3RD-ORDER RESONANT OPTICAL NONLINEARITY FROM TRANS-CIS PHOTOISOMERIZATION OF AN AZO-DYE IN A RIGID MATRIX

We investigated the trans-cis photoisomerization of an azo dye in a ri gid matrix and the resulting third-order resonant optical nonlinearity by means of the simple theoretical prediction of a two-energy-level s ystem, thin-layer chromatography and H-NMR studies. A methylorange (MO ), a methylred (MR), congored (CR) or a Disperse Red 1 (DR1) doped pol yvinyl alcohol (PVA) or silica film was used as nonlinear optical mate rial. The existence of equi-absorbing points, or isosbestic points in the absorbance spectrum change and the remarkable stationary transmitt ance to be independent of the action beam intensity enabled us to conf irm the photoisomerization even in a rigid matrix. Then, we measured t he third-order resonant optical nonlinearity of dichroism through the polarization absorbance spectrum measurement and determined the charac teristic optical parameters of the photoisomerization in the film such as the quantum yields phi(T), phi(C), the thermal reaction constant K and the photoisomerization time constant by fitting the theoretical c urve of the two-energy-level system to the observed temporal transmitt ance change after the action beam exposure of the MO/PVA film. The qua ntum yields were <(phi(T))over bar> = 0.36 and <(phi(C))over bar> = 0. 38, respectively. The photoisomerization time constant of MO embedded in the PVA film was a few seconds. The thermal reaction constant K dep ended on the excitation beam intensity.