Azopolymer Films Research Articles

Recording optical vortices in azo polymer films by applying holographic method

In this paper the optical vortices with topological charge <i>q</i> = –1, 1, 2, 4 are recorded in azo polymer films by using holographic technology. The forked holographic gratings formed by the Gaussian beam and optical vortex beam are recorded in the sample films, the original forked holographic grating and the recording rate are analyzed. The vortex beam is reconstructed by illuminating the sample film with a reference beam, and the recording quality is analyzed. Also the erasability and durability of the sample are tested. The experimental results show that the recording rates of vortex beams with different topological charges are relatively uniform, which means that the optical vortices with different topological charges can be recorded at the same speed. The forked holographic grating of the high-order optical vortex splits in the recording process due to the disturbances, such as anisotropic nonlinear light, atmospheric turbulence, and background light field. However, the split vortex beam still maintains a stable ring structure. The reconstructed optical vortex and the original optical vortex are highly consistent in morphology, and the interference fringes of the reconstructed optical vortices are highly consistent with the original vortex holographic gratings, indicating that the topological charge information in the optical vortices can be effectively recorded and read out. The recorded information can be erased by heating the sample to about 97 ℃, and new information can be re-recorded after cooling. There appears no fatigue in the sample after the information has been erased 100 times and good durability is still retained. Optical vortices theoretically have infinite states of topological charges, based on which great success is achieved in optical communication and information encoding. Therefore, storing and reading information of topological charges in optical vortices may have potential applications in optical information storage.

Polarization holographic gratings with enhanced parameters recorded in azopolymer based nanocomposite materials

We present study of the parameters of polarization holographic gratings recorded in thin films of the azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a composite PAZO-based organic/inorganic material with incorporated TiO2 nanoparticles with size 21 nm and different concentrations. The gratings are inscribed by two plane waves with left and right circular polarization from a He-Cd gas laser (442 nm) at recording angle 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that the diffraction efficiency for the hybrid samples is enhanced with respect to the pure azopolymer films. Comparison is also made between polarization response of samples deposited from water and from methanol solution.

Influence of optical forces induced by paraxial vortex Gaussian beams on the formation of a microrelief on carbazole-containing azopolymer films.

The paper discusses photoinduced microrelief formation in a film of an azopolymer. A theoretical study of the effect of laser beam polarization on the balance of optical forces acting under the direct action of paraxial Gaussian beams on the irradiated substance was made. We show that taking into account the gradient and scattering components of the force does not allow us to correctly describe the shape of the microasperities obtained on a carbazole-containing azopolymer. An approximation function is presented that describes the dependence of the microasperities' shapes on the non-gradient component of the optical force of laser radiation in the absence and presence of a vortex phase. A comparative analysis of the approximation results and experimentally obtained microreliefs was carried out.

Formation of half-period surface relief gratings in azobenzene containing polymer films

We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45° relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films.

Theoretical analysis on spatially structured beam induced mass transport in azo-polymer films.

The radiation force from paraxial beams possessing helical phase fronts causes twists on the surface of an azobenzene polymer sample, and leads to the formation of micro-scale structures. Here, we theoretically investigate the radiation force generated by spatially structured optical beams on a dispersive-absorptive substrate. We derive an analytical expression for the radiation force from spatially structured polarized beams, including, lemon, star, monstar and vector vortex beams in the paraxial regime. Finally, we extend our calculation for non-paraxial beams - optical beams under the tight-focusing regime - and simulate the transverse radiation forces numerically at the focal plane.

Deterministic Realization of Quasicrystal Surface Relief Gratings on Thin Azopolymer Films

AbstractSpatially structured UV–visible light fields generate topographic modulations on the surface of films of azobenzene‐containing polymers. The geometry of the surface reliefs depends on the spatiotemporal distribution of light over the sample. If multistep sequential irradiations are used, even illumination configurations as simple as the interference of two linearly polarized beams produce complex surface textures. This is the case of the quasicrystal geometries, obtainable as the superposition of multiple sinusoidal surface relief gratings oriented in different directions over the surface. The quantitative relief design would require a comprehensive theoretical description of the light‐induced relief formation mechanism, which is still elusive for azopolymers. Here, despite limiting the description at a phenomenological level, the deterministic design of the quasicrystal surface reliefs obtained in sequential light exposures of an azopolymer film is demonstrated. The model provides an excellent agreement between simulated and experimental relief textures, predicting also the dependence of relief structural features on illumination parameters as the number of exposure steps and the beam interference angle. The deterministic texture design allows the controlled tailoring of surface functionalities related to the relief geometry like light diffraction properties of the samples, which are here exploited to manipulate, split, and trap diffracted light.

Improvement of the photoinduced birefringence in PAZO azopolymer doped with ZnO via electrospray deposition

We present a study of the birefringence photoinduced in thin nanocomposite films of an azopolymer (PAZO) doped with different concentrations of nanoparticles (NPs) of ZnO with spherical shape and size in the range 40 – 100 nm. The nanocomposite thin films were prepared by electrospray deposition of ZnO over a thin azopolymer film with a thickness below 200 nm. The different NPs concentrations were obtained through different electrospray deposition times varied from 0 to 10 minutes. Birefringence was induced by a He-Cd laser at a wavelength of 442 nm and measured by a probe diode pumped solid state (DPSS) laser at 635 nm. The birefringence dependence on the ZnO NP deposition time reached its maximum for a deposition time of 90 s.

