Azopolymer Films Research Articles

Application of Surface Relief Hologram Using Azobenzene Containing Polymer Film

Surface relief holograms are fabricated by irradiation of laser interference patterns on an azobenzene polymer film. The hologram needs no post-treatment, and can be erased by heating the films to above its glass transition temperature. Rewritable Fresnel holograms are fabricated on azo-polymer film. Replication of the surface relief hologram is also demonstrated. An application for business cards is considered.

Effect of Solvent, Hydrogen Bonding, and thickness of Azopolymer Films on Surface Relief Grating

ABSTRACTEpoxy-based copolymers were synthesized with N,N-diglycidyl aniline (DGA) and aniline (An), called poly(DGA-co-An), where azo coupling reactions were performed using amino benzonitrile (ABN) and nitro aniline (NA). Two azopolymers were dissolved with both tetrahydrofuran (THF)/dioxane complex solvent and THF to compare the diffraction efficiencies according to solvent. The thin films spin-cast with THF/dioxane showed the better diffractive efficiency than with THF due to the high boil point of the residual dioxane. The azopolymers of two azo bonds were spin-coated at 800 and 1300 rpm where the thicker film showed the better diffractive efficiency. The epoxy-based copolymers synthesized with diglycidyl ether of bisphenol A (DGEBA) and aniline (An) or 3-hydroxyl aniline (HAn) were coupled with hydroxyl nitro amino benzene (HNAB). Hydroxyl groups in chromophores helped to form hydrogen bonding with the nitrogen atoms in the azo bonds and prevented photoisomerization, showing no surface relief gratings under a normal laser intensity of 100 mW/cm2. Polyurethane-based azopolymers were synthesized with disperse orange 17 (DO17) and toluene-2,4-diisocyanate (TDI), which were no hydroxide groups in the main chains, and showed the better diffractive efficiency than the epoxy-based azopolymers with nitro substituents.

Photoinduced Phenomena in Layer-by-Layer Films of Poly(Allylamine Hydrochloride) and Brilliant Yellow Azodye

The nanoscale interactions between adjacent layers of layer-by-layer (LBL) films from poly(allylamine hydrochloride) (PAH) and azodye Brilliant Yellow (BY) have been investigated, with the films employed for optical storage and the formation of surface-relief gratings. Using Fourier transform infrared spectroscopy, we identified interactions involving SO3- groups from BY and NH+ groups from PAH. These electrostatic interactions were responsible for the slow kinetics of writing in the optical storage experiments, due to a tendency to hinder photoisomerization and the subsequent reorientation of the azochromophores. The photoinduced birefringence did not saturate after one hour of exposure to the writing laser, whereas in azopolymer films, saturation is normally reached within a few minutes. On the other hand, the presence of such interactions prevented thermal relaxation of the chromophores after the writing laser was switched off, leading to a very stable written pattern. Moreover, the nanoscale interactions promoted mass transport for photoinscription of surface-relief gratings on PAH/BY LBL films, with the azochromophores being able to drag the inert PAH chains when undergoing the trans-cis-trans photoisomerization cycles. A low level of chromophore degradation was involved in the SRG photoinscription, which was confirmed with micro-Raman and fluorescence spectroscopies.

Nanometer Photothermal Heating and Cooling Dynamics of Azo Polymer Film Elucidated by Analyzing Nanosecond Laser-Induced Expansion/Contraction Behavior

The nanosecond transient expansion and successive contraction dynamics of an azobenzene-substituted urethane-urea copolymer film were studied by nanosecond time-resolved interferometry. In addition, the photoisomerization dynamics of azo dyes during morphological change was investigated by transient absorption measurement. A transiently generated cis isomer absorbed a few photons during laser irradiation and its energy was effectively converted to heat. When the pulse width of the irradiated laser was 8 or 14 ns full-width at half maximum (fwhm), the expansion of the polymer film started with 3–6 ns delay from the temperature elevation, while the film contracted exponentially accompanying a different absorbance change of the transient species. The expansion dynamics was well explained not in terms of volume change due to isomerization dynamics but in terms of thermal expansion involving glass–rubber transition. The film-thickness-dependent contraction dynamics could be mostly interpreted in view of the heat flow toward the quartz substrate, heat dissipation via the film itself, and phase transition of the polymer film from the rubbery to glassy states. The laser-induced transient morphological change of the urethane-urea copolymer film observed below the threshold could be well explained in terms of nanometer heating and cooling dynamics.

