LASER-INDUCED GRATINGS AND SPECTROSCOPY IN MONODOMAINS OF LIQUID-CRYSTALLINE POLYMERS

We present a detailed experimental characterization of an optical storage technique that uses monodomain films of liquid-crystalline polymers (LCP’s). This technique employs the trans-cis isomerization of cyanoazobenzene mesogenic groups of the LCP’s, which is photoinduced in the glassy state. Changes in the molecular geometry leading to nonmesogen moieties and to a reorientation of the optical axis were found to be responsible for the observed strong variations of the anisotropic optical properties. These variations were characterized by means of UV-visible spectroscopy with polarized light. The optically induced trans-cis isomerizations as well as the complete thermal relaxation back to the trans state were found to be similar to those for amorphous polymers containing azobenzene-type groups. However, permanent changes were observed in the irradiated LCP monodomains. An altered orientational distribution in the illuminated area is responsible for the observed long-time stability. The original uniform orientational order of the LCP monodomains could be fully restored by annealing the samples well above the glass-transition temperatures of the polymers. Laserinduced grating experiments were performed to characterize the formation of a periodic modulation of the anisotropic refractive index as a function of time and intensity. A kinetic model that describes the grating formation due to the isomerization reaction is introduced and experimentally confirmed. A very high resolving power (3000 lines/mm) combined with large diffraction efficiencies (50%o) could be achieved. Finally, the first hologram reversibly stored in a LCP is presented as practical example. Such a hologram has already lasted for more than two years at ambient. © 1990 Optical Society of America.