Fast, Light-Responsive, Metal-Like Polymer Actuators Generating High Stresses at Low Strain

Summary Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast ( 6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Modeling of Stripe Patterns in Photosensitive Azopolymers.

Placed at interfaces, azobenzene-containing materials show extraordinary phenomena when subjected to external light sources. Here we model the surface changes induced by one-dimensional Gaussian light fields in thin azopolymer films. Such fields can be produced in a quickly moving film irradiated with a strongly focused laser beam or illuminating the sample through a cylindrical lens. To explain the appearance of stripe patterns, we first calculate the unbalanced mechanical stresses induced by one-dimensional Gaussian fields in the interior of the film. In accordance with our orientation approach, the light-induced stress originates from the reorientation of azobenzenes that causes orientation of rigid backbone segments along the light polarization. The resulting volume forces have different signs and amplitude for light polarization directed perpendicular and parallel to the moving direction. Accordingly, the grooves are produced by the stretching forces and elongated protrusions by the compressive forces. Implementation into a viscoplastic model in a finite element software predicts a considerably weaker effect for the light polarized along the moving direction, in accordance with the experimental observations. The maximum value in the distribution of light-induced stresses becomes in this case very close to the yield stress which results in smaller surface deformations of the glassy azopolymer.

Picosecond optical vortex-induced chiral surface relief in an azo-polymer film

We report on the first demonstration of picosecond optical vortex-induced chiral surface relief in an azo-polymer film due to two-photon absorption isomerization. The chiral surface relief exhibits an extremely narrow defocusing tolerance without undesired outer rings due to the Airy pattern of highly focused light. Such chiral surface relief reflects a z-polarized electric field with an azimuthal helical phase caused by spin–orbital angular momentum coupling.

Influence of Photoanisotropies on Light-Controllable Structuration of Azopolymer Surface

The capability to optically control surface structuration of azopolymer films is an important goal in different research fields, enabling remote activation and tuning of associated processes mediat...

Quantitative Study of Morphological Features of Stem Cells onto Photopatterned Azopolymer Films.

In the last decade, the use of photolithography for the fabrication of structured substrates with controlled morphological patterns that are able to interact with cells at micrometric and nanometric size scales is strongly growing. A promising simple and versatile microfabrication method is based on the physical mass transport induced by visible light in photosensitive azobenzene-containing polymers (or azopolymers). Such light-driven material transport produces a modulation of the surface of the azopolymer film, whose geometry is controlled by the intensity and the polarization distributions of the irradiated light. Herein, two anisotropic structured azopolymer films have been used as substrates to evaluate the effects of topological signals on the in vitro response of human mesenchymal stem cells (hMSCs). The light-induced substrate patterns consist of parallel microgrooves, which are produced in a spatially confined or over large-scale areas of the samples, respectively. The analysis of confocal optical images of the in vitro hMSC cells grown on the patterned films offered relevant information about cell morphology—i.e., nuclei deformation and actin filaments elongation—in relation to the geometry and the spatial extent of the structured area of substrates. The results, together with the possibility of simple, versatile, and cost-effective light-induced structuration of azopolymers, promise the successful use of these materials as anisotropic platforms to study the cell guidance mechanisms governing in vitro tissue formation.

Recording Polarization Holograms in Polymer Films with Azo Dyes and Azo Polymers

Based on polymers with the addition of azo dyes as chromophores and azopolymers with the same covalently linked chromophores, recording media for polarization holography are created and their properties are studied when recording holograms of a plane wave front with parallel and perpendicular polarization of recording rays. It is found that the diffraction efficiency and the storage time of holograms are much longer in recording media with films of azo polymers in comparison with polymeric solid solutions of azo dyes. In the latter case, the energy of light is spent only for the trans–cis isomerization of azo dye molecules during exposure of holograms and the polymer chains do not undergo deformation with the formation of a surface relief in the polymer film. Therefore, it is preferable to use recording media based on solid solutions of azo dyes in dynamic polarization holography.

Analysis of characteristics of paraxial vector Gaussian beams affecting the formation of microstructures in an azopolymer

The paper considers paraxial Gaussian laser beams with different polarization states and their characteristics, such as the Poynting vector, gradient force, and force density, which are important for evaluating the effects of radiation on matter. An analysis of the considered characteristics of laser radiation from the point of view of the formation of photoinduced microrelief in an azopolymer film is carried out.

Photoinduced birefringence in azopolymer doped with Au nanoparticles

A study of the photoinduced birefringence in nanocomposite thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with gold (Au) nanoparticles (NPs) is presented. The NPs are with a spherical shape and mean size 10 nm. Transmission electron microscopy (TEM) and Dynamic Light Scattering (DLS) analyses of the particles have also been made. Samples were prepared with five different concentrations of the NPs in the azopolymer films varying from 0 (non-doped azopolymer film) to 4 a.u. (1 a.u. = 0.015 wt %). Birefringence was induced by a He-Cd laser with a wavelength 442 nm and measured with probe Diode Pumped Solid State (DPSS) laser at 635 nm. An increase of the photoinduced birefringence in doped azopolymer nanocomposite films compared to samples of undoped azopolymers was observed. The maximal enhancement was achieved for films with 2 a.u. of Au NPs.

