Photoinduced Anisotropy Research Articles

The question of temperature-dependent intrinsic dipoles in chalcogenide glasses

The claim that the kinetics of photoinduced anisotropy in chalcogenide glasses prove the presence of a temperature-dependent concentration of electric dipoles is re-examined. It is concluded that neither a temperature dependence of electric dipoles nor even the presence of such dipoles is necessary to explain the experimental observations.

Linearly polarized light induced changes of domain structure in cobalt doped YIG films

Experimental and theoretical results of the investigation of photoinduced changes of magnetic domain structures (DS) inside the irradiated region of an epitaxial YIG : Co film are given. Domain reconstruction was studied under argon laser light pulses. Velocity of photoinduced magnetic anisotropy (PMA) growth was much faster than PMA decay velocity.

Diffraction-efficiency oscillations in amorphous As_2S_3 films

An experimental study of the holographic gratings recorded in nonannealed, thermally with time relaxed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. The dependences of the maximal first-order diffraction efficiency on the holographic grating period was studied in a wide range of periods, from 0.40 to 70.0 µm. A peculiar oscillatory diffraction-efficiency temporal behavior occurring under certain conditions is reported. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The diffraction-efficiency oscillations are explained by the model of annihilating defects and by multiwave mixing in a thin dynamic hologram.

Synthesis and investigation of photoinduced anisotropy of a series of liquid crystalline copolymers with azo groups

A series of liquid-crystalline copolymers poly{6-[4-(4-hexyloxy benzoyloxy)phenoxy]hexyl methacrylate}-co-{6-[4-( S-2-methyl-1-butyloxycarbonylphenylazo)phenoxy]hexyl methacrylate} with an azobenzene moiety as photoreactive mesogenic unit were prepared. Their liquid crystalline properties and photoinduced anisotropy were investigated. The results showed that these polymers exhibited layered smectic phases and showed good photoinduced anisotropy both in the glass states and in the mesophase states.

Photoinduced anisotropy and polarization holography in azobenzene side-chain polymer

Photoinduced anisotropy and polarization holographic recording have been studied in an amorphous tripolymer containing azo-side groups. It is shown that diverse mechanisms should be invoked to explain the experimental results. The polymer possesses a large value of photoinduced birefringence (Δ n=7.8×10 −3). A reversible polarization hologram with high efficiency (19.3%) has been recorded with a CW 532 nm laser at relatively low light intensity in the thin polymer film.

Dynamics of photoinduced orientational order of azo‐dyes in polymer films

AbstractDynamical and spectral behavior of photoassisted electrical poling and photoinduced anisotropy are studied in films of DR1‐doped PMMA, DR1‐PMMA copolymer and new DR1‐Polyimide copolymer. The orientational mobility of DR1 is characterized both in ordering processes (angularly selective trans‐cis photoisomerization) and in disordering relaxation processes. Mobility is increased by the photoisomerization and decreases slowly with time, after the end of optical pumping. The transient dichroism recorded simultaneously on six probe wavelengths shows a spectral inhomogeneity of the absorption band of DR1 and of the photoinduced anisotropy (particularly in polyimide films).

In-Situ Observation of Thermal Amplification of Photogenerated Optical Anisotropy of Films of Azobenzene Polymers

ABSTRACTThin films of polymers containing p-cyanoazobenzene side chains were studied to reveal its applicability for optical storage media, surface grating optics and aligning films for liquid crystal (LC) display devices. We followed the generation of optical anisotropy of LPLirradiated films of three polymethacrylates with azobenzene side chains exhibiting amorphous, crystalline and liquid-crystalline natures during heat treatment by mean of an ellipsometry and polarized UV-visible spectroscopy. Whereas photogenerated optical anisotropy of amorphous polymer films disappeared above glass transition temperature, the photo-induced anisotropy of films of liquid crystalline and crystalline polymer films was significantly amplified by heat treatment. In case of crystalline polymers amplified optical anisotropy was maintained even at 300°C whereas partial decomposition was observed.

Mechanism of nonlinear photoinduced anisotropy in bacteriorhodopsin and its derivatives.

Polymer films made with photosensitive chromophore protein bacteriorhodopsin (BR) from the extreme halophile Halobacterium salinarium as well as films made with BR derivatives exhibit a nonlinear photoinduced anisotropy. Two different methods can be used to induce anisotropy in polymer BR films. The first method is based on the anisotropic properties of the initial form of the photocycle, BR570 (B-type anisotropy). Another method is based on the anisotropic properties of the longest-lived photocycle intermediate M412 (M-type anisotropy). CW gas lasers were employed to induce a reversible anisotropy in polymer BR films. Nonlinear photoinduced anisotropy is discussed in the context of a model for the anisotropic photoselection of BR molecules under linearly polarized light. A comparison of the experimental dependencies of nonlinear photoinduced anisotropy on laser intensity with similar calculated dependencies enables one to determine the molecular dichroism of BR and its derivatives not only for the initial form of the photocycle, B but also for the longest-lived intermediate M. Here we present the data showing the correlation between the laser induced nonlinear anisotropic properties and chromophore/protein interactions in BR. The effect of polymer binder on the nonlinear photoanisotropic properties of polymer BR films is also described.