Third-harmonic and sum-frequency generation in a quadratically nonlinear polymer by time-ordered ultrashort laser pulses

Third-harmonic and sum-frequency generation in quadratically nonlinear azopolymer films is experimentally studied using femtosecond chromium forsterite laser pulses. A noncollinear geometry of sum-frequency and third-harmonic generation developed and implemented in this work allows the influence of the time ordering of ultrashort laser pump pulses on nonlinear-optical phenomena to be experimentally observed. Femtosecond laser pulses induce transitions of azopolymer molecules to an electronically excited state and produce vibrational wave packets, leading to an asymmetry in the dependence of the efficiency of second-and third-order nonlinear-optical processes on the delay time between the pump pulses.

Polarization storage by nonlinear orientational hole burning in azo dye-containing polymer films

We present experimental evidence of polarization storage by nonlinear orientational hole burning in films of poly(methylmethacrylate) containing disperse red one (DR1). The data are written by polarized nonlinear excitation of DR1 by a tightly focused Ti: Sapphire laser and read by polarized confocal laser scanning microscopy in the reflection mode. The bits that are imaged with a light polarization which is parallel to the excitation laser polarization reflect light ∼2.5 times more efficiently than those which are imaged with a light polarization which is perpendicular to it. This finding is consistent with orientational hole burning of chromophores by two-photon excitation.

Design of Dye Concentrations in Azobenzene-Containing Polymer films for Volume Holographic Storage

Diffraction efficiencies of holograms in thick azo polymer films were improved by designing optical densities of the films. A nonphotosensitive polymer was used as a binder for controlling the dye concentrations. We fabricated 250-µm-thick films by changing the concentrations. While the optically induced birefringence decreased with decreasing concentration, reduction of absorption loss of the reading beam increased the diffraction efficiency. Thus, owing to the competition between the decrease in birefringence and the increase in efficiency, there exist appropriate concentrations for improving both sensitivity and magnitude of the diffraction efficiency. Consequently, the diffraction efficiency reached 25% without degradation of the sensitivity. We also demonstrated holographic multiplexing using the film. A set of 20 data pages (66 kbits per page) was successfully multiplexed and retrieved with a low bit-error rate of 1.3×10-3. The use of an error correction coding would allow error-free recovery of the data pages.

Bifocal-polarization holographic lens.

We present a holographic lens with novel features recorded in an azopolymer film. Two holographic modulations, bulk birefringence and surface relief, are induced in the medium at the same time. The resultant holographic element has two focal planes, and the polarization of light in the focal points depends on the polarization of the incident light. Applications of this device for writing-reading information in two planes simultaneously or separately are described.

Temperature dependence of photo-induced birefringence in azo-doped polymers containing different substitutions

The photo-induced birefringence of three new azo polymer films containing different substitutions was studied. Under a pumping light with the same polarization and intensity, the samples showed different profiles of birefringence as a function of temperature. It was found that the smaller azo molecules possess higher photo-induced birefringence but less stability. These phenomena have been explained in terms of the molecules' thermal motion and structure.

LC Alignment Controlled by Photoordering and Photorefraction in a Command Substrate

As command substrates in liquid crystal (LC) panels, azopolymer films have been widely studied over past decade. These studies were mainly caused by photoordering and photorefraction effects allowing to control LC alignment. Earlier, the mentioned effects were used separately. In the present study, a layer of azopolymer acts both as photoaligning and photorefractive substrate. The photoordering effect is used to align LCs homogeneously, whereas effect of the specific photorefraction is employed to modify boundary conditions. The latter ones were spatially modulated by the sample exposure to interference patterns formed by two coherent laser beams. Multiple orders of diffraction are observed in samples tilted with respect to the beam incidence bisector when the samples are biased by a dc external electric field with the intensity close to the Frederiks threshold. Possible grating recording mechanisms are discussed.

