Photoinduced Birefringence Research Articles

Cyclic Side‐Chain Phenylazo Naphthalene Polymers: Enhanced Fluorescence Emission and Surface Relief Grating Formation

Well-defined cyclic-polymers (cyclic-PAzoMMAs), bearing side-chain phenylazo naphthalene chromophore, were successfully synthesized by the combination of atom transfer radical polymerization (ATRP) and copper(I)-catalyzed azide/alkyne cycloaddition "click" reaction, as verified by GPC, (1) H NMR, FTIR, and MALDI-TOF mass spectrometry. The cyclic-PAzoMMA showed higher glass transition temperatures than the linear-PAzoMMA with the same molecular weight. Interestingly, the cyclic-PAzoMMA exhibited deeper modulation depth (M.D.) induced by SRG, larger value of the photoinduced birefringence, increased fluorescence emission, and longer fluorescence lifetime in comparison with its linear counterpart.

Photoinduced phenomena in azo-dyed gelatine films

In this work a photoinduced properties of azo-dye doped gelatine films were studied. Gelatine is the oldest polymeric matrix, which is used in holography and photography. Preparation of these films is simple and cheap, but ingredients aren't toxic. Films were synthesized using gelatine and azo-dye -Disperse Red13. Non- aqueous solvents - dimethyl sulfoxide and acetic acid, were used. Films were coated on glass plates using applicator with layer thickness 30, 60, 90 μm and then were heated in vacuum chamber. Azo-dyed gelatine films were illuminated by 532nm laser radiation. Photoinduced birefringence and photoinduced dichroism dependence on films ingredients ratio, laser light intensity and sample treatment temperature were investigated. Holographic recording on these films was performed in dependence on recording beam intensity and polarization. Volume polarization gratings and direct surface relief formation were studied by measuring diffraction efficiency with 660nm diode laser radiation. The obtained surface relief was investigated by AFM.

Holographic studies of photoinduced anisotropy in molecular glassy films containing diphenylamine azochromophores

Photoinduced anisotropy (PA) is experimentally studied in three molecular glassy films synthesized in our Faculty and containing diphenylamine based azochromophores {4-((4-nitrophenyl)diazenyl)-N-(4-((4-nitrophenyl)diazenyl)phenyl)-N-(2-trityloxy)ethyl) benzeneamine (shortly K-D-24), 4-((2-chloro-4-tritylphenyl)diazenyl)-N-(4-((2-chloro-4-tritylphenyl)diazenyl)phenyl)-N-(2-(trityloxy)ethyl)benzenamine (shortly K-D-25), 4,4'-(2-(trityloxy)ethylazanediyl)bis(4,1-phenylene)bis(diazene-2,1-diyl)dibenzonitrile (shortly K-D-32) } and, for comparison, in an a-As2S3 film. Holographic method enabling simultaneous measurements of s- and p-polarized diffracted light powers was applied in both transmission and reflection modes. Holographic grating recording with the period of 2 μm was made by two equally strong 532 nm p- polarized laser beams with total light intensity of 0.81 W/cm2. The readout was made by 633 nm circularly polarized beam, and the s- and p-polarized components of diffracted light were separated by polarization beam splitter prism. PA was characterized by diffracted power difference (DPD), Ps − Pp, and by anisotropy contrast A = (ns − np)/(ns + np), ns and np being the diffraction efficiencies for s- and p-polarized light. DPD exhibited markedly different kinetic behaviour in transmission and reflection modes. There was a negative minimum in transmission mode in all samples, and a growth with oscillations up to saturation in reflection mode. Small initial DPD maxima were specific to a-As2S3 film. The highest PA was found in K-D-24 film in transmission mode (A=-0.23) and in K-D-25 film in reflection mode (A=0.49). Holographic recording efficiency and PA do not correlate. In our opinion, PA in transmission mode of molecular glasses is due to the photoinduced volume processes including trans-cis photoisomerization, chromophore orientation and mechanical stress modulation. In the case of a-As2S3 film these photoinduced volume processes include photoinduced structural changes, D-centre orientation and, again, mechanical stress modulation. All these processes lead to a photoinduced birefringence. Surface relief grating recording and polarization-dependent reflection are responsible for diffractive anisotropy in reflection mode.

