Inscription Of Surface Relief Gratings Research Articles

Viscoplastic Modeling of Surface Relief Grating Growth on Isotropic and Preoriented Azopolymer Films.

We report on solving of two intriguing issues concerning the inscription of surface relief gratings within azopolymer thin films under irradiation with SS, PP and RL interference patterns. For this, we utilize the orientation approach and viscoplastic modeling in combination with experimental results, where the change in surface topography is acquired in situ during irradiation with modulated light. First, the initial orientation state of polymer backbones is proved to be responsible for the contradictory experimental reports on the efficiency of the SS interference pattern. Different orientation states can influence not only the phase of SS grating but also its height, which is experimentally confirmed by using special pretreatments. Second, the faster growth of gratings inscribed by the RL interference pattern is shown to be promoted by a weak photosoftening effect. Overall, the modeled results are in good agreement with the order of relative growth efficiency: RL-PP-SS.

Molecular-Level Photo-Orientation Insights into Macroscopic Photo-Induced Motion in Azobenzene-Containing Polymer Complexes.

As part of continuing efforts to deepen the understanding of photo-induced mass transport in azo-containing polymers, we compared the diffraction efficiency (DE) during surface-relief grating (SRG) inscription, photo-induced molecular orientation (<P2>), and thermal stability in two sets of supramolecular azopolymer complexes, namely, hydrogen-bonded (H-bonded) and ionically bonded (i-bonded) complexes, both as a function of the polymer degree of polymerization (DP). To that end, poly(4-vinylpyridine) (P4VP) polymers with DPs of 41, 480, and 1900 were H-bonded at an equimolar ratio with 4-hydroxy-4'-dimethylaminoazobenzene (azoOH), and the fully quaternized derivatives of the three P4VPs (P4VPMe) were i-bonded via ion exchange to sodium 4-[(4-dimethylamino)-phenylazo]benzene sulfonate (azoSO3), also known as methyl orange, where the OH functionality of azoOH is replaced by a sulfonate group. The i-bonded complexes show much better DE performances and <P2> levels than those of H-bonded complexes, which we relate to the liquid crystal structure of the former complexes. Fitting the <P2> curves by a biexponential equation leads to two parameters associated with a fast trans-cis or angular hole burning (AHB) process and a slow angular redistribution (AR) process of the azo, respectively. It is found that AHB is predominant in the H-bonded complexes, whereas the AR contribution is much greater in the i-bonded complexes, assuring their superior SRG efficiency that is enabled by the anisotropic free volume created mainly by the AR process. In each set of complexes, the SRG efficiency is much better for the lowest DP complex, while the AR contribution is constant (and low) for the H-bonded complexes and increases roughly linearly with the decrease in DP for the i-bonded complexes. The latter difference might be related to the presence of entanglements in the complexes with DPs 480 and 1900, which slow down the macroscopic movement during SRG inscription but not the molecular-scale movement in photo-orientation.

Plasmon-driven light harvesting in poly(vinyl alcohol) films for precise surface topography modulation.

Efficient light harvesting is essential for advanced photonic devices. Complex micro/nano surface relief structures can be produced via light-triggered mechanical movement, but limited in organic active molecular units. In this Letter, we propose to embed noble-metal particles into light-inactive polyvinyl alcohol matrix to construct a light harvesting system driven by plasmon for inscription of surface relief gratings. Ultra-small-sized silver nuclei are generated in the polymer by pre-thermal treatment, acting as an accelerator for the subsequent photoinduced particle growth, hydrogen group cleavage, and matrix softening. Based on such properties, a complex plasmonic array carrying ultra-high-density information is achieved with peristrophic multiplexing holography. This Letter paves a bright way to realize data storage, information encryption, and optical microcavity.

Modeling of Nonlinear Optical Phenomena in Host-Guest Systems Using Bond Fluctuation Monte Carlo Model: A Review.

We review the results of Monte Carlo studies of chosen nonlinear optical effects in host-guest systems, using methods based on the bond-fluctuation model (BFM) for a polymer matrix. In particular, we simulate the inscription of various types of diffraction gratings in degenerate two wave mixing (DTWM) experiments (surface relief gratings (SRG), gratings in polymers doped with azo-dye molecules and gratings in biopolymers), poling effects (electric field poling of dipolar molecules and all-optical poling) and photomechanical effect. All these processes are characterized in terms of parameters measured in experiments, such as diffraction efficiency, nonlinear susceptibilities, density profiles or loading parameters. Local free volume in the BFM matrix, characterized by probabilistic distributions and correlation functions, displays a complex mosaic-like structure of scale-free clusters, which are thought to be responsible for heterogeneous dynamics of nonlinear optical processes. The photoinduced dynamics of single azopolymer chains, studied in two and three dimensions, displays complex sub-diffusive, diffusive and super-diffusive dynamical regimes. A directly related mathematical model of SRG inscription, based on the continuous time random walk (CTRW) formalism, is formulated and studied. Theoretical part of the review is devoted to the justification of the a priori assumptions made in the BFM modeling of photoinduced motion of the azo-polymer chains.