Liquid crystal alignment by Holographic surface relief grating inscribed on azo-polymer film

We found that the surface relief grating(SRG)made the nematic liquid crystal(LC)align along the grating direction. The SRGs were photo-fabricated on a liquid crystalline azobenzene side-chain polymer film by the conventional holographic method. The twisted nematic liquid crystal(TNLC)cell was prepared using two SRG substrates or two rubbed polyimide substrates as alignment layers, and the LC alignment was investigated by measuring the transmittance of a 90°-TNLC cell located between crossed polarizers. The LC aligning process was studied in-situ using the hybrid LC cell with the rubbed polyimide substrate and the SRG substrate as the reference and the test alignment layers, respectively. The in-situ transmission data show that the topological effect by the SRG formation and the intermolecular interaction effect by the azo chromophore alignment compete with each other.

Temperature modeling of laser-irradiated azo-polymer thin films.

Azobenzene polymer thin films exhibit reversible surface mass transport when irradiated with a light intensity and/or polarization gradient, although the exact mechanism remains unknown. In order to address the role of thermal effects in the surface relief grating formation process peculiar to azo polymers, a cellular automaton simulation was developed to model heat flow in thin films undergoing laser irradiation. Typical irradiation intensities of 50 mW/cm2 resulted in film temperature rises on the order of 5 K, confirmed experimentally. The temperature gradient between the light maxima and minima was found, however, to stabilize at only 10(-4) K within 2 micros. These results indicate that thermal effects play a negligible role during inscription, for films of any thickness. Experiments monitoring surface relief grating formation on substrates of different thermal conductivity confirm that inscription is insensitive to film temperature. Further simulations suggest that high-intensity pulsed irradiation leads to destructive temperatures and sample ablation, not to reversible optical mass transport.

Photofabrication of Superhelix‐Like Patterns on Azobenzene Polymer Films

Unusual superhelix‐like patterns on the surface of azopolymer films (see Figure) have been directly photofabricated by exposure to a laser interference pattern. Each superhelix‐like pattern is aligned parallel to the others, with a diameter of micrometer scale that could be controlled by the laser interference periodicity.

Surface-relief gratings and photoinduced birefringence in layer-by-layer films of chitosan and an azopolymer.

Layer-by-layer (LBL) films of chitosan alternated with an azopolymer, PS119, have been used for optical storage and fabrication of surface-relief gratings. The optical properties stem from the trans-cis-trans isomerization cycles undergone by the azochromophore, with a kinetics for writing the birefringence pattern that is much slower than in the spin-coated or cast films of azopolymers. The long writing times, of the order of 100 s, are due to the electrostatic interactions between adjacent chitosan and PS119 layers. Such interactions are also responsible for other features in the LBL films, namely the increase in the amount of adsorbed material when the pH of the preparation solution is decreased and the large residual birefringence after the writing laser is switched off. Gratings could be inscribed with s-polarized but not with p-polarized light, indicating a mass transport process associated with photodegradation.

Photodynamic Properties of Azobenzene Molecular Films with Triphenylamines

A new type of photochromic molecule containing three azobenzene groups, tris[(4-(phenylazo)diphenylamine) phenylamine] (TPPA), was synthesized and its photoinduced behavior was examined. Good optical quality TPPA films could be prepared by spin-coating. The photoinduced behavior of the TPPA films, such as photoinduced birefringence and surface relief grating (SRG) formation, was investigated. Compared to an azobenzene-functionalized polymeric PDO3 film, the TPPA molecular film showed a rapid response for photoinduced birefringence and SRG formation, but the saturated value for the birefringence was much smaller than that of the PDO3 azopolymer film. The dynamic behavior of SRG photofabrication was compared for two TPPA films with different morphologies: an amorphous film prepared by spin coating and a semicrystalline film prepared by vacuum thermal deposition. The crystallinity associated with the TPPA film strongly hampered photoinduced mass transport in the fabrication of SRG.