Computer-generated holograms for complex surface reliefs on azopolymer films

The light-driven superficial structuration observed on the surface of films of azobenzene-containing polymers follows the optical field distribution of the illuminating light pattern, i.e. the light polarization state and the intensity distribution. The ability to precisely manipulate the illuminating intensity pattern can hence provide a new level in the range of complex light-induced superficial textures accessible onto azopolymer film surfaces. In this respect, digital holography, based on the principles of the Computer-Generated Holograms (CGHs), and actually implemented by means of a versatile liquid crystal spatial light modulator, can represent a unique experimental tool in the field of the light-induced mass migration in azo-materials. In the present work, we demonstrate the possibility to precisely control the features and the quality of complex light patterns generated through CGHs in order to induce arbitrarily complex surface reliefs onto the surface of an azopolymer. The results shown here can potentially broaden the range of possible applications of photo-responsive azopolymer films in the fields of surface engineering, biology and photonics.

Direct Fabrication of Nanodomes‐Combined Surface Relief Gratings on Azobenzene Polymer Films with Controlled Shapes and Sizes

ABSTRACTIn this study, we present nanodomes‐combined surface relief gratings (SRGs) of azopolymer films with controlled shapes and sizes. We investigate the effect of the polarization mode of light interference on leading nanodomes in the conventional SRG patterns. In addition, we also systematically study the relationship between Bragg distance of light interference and shapes of nanodomes. From this, we explain the anisotropic self‐assembled behavior nanodomes in photoaddressable azopolymer films regarding polarization modes as well as spatial confinement effect. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 731–737

Photocontrollable Wrinkle Morphology Evolution on Azo-Based Multilayers for Hierarchical Surface Micropatterns Fabrication.

Inspired by nature, comprehensive understanding and ingenious utilization of the self-organized wrinkling behaviors of the sandwiched multilayer bonded on substrates are important for engineering and/or functional laminated devices design. Herein, we report a facile and effective strategy to regulate the wrinkles morphology evolution and the resultant hierarchical surface micropatterns on azobenzene-based laminated multilayers by visible-light irradiation. Revealed by systematic experiments, the photocontrolled dynamic wrinkle evolutions are triggered by the reversible photoisomerization of azobenzene in the top azopolymer film and are strongly dependent on the intermediate photoinert layers (e.g., polystyrene and oxygen plasma-induced SiO x layer) with the wrinkle-reinforcing effect or the stress relaxation acceleration effect. Interestingly, large-area well-defined hierarchical surface wrinkle patterns could be fabricated on the multilayers upon selective exposure. In the unexposed region, the wrinkles evolved into highly oriented patterns, whereas in the exposed region, they were fully erased or evolved into smaller-wavelength wrinkles. This study not only sheds light on the morphological evolution of the wrinkling laminated composites in engineering and nature but also paves a new avenue to conveniently and controllably realize the hierarchical stimulus-responsive surface patterns.

Запись поляризационных голограмм в полимерных пленках с азокрасителями и азополимерами

Abstract Based on polymers with the addition of azo dyes as chromophores and azopolymers with the same covalently linked chromophores, recording media for polarization holography are created and their properties are studied when recording holograms of a plane wave front with parallel and perpendicular polarization of recording rays. It is found that the diffraction efficiency and the storage time of holograms are much longer in recording media with films of azo polymers in comparison with polymeric solid solutions of azo dyes. In the latter case, the energy of light is spent only for the trans – cis isomerization of azo dye molecules during exposure of holograms and the polymer chains do not undergo deformation with the formation of a surface relief in the polymer film. Therefore, it is preferable to use recording media based on solid solutions of azo dyes in dynamic polarization holography.

Improvement of the photoinduced birefringence in azopolymer PAZO doped with TiO2 nanoparticles via thermal treatment

Abstract We present a study of the photoinduced birefringence in nanocomposite films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfon amido]-1,2-ethanediyl, sodium salt]) doped with TiO2 nanoparticles (NP) with different concentrations before and after thermal annealing. The NP represent nanopowder with primary particle size 21 nm. The concentration of the NP was varied from 0% (non-doped azopolymer film) to 5 wt%. The thermal process, applied to the nanocomposite films, includes 1 h heating at 200°C. Previous studies of PAZO show that the polymer is stable up to 270°C. We study the dependence of the maximal birefringence induced with He-Cd laser (λ= 442 nm) on the concentration of the TiO2 NP in the azopolymer thin films as well as thermal effect on the absorbance spectra of the thin films. As indicated by our results, the birefringence is higher for the thermally annealed samples. An increase of the photoinduced birefringence is also observed for the nanocomposite layers with 1% NP for the non-annealed films, and with 2% NP for the annealed films.