Experimental study of the second-order non-linear optical properties of tetrathia-[7]-helicene

Femtosecond hyper-Rayleigh scattering (HRS) experiments as a function of amplitude modulation frequency have been performed on tetrathia-[7]-helicene. The apparent first hyperpolarizability βapp(ω) is a decreasing function of increasing frequency ω, due to the two-photon fluorescence (2PF) contribution. We observe, however, for the first time, also a frequency-dependent HRS depolarization ratio ρ(ω). An accurate value for ρ is important, since it offers an indication for the symmetry of the chromophore (5, resp. 1.5, for dipoles, resp. octopoles, but also independent of frequency, since identical for HRS and 2PF). For helicoidal symmetry ρ(ω) is increasing for higher frequencies, in agreement with theory.