Z-scan measurements of the nonlinear refraction in retinal derivatives

The nonlinear refractive index, n2, of 13-cis retinal aldehyde, 13-cis retinoic acid and all-trans retinoic acid were determined. For 13-cis retinal aldehyde solutions which were left to age for a few days, an increase of over one order of magnitude for n2 was observed. With basis on the comparison of infrared spectra of freshly prepared and aged solutions, we suggest that the enhancement of the nonlinear response is associated to a spontaneous cis to trans isomerization of the retinal molecule. The second order hyperpolarizabities of all the retinal derivatives were also inferred from the measurements.