Laser-induced gratings and spectroscopy in monodomains of liquid-crystalline polymers

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Abstract
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. Laser-induced 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%) 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.