Investigation of circular Bragg reflection in an azo polymer with photoinduced chirality

There has been recent interest in photoinduced chirality in azo polymers upon irradiation with circularly polarized light. The mechanism of ordering is thought to involve successive twisting of the achiral photoactive azo-containing side chains in response to the handedness of the pump polarization. In this study, a thin film of a liquid-crystalline azo polymer with a room-temperature nematic phase is irradiated with circularly polarized light and a circular Bragg reflection is observed in the absorption spectrum. When probed at normal incidence, the evolution of this selective reflection is monitored with increasing exposure. Then, the probing is carried out for various angles of incidence and the change in peak shape is studied. In the last experiment, the position of the Bragg reflection is monitored as the wavelength of the pump laser is varied. We propose that while the induced chirality is due to the circularly polarized light, the associated pitch may be large; the induced periodicity corresponding to the Bragg reflection may be the result of partial erasure due to a standing wave created by back reflection in thin films.

Photoinduced three-dimensional orientational order in side chain liquid crystalline azopolymers.

We apply experimental technique based on the combination of methods dealing with principal refractive indices and absorption coefficients to study the photoinduced three-dimensional (3D) orientational order in the films of liquid crystalline (LC) azopolymers. The technique is used to identify 3D orientational configurations of trans azobenzene chromophores and to characterize the degree of ordering in terms of order parameters. We study two types of LC azopolymers which form structures with preferred in-plane and out-of-plane alignment of azochromophores, respectively. Using irradiation with the polarized light of two different wavelengths, we find that the kinetics of photoinduced anisotropy can be dominated by either photoreorientation (angular redistribution of trans chromophores) or photoselection (angular selective trans-cis isomerization) mechanisms depending on the wavelength. At the early stages of irradiation, the films of both azopolymers are biaxial. This biaxiality disappears on reaching a state of photosaturation. In the regime of photoselection, the photosaturated state of the film is optically isotropic. But, in the case of the photoreorientation mechanism, anisotropy of this state is uniaxial with the optical axis dependent on the preferential alignment of azochromophores. We formulate the phenomenological model describing the kinetics of photoinduced anisotropy in terms of the isomer concentrations and the order parameter tensor. We present the numerical results for absorption coefficients that are found to be in good agreement with the experimental data. The model is also used to interpret the effect of changing the mechanism with the wavelength of the pumping light.

Holographic storage in carbazole-azo binary compound doped polymer

A series of novel materials of carbazole-azo binary compound doped polymer were introduced for holographic storage. Optical absorption changes orientation distribution of trans isomer of azobenzene moiety that results in the modulation of the refractive indices of azo doped polymer film without additional pump light. The diffraction efficiency of the 15 μm thick film of 4 ′ -carbazolylhexoxyl-2-methyl-4-nitro azobenzene is about 0.65%.

Design of Holographic Recording Conditions with Optical Densities of Thick Azopolymer Films

ABSTRACTWe evaluated the relation between the degradation of the Bragg selectivity and the optical densities of the films by use of the shift-multiplexing method with a spherical reference wave. Our recording media were prepared by blending methylazobenzene-containing polymer (MAP) and cyanobiphenyl-containing polymer (CBP). We fabricated four kinds of 250μm-thick films whose optical densities were increased with MAP concentrations that ranged from 10wt% to 70wt%. As a result of the evaluation, the film with a higher optical density degraded the Bragg selectivity more significantly. The degradation of the Bragg selectivity affected the bit error rate (BER). In addition, we obtained a guide to holographic recording conditions for achieving a low BER as to the films with non-negligible optical densities.

Synthesis and characterization of a novel photoprocessible hyperbranched azo polymer

A novel hyperbranched polymer containing pseudo-stilbene type azo chromophores throughout the hyperbranched architecture has been synthesized by ester-interchange reaction of ethyl-4-{4′-[bis(2-hydroxyethyl)amino]phenylazo}benzoate. The polymer synthesized was characterized by using spectroscopic methods and thermal analysis. With interfering illumination of Ar + laser beams (488 nm) at modest intensities (150 mW/cm 2), significant surface-relief-gratings formed on the hyperbranched azo polymer films were observed. The first order of diffraction efficiency reached 27% after 60 min irradiation.