Large inverse relaxation of photoinduced birefringence in an azobenzene-containing ionic self-assembly complex

Photoinduced anisotropy and polarization holographic recording were investigated in an azobenzene-containing ionic self-assembly complex. The complex film was found to possess large photoinduced dichroism (D∼0.36) and photoinduced birefringence (Δn∼10−2). A noteworthy observation was that the photoinduced birefringence did not show any decay, but strongly increased after switching off the pump light at room temperature. The diffracted signals of the polarization holographic gratings, written by two orthogonally linear and circular polarized beams respectively, exhibited similar inverse relaxation behavior as the photoinduced birefringence after switching off the writing lights. A discussion about the mechanism accounting for the abnormal relaxation process of photoinduced birefringence is presented.

Increase of photoinduced birefringence in a new type of anisotropic nanocomposite: azopolymer doped with ZnO nanoparticles

We report a significant increase of about 50% of the photoinduced birefringence in nanocomposite films of azopolymers doped with ZnO nanoparticles compared with samples made from nondoped azopolymers. This increase is most pronounced at small concentrations of the nanoparticles of 0.5% and for the amorphous polymers used in our study. We observe also an improvement of the response time of more than 25% in some of the polymers, which allows for faster and more effective polarization optical recording.

Determination of the photoinduced birefringence and of the refractive index by Brewster method of epoxy resin doped with CdS

This work describes the synthesis of polymeric composites based on epoxy resin doped with different concentrations of cadmium sulfide (CdS). We studied the photoinduced birefringence and the refractive index by Brewster method as a function of the CdS concentration. UV–Vis spectroscopy was used to determine the absorption bands of the samples. An experimental arrangement to ascertain the photoinduced birefringence used a writing LASER (ion Ar+ of λ=457nm) and a reading LASER (He–Ne of λ=632.8nm). The results were quantified using the theoretical model purposed by Debye, where it was observed that the pure epoxy resin presented photoinduced birefringence, which increased with the addition of the CdS. Therefore, it was verified that the variation of the power of the writing LASER directly contributes to the amount of photoinduced birefringence. In addition, the behavior of the refractive index of the composite as a function of the addition of CdS was verified by the Brewster angle technique to provide a wavelength of 650nm using a radiation source with a monochromator. We observed a linear increase in the refractive index with the addition of CdS.

Polyurethane–epoxy based azopolymers: Influence of chemical structure over photoinduced birefringence

Abstract A comparative study of the influence of the competitive reactions that can take place in epoxy–isocyanate based azo systems over optical behavior was developed. Diverse disperse red 19 (DR 19) azo urethane oligomers (PUs) were synthesized and characterized using size exclusion chromatography (SEC), Fourier transform infrared (FTIR), and UV–visible spectroscopy. Based on these urethane oligomers, epoxy-PU networks having isocyanurate and oxazolidone rings in their structures were prepared in different isocyanate/epoxy stoichiometric ratios, r = eq. NCO/eq. epoxy (0.5, 1, and 2). Photoinduced anisotropy (Δ n ), remnant birefringence (RB) and dichroism were measured for the resulting polymers and evaluated as function of their T g s and chemical structures. Final materials exhibited very high values of Δ n and RB being promised materials for using in optical storage information devices.

Long-Term Stability at High Temperatures for Birefringence in PAZO/PAH Layer-by-Layer Films

Optical memories with long-term stability at high temperatures have long been pursued in azopolymers with photoinduced birefringence. In this study, we show that the residual birefringence in layer-by-layer (LbL) films made with poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) alternated with poly(allylamine hydrochloride) (PAH) can be tuned by varying the extent of electrostatic interactions with film fabrication at different pHs for PAH. The dynamics of both writing and relaxation processes could be explained with a two-stage mechanism involving the orientation of the chromophores per se and the chain movement. Upon calculating the activation energies for these processes, we demonstrate semiquantitatively that reduced electrostatic interactions in films prepared at higher pH, for which PAH is less charged, are responsible for the longer stability at high temperatures. This is attributed to orientation of PAZO chromophores via cooperative aggregation, where the presence of counterions hindered relaxation.

