Polarization holography reveals the nature of the grating in polymers containing azo-dye

To study the origin of reversible holographic recording in three polymers c ontaining the same ate-dye, we have measured the diffraction efficiency and analyzed the gratings characteristics for various writing beams polarizati ons. The amplitude of the holographic grating, as well as the ratio between index and absorption modulations, have been investigated by gratings shift ing. The total amount of diffracted power and the diffraction efficiency ve rsus the reading beam polarization has been measured by non-degenerated fou r waves mixing. These experiments have revealed that the molecular mechanis ms of holographic recording in the studied compounds are different. The pho toinduced orientation of the chromophores is predominant for C6-C11-DMNPAA; so, in C11-CG-DMNPAA (DMNPAA: 2,5-dimethyl-4(p-nitrophenylazo)anisole), th e refractive index variation comes from the presence of both trans and cis populations generated by photoisomerization. The behavior of the PVK:DMNPAA is included between these extreme cases since both phenomena act. (C) 2000 Elsevier Science B.V. All rights reserved.