Light mediated emergence of surface patterns in azopolymers at low temperatures.

Polymer thin films doped with azobenzene molecules do have the ability to organize themselves in spontaneous surface relief gratings (SRG) under irradiation using a single polarized beam. To shed some light on this still unexplained phenomenon, we use a new method that permits us to access experimentally the very first steps of the pattern formation process. By decreasing the temperature, we slow down the formation and organization of patterns, due to the large increase in the viscosity and relaxation time of the azopolymer. As a result, decreasing the temperature allows us to access and study much shorter time scales, in the physical mechanisms underlying the pattern formation, than those previously reported. We find that the patterns organize themselves in sub-structures which size increases with the temperature, following the diffusion coefficient evolution of the material. This result suggests that the pattern formation and organization are mainly governed by diffusive processes, in agreement with some theories of SRG formation. Upon decreasing the temperature further, we observe the emergence of small voids located at the junction of the sub-structures.

Anisotropic media effect on the dipole moment of some coumarin dyes.

The ground state (µ(g)) and the excited state (µ(e)) dipole moments of two coumarin laser dyes, C500 and C503, were studied at room temperature in various solvents, viz., aprotic solvents, alcohols and liquid crystals at 298 K. We report dipole moment of laser dyes in different anisotropic (liquid crystals) and isotropic environments. The dipole moments values in different media help to investigate environment effects on the molecular dipole moment and provide a straightforward method for comparing their properties. Ground and excited state dipole moments of coumarin dyes were evaluated by means of solvatochromic shift method. It was observed that dipole moment values of excited states (µ(e)) were higher than corresponding ground state values (µ(g)), indicating a substantial redistribution of the π-electron densities in a more polar excited state for the dyes investigated.

Substituent and solvent effects on the photo-physical properties of some coumarin dyes.

Absorption and fluorescence spectra of three coumarin dyes with various substituents and alkyl groups have been recorded in solvents in the range of 200-730 nm. The photo-physical behavior of dissolved dye depends on the nature of its environment, i.e. the solvatochromic behaviors of coumarin dyes and solvent/solute hydrogen bonding interactions can be analyzed by means of linear solvation energy relationships concept proposed by Kamlet and Taft. The intensity, shape, and maximum wavelength of the absorption and fluorescence band of these dyes in solution depend strongly on the solvent-solute interactions and solvent nature. Hydrogen bonding interactions (specific solute-solvent interactions) between these dye-solvent complex and dipolarity/polarizability (non-specific solute-solvent interactions) control reorientation of solvent molecules around the dye.

Effect of polyacrylamide hydrophilic gel composition on photo-physical behavior of Oxazine 750.

Electronic absorption spectra of Oxazine 750 dye in aqueous solutions and in polyacrylamide hydrogel matrix with different structural composition were studied at room temperature. The transport and the solute-solute interactions of the ionic dye in aqueous solutions across the hydrophilic gels were investigated via exploring spectral properties of the dye-loaded hydrogel. The nature of the dye pair interactions in these media was discussed using the Kasha exciton theory. In addition, the monomer-dimer equilibrium of Oxazine 750 in hydrogels with different compositional percentage has been determined by means of UV-vis spectroscopy and least square fitting methods.

Effects of surfactants on the molecular aggregation of rhodamine dyes in aqueous solutions.

The aggregative properties of rhodamine 6G (R6G) and rhodamine B (RB) in aqueous solutions of cationic and anionic surfactants were studied using the absorption and fluorescence spectroscopic methods. The dye-surfactant interactions in aqueous and surfactant solutions were also studied. The spectroscopic studies were extended in order to determine the effect of molecular structure of the surfactants on the aggregative behavior of the rhodamine dyes. The aggregate structures and the nature of the interacting pairs in these dyes were discussed using the exciton theory.

Electronic absorption spectra of cresyl violet acetate in anisotropic and isotropic solvents.

The isotropic and anisotropic solvation characteristics of cresyl violet acetate (CVA) were investigated in isotropic liquid solutions and in polar nematic matrices as a function of the solvent type and concentration. The interaction of the ionic dye with the anisotropic surrounding and with that of the isotropic solvents was investigated and compared. The experimental result suggests that the nematic liquid crystalline solvents might create stronger solvation than the isotropic solvents. The spectral shifts were correlated by the solvent permittivity and Kamlet-Taft parameters. The polarized absorption spectra of cresyl violet acetate were measured between 400 and 800 nm and the dichroic ratio R and degree of anisotropy S of this dye in the liquid crystalline host determined.

On the photo-physical behavior and electro-optical effect of oxazine dyes in anisotropic host.

The optical characterization and polarized spectroscopy of oxazine 720, oxazine 750, and Nile blue were obtained in a nematic matrix. The interaction of these ionic dyes with the anisotropic surrounding and with that of the isotropic organic solvents was investigated and compared. The influence of the host nature on the molecular interaction of the guest molecules was also investigated. Dichroic ratios R and degree of anisotropy S of these ionic dyes in the liquid crystalline host were investigated by measuring the intensity of the absorption bands in the visible region of parallel aligned samples. The electro-optical effects of the guest-host systems were also investigated in an electro-optical system using polarized spectroscopic method based on homogeneous-homeotropic alignment.

Rotating and Fugitive Cavity Solitons in semiconductor microresonators.

We describe two different methods that exploit the intrinsic mobility properties of cavity solitons to realize periodic motion, suitable in principle to provide soliton-based, all-optical clocking or synchronization. The first method relies on the drift of solitons in phase gradients: when the holding beam corresponds to a doughnut mode (instead of a Gaussian as usually) cavity solitons undergo a rotational motion along the annulus of the doughnut. The second makes additional use of the recently discovered spontaneous motion of cavity solitons induced by the thermal dynamics, it demonstrates that it can be controlled by introducing phase or amplitude modulations in the holding beam. Finally, we show that in presence of a weak 2D phase modulation, the cavity soliton, under the thermally induced motion, performs a random walk from one maximum of the phase profile to another, always escaping from the temperature minimum generated by the soliton itself (Fugitive Soliton).