Photo-induced birefringence in a nematic liquid crystal mixture doped with light-switchable mesogenic azobenzene derivatives

We report on photoinduced effects observed in azo-mesogene doped nematic liquid crystal mixture using birefringence measurement method. In Optical Kerr Effect configuration the photoinduced birefringence observed in the studied planarly aligned liquid crystal cell is equivalent to very high n2≈13cm2/W nonlinear optical coefficient. Experimental results are explained on a basis of light-induced modification of the surface conditions due to trans-cis isomerizations that affect the bulk effective electric field acting on molecules. The measurements are supported by a simple model of the phenomenon and Jones matrix calculus. The response of the system is composed of two dynamically different contributions: the fast and the slow one.

Epoxy-based azo polymers with high chromophore density: Synthesis, characterization and photoinduced birefringence

Three epoxy-based azo polymers (PEP-AZ-Cl, PEP-AZ-CN and PEP-AZ-NT) with high chromophore density were synthesized by using post-polymerization azo-coupling reactions between epoxy-based precursor polymer (PEP-AN) and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile and 4-nitroaniline, respectively. The structures and properties of the azo polymers were characterized by using 1H-NMR, FT-IR, UV-Vis and thermal analyses. The photoinduced birefringence of the azo polymers was studied by irradiating spin-coated films of the polymers with laser beam at three different wavelengths (488, 532, and 589 nm). The results indicate that the photoinduced birefringence of the azo polymers is related with the electron-withdrawing group on azo chromophores and the excitation wavelength. The excitation wavelength that can cause the efficient responses is determined by the absorption band positions of the azo chromophores, which are mainly affected by the electron-withdrawing group on the chromophores. Therefore, the azo polymers containing chromophores with different electron-withdrawing groups show different responsive behavior to the irradiation light at different wavelengths. When irradiated with 488 nm light, PEP-AZ-Cl shows the shortest time to reach the saturated birefringence but with the lowest saturation birefringence level compared with the other two azo polymers. When irradiated with 532 nm light, PEP-AZ-CN shows the shortest time to reach the saturated birefringence. When irradiated with 532 and 589 nm light, PEP-AZ-NT shows the highest saturation birefringence level.

Surface-Relief Gratings and Stable Birefringence Inscribed Using Light of Broad Spectral Range in Supramolecular Polymer-Bisazobenzene Complexes

We report on phenol–pyridine hydrogen-bonded supramolecular polymer–azobenzene complexes made from a newly designed polar bisazobenzene chromophore. Because of the substitution with a polar nitro group, the chromophore possesses an extremely broad absorption band, spanning from near-UV up to 650 nm. Moreover, the inclusion of two methoxy groups to the central benzene ring prevents excessive chromophore–chromophore intermolecular interactions and provides advantageous size-related properties. Together, these features of the prepared photoresponsive polymer materials enable efficient inscription of (i) photoinduced birefringence with outstanding stability at various chromophore concentrations and (ii) surface-relief grating formation over a wide range of writing wavelengths from 405 to 633 nm. The photoresponsive behavior is compared to that of Disperse Yellow 7-based supramolecular complexes.

