of increasing conjugation length (from ethenyl to decapentaenyl) have been measured. The inherent β-value of these chromophores shows a maximum of (2045±35)×10−30 esu for the hexatrienyl conjugation length.