Photoinduced Anisotropy in a Polymer Doped with Azo Dyes in the Photostationary State Studied by Polarized FT-IR Spectroscopy

Photoinduced optical anisotropy in poly(methyl methacrylate) (PMMA) films doped with Disperse Orange 3 (DO3, NO2–C6H4–N=N–C6H4–NH2) was investigated by polarized FT-IR spectroscopy. Symmetric (NOs2) and antisymmetric (NOas2) stretching modes of the NO2 group and the C–N stretching mode of C–NH2(C–N) in the infrared absorption spectra of DO3 were found to be useful in order to evaluate the degree of orientation of DO3 molecules in the polymer matrix. Infrared dichroism was observed in the NOs2, NOas2, and C–N bands under the stationary irradiation by visible light. The orientation factors, K(n)Zf (f = x, y, z, n = trans, cis), determined from the observed dichroism of each isomer in the photostationary state indicate that the p-NO2–C6H4 group is less mobile than the p-NH2–C6H4 group in PMMA during trans-cis-trans isomerization. This dynamic behavior of the DO3 molecules can be interpreted in terms of restriction of rotation within the free volume of PMMA. This study shows that polarized FT-IR spectroscopy is one of the most powerful methods available for the analysis of the physical mechanisms of photoinduced anisotropy.

Article

This paper concerns itself with photoinduced anisotropy in two azobenzene-functionalized polymers. A simple physical model of the process that considers only three possible states of azo chromophores is proposed. This model predicts an equilibrium level of anisotropy that depends on the pump irradiance and temperature. Each of these influences is examined in turn, with experimental illustrations from the two polymers poly[4-[2-(methacryloxy)-ethyl-ethyl]azobenzene] (pMEA) and poly[4prime-[[2-(methacryloxy)ethyl]-ethyl]amino-4-nitroazobenzene] (pDR1M).Key words: azobenzene polymers, photoinduced orientation, irradiance, temperature.

Quantum theory and experimental studies of absorption spectra and photoisomerization of azobenzene polymers

The microscopic properties of azobenzene chromophores are important for a correct description of optical storage systems based on photoinduced anisotropy in azobenzene polymers. A quantum model of these properties is presented and verified by comparison to experimental absorption spectra for trans and cis isomers of cyano methoxy azobenzene. In addition, the trans→cis quantum efficiency is measured, and hence the combined experimental and theoretical work allows one to determine the essential molecular properties, including magnitude and anisotropy of the absorption cross section and various components of the polarizability tensor for both trans and cis isomers. It is shown that the trans isomer is almost perfectly anisotropic, whereas the cis isomer is approximately isotropic in the plane containing the central C—N=N—C azobridge. The implications for models of the storage mechanism are discussed.

Investigation of the Photoinduced Optical Anisotropy of Azo Dye Mesophases

The photoinduced optical anisotropy (POA) of novel aqueous azo dye systems which form chromonic mesophases upon addition of methanol or sodium chloride was investigated. The phases were characterized between crossed polarizers. For further characterization rheological and holographic transient relaxation experiments were carried out. Distinctive differences in the hologram formation and decay were found for phases which exhibit the same texture in polarization microscopy. The generated holographic gratings could be assigned to either photoinduced phase transitions or an alignment of optical axes. In the present study we concentrate on a comparison of phases with the same macroscopic birefringence; results are discussed in comparison with models in the literature.

Photoinduced anisotropy and polarization memory of the photoluminescence in germanium sulfide glasses

Optical anisotropies, caused by the illumination of chalcogenide glasses with polarized light, have been observed experimentally through measurements of, for instance, dichroism and birefringence of an illuminated sample. Polarization memory of the photoluminescence has been considered in the past as a further way to obtain information about the photoinduced anisotropies. It is shown that the latter is not correct and that, consequently, photoluminescence polarization memory cannot be used to help discern between various proposals for the structural rearrangements that underlie the photoinduced changes in the optical properties.

Photoinduced dichroism and optical anisotropy in a liquid-crystalline azobenzene side chain polymer caused by anisotropic angular distribution of trans and cis isomers

Photochromism has been studied for two comb-like liquid-crystalline copolymers (I) and (II) containing azobenzene chromophores in their side chains. In a smectic glass phase of both copolymers, upon short-time irradiation by UV light, long-living cis isomers are observed. Both copolymers manifest the photoinduced anisotropy, the physical mechanisms of which seem to be quite different. In spin-coated films of polymer (II), the origin of the anisotropy is a strong stable dichroism, which is due to an enrichment and depletion of the chosen angular direction, correspondingly, with trans and cis isomers of the azobenzene chromophores. Polymer (I) manifests no dichroism at all, and its induced optical anisotropy may be accounted for by a rather slow chromophore reorientation. In copolymer (II) a considerable reorientation of the mesogenic groups also occurs as a secondary phenomenon at the stage of the cis isomer formation only. This observation shed more light on the general process of the light-induced molecular reorientation in polymers, liquid crystals, and Langmuir–Blodgett films, which is of great importance for holographic information recording.