Photoinduced optical anisotropy in azobenzene containing block copolymer–homopolymer blends. Influence of microstructure and molecular weight

Microstructure in two diblock methacrylic azo polymers and in some of their blends with PMMA of different molecular weights as well as their photoinduced anisotropy have been investigated. The block copolymers have similar structure but different azo content and degree of polymerization. A synthetic strategy based on a controlled radical polymerization (ATRP) of polymeric blocks and their coupling by click chemistry has been applied to obtain an azo block copolymer of high molecular weight. Microphase segregation has been observed in the block copolymers and in most of the blends. In blends of the block copolymer with lower degree of polymerization (Block 1) azo microdomains change from lamellar to spherical morphology when the azo content decreases from 24 to 3wt.%. In the block copolymer with higher degree of polymerization (Block 2) and its blends, down to 3wt.% azo content, spherical azo microdomains have been found. A decrease of the order parameter (η) and the photoinduced birefringence normalized to the azo content (|Δn|norm) has been found in blends of Block 1 when the azo content decreases. However, |Δn|norm and η values similar to those in the azo homopolymer have been observed in Block 2 and its blends. These blends can be used to lower the azo content while keeping a photoinduced response similar to that in the azo homopolymer.

Photoinduced orientation in nunchaku-like azo molecular glass studied by birefringence characterization and FT-IR spectroscopy

Photoinduced orientation of amorphous azo materials based on nunchaku-like molecules was studied by birefringence characterization, polarized infrared spectroscopy, and 2D-IR correlation spectroscopy. The nunchaku-like molecules (AZBP-CA and AZBP-CN) contained both an azo chromophore and a rigid biphenylene unit linked with the flexible spacer. Upon irradiation with a linearly polarized Ar+ laser beam (488 nm), the birefringence growth and unique relaxation behavior were investigated for solid thin films of the materials. The results show that the high birefringence of AZBP-CA upon the light irradiation is related with the hydrogen-bonding in the material as indicated by IR spectroscopy. The nunchaku-like molecule films show the unique relaxation behavior when switching off the exciting laser, i.e. the photoinduced birefringence further increases instead of partial decay, which is typically observed for most azo polymer films. The amount of the post-irradiation birefringence growth is positively correlated with the illumination time during the writing process. Polarized infrared spectroscopic investigation indicates that the orientations of the azo chromophore and biphenylene unit are strongly correlated with each other. As revealed by the two-dimensional infrared correlation spectroscopy, the azo chromophore and biphenylene unit move cooperatively almost at the same time. The flexible spacer of the molecules prefers to align in the same direction as the azo chromophore after the light irradiation, which suggests that the photoinduced orientation of the azo chromophore transfers through the flexible spacer to the biphenylene unit.

Poly(etherimide)s and poly(esterimide)s containing azobenzene units: Characterization and study of photoinduced optical anisotropy

In this paper novel processable, photoresponsible aromatic polymers with imide rings and covalently boned azobenzene units are presented. Prepolymerization strategy based on polycondensation of two diamines with azobenzene group, that is, 2,4-diamino-4′-cyanoazobenzne and 2,4-diamino-4′-nitroazobenzene and two dianhydrides, was utilized for preparation of the new photochromic polyimides. The obtained polymers differ in the chemical structure of polymer backbone and the kind of substituents on azobenzene moieties. Polymers were characterized and evaluated by FT-IR, 1H NMR, X-ray, UV-vis, DSC, and TGA methods. The synthesized azopolymers exhibited glass transition temperatures in the range of 140–170 °C, thermal stability with initial decomposition temperatures ( T d) in the range of 266–290 °C, and excellent solubilities in common organic solvents, providing optical-quality films. The light-induced optical anisotropy was studied in obtained polymers by photoinduced birefringence measurements. The birefringence was generated by the femtosecond pulses at 400 nm via a linear absorption of the trans-azobenzene. A large light-induced birefringence (Δ n = 0.02) was achieved in poly(esterimide)s contrary to poly(etherimide)s. The birefringence exhibited a very stable behavior after switching off the writing light indicating a permanent storage capacity.

Different chromophore concentration dependence of photoinduced birefringence and second-order susceptibility in all-optical poling

We study photoinduced axial and polar ordering in Disperse Red 1 azobenzene–poly(4-vinylpyridine) polymer systems by monitoring both birefringence and second-harmonic generation during all-optical poling. The two responses are found to exhibit very distinct dependences on chromophore concentration: the photoinduced birefringence increasing up to 51 wt. % concentration and the second-order response reaching its peak already at 23 wt. %. The results show that the polar order required for second-order response is highly sensitive to chromophore-chromophore intermolecular interactions, whereas the birefringence is much more robust against such effects.

