Organic Compounds with Large and High-Contrast pH-Switchable Nonlinear Optical Response

Measurements of the nonlinear optical (NLO) response (molecular first hyperpolarizability, beta) of two pH-switchable dyes were performed by means of hyper-Rayleigh scattering (HRS) over a wide range of laser wavelengths and pH values. The extensive wavelength-dependent NLO data combined with linear optical spectroscopy allow the results to be related to the electronic structures of the push-pull systems and the NLO switching contrast to be optimized. The results are modeled theoretically using a vibronic beta dispersion model including both homogeneous and inhomogeneous broadening, yielding a full wavelength dependence of beta, as well as of its switching contrast. The hyperpolarizabilities measured for the basic "on"-states are the highest ever reported for such compounds, reaching over 200 X 10(-30) esu in the static limit and over 2700 X 10(-30) esu at resonance. By combining a high intrinsic contrast in the static hyperpolarizability beta(0) with a large shift in resonance, an extremely large range in contrasts of the HRS signal is reached of more than 2 orders of magnitude at wavelengths accessible with the widely used Ti:sapphire laser and moreover within a biologically compatible pH range.