Time-resolved electric-field-induced second harmonic: simultaneous measurement of first and second molecular hyperpolarizabilities.

The standard electric-field-induced second-harmonic (EFISH) technique for measurement of the first hyperpolarizability (beta) of nonlinear optical molecules is limited by the fact that the second hyperpolarizability (gamma) also contributes to the second-harmonic signal from which beta is deduced. We present a modified time-resolved EFISH in which the first and the second hyperpolarizabilities can be determined separately and accurately in the same experiment. We studied para-nitro aniline dissolved in a highly viscous solvent, glycerol, under conditions whereby the electric field was applied faster than the characteristic time for molecular rotation. This technique enabled the gamma contribution to the signal to be resolved separately from the beta contribution. The results confirm that for this molecule gamma contributes only ~10% of the total EFISH hyperpolarizability.