Optical damage resistance of Ce:Fe:LiNbO3 crystals with various Li/Nb ratios

Ce:Fe:LiNbO3 crystals were grown by the Czochralski technique with various ratios of Li/Nb=0.94, 1.05, 1.20 and 1.38 in the melt. Their UV–Vis absorption spectra were measured in order to investigate their defect structures and their optical damage resistance was characterized by the photoinduced birefringence change and transmission facual distortion method. The optical damage resistance of Ce:Fe:LiNbO3 crystals improves with the Li/Nb ratio increases. The dependence of the optical damage resistance on the defect structure of Ce:Fe:LiNbO3 crystals is discussed in detail.

A new mechanism of stable optical birefringence recording under polarized UV radiation inphotocrosslinking polymeric materials

The photoinduced birefringence in glassy layers of the new photocrosslinkingpolymers containing benzaldehyde groups in the side chain is experimentallyinvestigated. The kinetics of the birefringence recording is revealed to have anuntypical shape compared to the known photosensitive materials of the same type.Under polarized UV exposure, the negative and positive birefringence are formedconsecutively, the positive one being stable to the influence of liquids and heating (up to120 °C). A new qualitative mechanism explaining the features revealed is proposed and verified bynumerical simulation. According to the assumptions made, the positive birefringence isformed by anisotropic photocrosslinks which ensure its stability, which is of greatimportance for the practical application of the polymers under study, including liquidcrystal photoalignment.

Volume polarization holographic recording in thick phenanthrenequinone-doped poly(methyl methacrylate) photopolymer

Volume polarization holographic recording in phenanthrenequinone-doped poly (methyl methacrylate) photopolymer is obtained. Photoinduced birefringence in a 2 mm thick sample is measured by a phase-modulated ellipsometry. The birefringence induced in this material by linearly polarized beam at 514 nm reaches 1.2×10(-5). In addition, ability for recording volume polarization grating using two different polarization configurations is demonstrated and compared. The experimental results show that the diffraction efficiency of the hologram reaches to ∼40% by using two orthogonal circularly polarized beams.

Optical damage resistance of Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

AbstractHf:Fe:LiNbO3 crystals were grown in air by the Czochralski technique with various [Li]/[Nb] ratios ([Li]/[Nb]=0.94, 1.05, 1.20) in melt. The defect structure and location of doped ions were analyzed by the UV‐visible absorption spectra. The optical damage resistance of Hf:Fe:LiNbO3 crystals was investigated by the photoinduced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO3 crystals decreases with the increase of the [Li]/[Nb] ratio. The dependence of the optical damage resistance of Hf:Fe:LiNbO3 crystals on the defect structure is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Birefringence dynamics of poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt} cast films

Photoinduced birefringence creation/decay dynamics in poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt}cast films, has been characterized in the 5 to 100 mW writing laser power range at the wavelength of 514 nm. The maximum birefringence magnitude increased with laser beam power, being the largest value of 0.03, measured at 632.8 nm. Birefringence creation kinetics followed a biexponential behavior, where a slow process and a fast process could be clearly distinguished. The fast process accounted for more than 60% of birefringence. The time constants for both fast and slow processes decreased with increasing power of writing beam. Birefringence relaxation after removal of writing laser beam revealed to be independent of beam power and, was found to be ruled by two processes, described by a biexponential curve plus a constant term accounting for residual birefringence. A high residual birefringence signal of 85 ± 3% was attained, with the fast process contributing to 67% of decay from maximum value. The large values obtained for time constants for both birefringence creation and relaxation processes are believed to arise from stabilization of azo-group either by the formation of an ionic network, or by local short range interactions that are coming through as a result polyelectrolyte coiling due to its high degree of ionization.