Measurements of the first hyperpolarizabilities of thiophene-based charge-transfer chromophores with hyper-Rayleigh scattering at 1064 and 1907 nm Original Research Article

The first hyperpolarizability β is determined for 10 new thiophene-based charge-transfer (CT) chromophores using the hyper-Rayleigh scattering (HRS) technique at the wavelengths of 1064 and 1907 nm. Exceptionally high hyperpolarizabilities for several chromophores are obtained, exceeding 1700×10−30 esu at the long wavelength of 1907 nm. The “intrinsic hyperpolarizability” β0 is calculated using the Oudar–Chemla equation from the β values obtained at these two excitation wavelengths. For some chromophores β0 calculated from 1064 nm HRS data differ from the 1907 nm results. Using the two-state model, we show that self-consistent intrinsic hyperpolarizability β0 and CT transition frequency ω0 can be obtained for data from these two-excitation wavelengths, provided that one does not identify ω0 with the maximum of the linear absorption spectrum. The discrepancy is attributed to the effect of two-photon resonance associated with the 1064 nm radiation employed in the HRS experiment. We show that the β0 discrepancy and difference in ω0 can be removed if both damping (or dephasing) and vibrational detuning mechanism are incorporated in the two-state model. The structure–function relationship for the various chromophores investigated is also elucidated.