Two-color holographic-grating formation in amorphous As_2S_3 films

A detailed experimental study of the holographic gratings recorded in nonannealed amorphous As2S3 films by 514.5-nm light in the presence of 632.8-nm readout light is carried out. A strong influence of a continuous 632.8-nm readout is found. The dependences of the maximal first-order diffraction efficiency and the corresponding specific recording energy on the holographic grating period were studied in a wide range of periods from 0.40 to 70.0 μm for 2-yr-old films. The obtained results are discussed in terms of photoinduced structural changes, relaxational structural changes, photoinduced anisotropy, and photoinduced recharging of the localized states in the bandgap. The photoinduced sulphur related D-- and D+-center orientation mechanism is proposed to explain the stimulating action of 632.8-nm light.

A model of photoinduced anisotropy in glassy semiconductors

A model of photoinduced optical anisotropy in amorphous semiconductors is studied theoretically. The change in the optical characteristics of a sample is associated with the photogeneration of geminate electron-hole pairs. If the pairs are generated by linearly polarized light, then the dipole moments of the geminate pairs lie mainly in the polarization plane, making the sample optically anisotropic. The model associates the optical anisotropy of the sample with the mean-square projection 〈Pz2〉 of the dipole moment per unit volume on the polarization axis of the radiation. The evolution of 〈Pz2〉 is determined by the kinetics of the drift and recombination of carriers in geminate pairs. The kinetics of the photoinduced anisotropy under continuous irradiation of the sample and its relaxation as a result of irradiation with pulsed polarized light is calculated.

Laser-induced anisotropic absorbtion, reflection, and scattering of light in chalcogenide glassy semiconductors

Anisotropy induced in thin-film and bulk samples of chalcogenide glassy semiconductors by the linearly polarized light of different spectral ranges is studied. Three different ranges of exciting photon energy can be distinguished. 1) Above-band-gap light excitation is studied in film samples, two distinct processes are identified in this range: creation of photoinduced defects and their photostimulated orientation and reorientation; a “defect-based” model of photoinduced anisotropy is further developed. 2) Subband-gap light excitation is studied in bulk samples; creation of anisotropically scattered centers is assumed to be the basis of all photoinduced vector phenomena in that spectral range. 3) Superband-gap light excitation is studied in film and bulk samples because the application of differential reflectance spectroscopy; it was shown that not only defects but also main covalent bonds of the glass can be oriented and reoriented by the linearly polarized light that generates the photoinduced dichroism in this spectral interval.

High-temperature photomagnetism in Co-doped yttrium iron garnet films

Magnetization processes in $(\mathrm{YCa}{)}_{3}(\mathrm{FeCoGe}{)}_{5}{\mathrm{O}}_{12}$ epitaxial films induced by linearly polarized argon laser light $(\ensuremath{\lambda}=0.488\phantom{\rule{0ex}{0ex}}\ensuremath{\mu}\mathrm{m}$) were observed at room temperature. These processes happened through a displacement of boundary between magnetic domain phases with different in-plane magnetization components. The direction and amplitude of the displacement depended on the polarization direction and light intensity. The results are explained within the scope of a phenomenological theory of photoinduced anisotropy. The microscopic origin of the photomagnetic effect in investigated materials is qualitatively discussed.

Photoinduced anisotropy in Pr-doped sulfide glasses with varying composition and Pr content

Metastable photoinduced anisotropy (PA) generated by linearly polarized sub-bandgap light is reported for undoped and Pr-doped Ge–S–I, Ge–Ga–S and As–Ga–S glasses with varying composition and Pr content. Compositional trends of PA are related to the light-stimulated reorientation of intrinsic anisotropic centers and their environments, which vary with composition. These centers can be modeled with pairs of over- and under-coordinated sulfur atoms: the valence alternation pairs (VAPs). The addition of Pr up to a level of about 1000 ppm affects PA, suggesting that this concentration is comparable with the concentration of VAPs. Microscopic models are proposed for the centers responsible for PA in these glasses.

Photoinduced reflectance anisotropy in chalcogenide glassy semiconductors

Photoinduced anisotropy (PA) in films and bulk samples of chalcogenide glasses As 2S 3, As 50Se 50 and Ge 20As 20S 60 has been studied in a wide spectral range (1.5 to 5.5 eV) using reflectance difference spectroscopy (RDS). PA was observed in the energy range much exceeding the energy of inducing photons and also in the range 4.5–5.5 eV energies greater than band gap energy for our samples. The sign of reflectance anisotropy induced by the near-band-gap light was shown to be opposite at smaller and larger energies. The conclusion is made that not only defects and scattering centers but also interatomic covalent bonds can be oriented and reoriented with linearly polarized light. A model of redistribution between bonding and lone-pair electrons is proposed.