Raffaella Raschellà

Polarization holographic gratings in hybrid solgel films doped with Disperse Red 1.

Polarization holographic gratings in sp configuration are written at 488 nm in photorefractive organic-inorganic films based on SiO2. The films, prepared by a solgel technique, contain Disperse Red 1, carbazole units, and 2,4,7-trinitro-9-fluorenone. The gratings are characterized by their diffraction efficiency for a 632.8-nm probe. The polarization gratings act as a half-wave plate, and the diffraction efficiency is independent of the polarization direction of the probe.

Modeling and experimental study of photoinduced anisotropy in hybrid solgel films

The mechanism of photoinduced anisotropy through photoisomerization has been studied by measuring photoinduced dichroism and birefringence in solgel silica-based glasses containing an azo dye (Disperse Red 1) and a plasticizer (carbazole). The equations commonly adopted to describe the molecular orientation (Sekkats theory) have been solved numerically and analytically within two different approximations. The results have been applied to photoinduced birefringence measured with expanded beams, and the dominant mechanisms driving the azo-dye molecular orientation in the investigated materials during both illumination and relaxation have been inferred. The experiments of photoinduced dichroism, performed with nonexpanded beams, suggest that the distribution of times describing the kinetics of the anisotropy formation, usually attributed to inhomogeneities in the chromophore environment, may be ascribed instead to the nonuniform profile of the beams.

Hybrid sol-gel films for holographic applications

Films based on SiO2 and containing Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF) have been prepared by a sol-gel technique. By photoinduced birefringence and photorefractive effect, holographic gratings have been produced using 488.0 mn and 632.8 nm light, respectively. Pure polarization gratings show diffraction efficiencies higher than those obtained by light intensity modulation. Various processes concerning the DR1 molecular orientation such as angular hole burning (AHB), angular redistribution (AR), Cis molecules decay and rotational diffusion are discussed to explain the temporal behaviour of the diffraction efficiencies. A poling field was not necessary to obtain photorefractive gratings, which show Asymmetric Energy Exchange (AEE) in two-beam coupling (2BC) experiments.

Polarization gratings in sol-gel thin films investigated through the moving grating technique

Intensity and pure polarization holographic gratings in silica-based sol-gel thin films, containing carbazole group and Disperse Red 1, are obtained by photoisomerization at 488 nm and are investigated by means of the moving grating technique (MGT). General equations relating the modulated MGT signal to amplitudes and phase shifts of the index and absorption gratings are derived for thin films in the case of different polarization configurations. It is shown that the technique, commonly used to characterize photorefractive gratings, is a powerful tool to investigate the orientation mechanisms responsible for the holographic polarization gratings formation.

Holographic gratings in hybrid sol-gel films

Organic-inorganic films based on SiO2, containing Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF), have been prepared by a sol-gel technique. Diffraction gratings have been produced through different effects: photoinduced birefringence and photorefractivity, using 488.0 nm and 632.8 nm light, respectively. Pure polarization holographic birefringence gratings have been investigated and diffraction efficiencies higher than those obtained by light intensity modulation have been measured. The study of the temporal behavior of the diffraction efficiencies makes possible the identification of different processes involved in the DR1 molecular orientation: angular hole burning (AHB), angular redistribution (AR), Cis molecules gain has been determined by two-beam coupling (2BC) measurements. The effect of the polarization of the writing beams on the grating and of a circularly polarized photoisomerizing radiation during grating erasure has been interpreted in terms of an orientation contribution to the grating formation.