Photoresponsive Behavior of Heterocyclic Azo Polymers with Various Functional Groups

Here, we report on efficiency of light-induced processes in a series of methacrylic polymers containing heterocyclic azo moieties covalently attached to the polymer backbone. The optical response of the polymers was investigated in experiments on photoinduced birefringence measurements and surface relief gratings inscription upon irradiation with polarized blue light. The experimental results indicate that the presence of acceptor-type (Br) substituent in the side benzene ring linked to the heterocyclic ring has a beneficial effect on efficiency of birefringence generation, while incorporation of donor-type (CH3, OCH3) substituents lowers the level of photoinduced birefringence. Moreover, the process of surface relief structure formation was successfully observed for all studied polymers. The obtained surface-modulation depth reached up to 100 nm for two out of four investigated materials. No straightforward relation between the induced azo chromophore alignment level and the efficiency of mass-transport process is observed.

Complex Dynamics of Photoinduced Mass Transport and Surface Relief Gratings Formation.

The microscopic and semi-macroscopic mechanisms responsible for photoinduced mass transport in functionalized azo-polymers are far from deeply understood. To get some insight into those mechanisms on “microscopic” scale, we studied the directed photoinduced motion of single functionalized polymer chains under various types of polarized light illumination using Monte Carlo bond fluctuation model and our kinetic Monte Carlo model for photoinduced mass transport. We found sub-diffusive, diffusive and super-diffusive regimes of the dynamics of single chains at constant illumination and mostly super-diffusive regime for directed motion in the presence of the gradient of light intensity. This regime is more enhanced for long than for short chains and it approaches the ballistic limit for very long chains. We propose a physical picture of light-driven inscription of Surface Relief Gratings (SRG) as corresponding to a dynamical coexistence of normal and anomalous diffusion in various parts of the system. A simple continuous time random walk model of SRG inscription based on this physical picture reproduced the light-driven mass transport found in experiments as well as the fine structure of SRG.

Photoinduced Liquefaction of Azobenzene-Containing Polymers.

Light can liquefy some solid azobenzene-containing polymers (azopolymers) by photoisomerization. Two types of photoinduced liquefaction of azopolymers have been reported: (1) polarized light can guide solid azopolymers "flow" along the polarization direction, which is called directional photofluidization and has been used for inscription of surface relief gratings (SRGs); (2) recently, some of us found that light can switch the glass transition temperatures (Tg ) of azopolymers and induce reversible solid-to-liquid transitions of these azopolymers. This Minireview demonstrates and compares directional photofluidization and photoinduced reversible solid-to-liquid transitions of azopolymers. Potential applications based on photoinduced liquefaction of azopolymers are highlighted and some remaining challenges in the field of photoinduced liquefaction of azopolymers are discussed.

Analyses of surface relief gratings inscription in epoxy‐azo linear and crosslinked polymers

ABSTRACTThis work evaluates the inscription of surface relief gratings (SRG) in a series of epoxy‐azo prepolymers and networks, both based on the same azo chromophore. Variables such as matrix structure (thermoplastic/thermoset), film thickness, exposure time, power of the incident laser beam, and content of azobenzene were analyzed on the final modulation depth of the gratings. The main goal of the article is to achieve the efficient formation of SRGs on crosslinked azopolymer films in a single step, in a reasonable time, and to a significant extent. The progressive erasure of the obtained structures with increasing temperature showed that part of the photoinduced information remained encoded in the material: as the relief partially relaxed above the glass transition temperature of the bulk material (116 °C) and up to 200 °C, its modulation remained in about one third of the initial height, and did not completely disappear. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1542–1552

SRG Inscription in Supramolecular Liquid Crystalline Polymer Film: Replacement of Mesogens

The photoinduced surface relief formation via mass transfer upon irradiation with patterned light has long been a subject of extensive investigation. In azobenzene-containing liquid crystalline materials, UV light irradiation that generates the cis isomer leads to the liquid crystal to isotropic photochemical transition. Due to this phase change, efficiency of the mass transfer to generate a surface relief grating (SRG) becomes markedly greater. We have previously indicated that azobenzene-colored SRG-inscribed film can be bleached by removing a hydrogen-bonded azobenzene mesogen. However, this process largely reduces the height feature of the SRG corrugation. Herein, we propose an extended procedure where a colorless mesogen is filled successively after the removal of the azobenzene side chain. The process involves four stages: (i) SRG inscription in a hydrogen-bonded supramolecular azobenzene material; (ii) crosslinking (insolubilization) of the SRG film; (iii) removal of azobenzene mesogen by rinsing with a solvent, and (iv) stuffing the hollow film with a different mesogen. Although the final stuffing stage was insufficient at the present stage, this work demonstrates the possibility and validity of the strategy of mesogen replacement.

Photoactive/Passive Molecular Glass Blends: An Efficient Strategy to Optimize Azomaterials for Surface Relief Grating Inscription.

Irradiation of azomaterials causes various photophysical and photomechanical effects that can be exploited for the preparation of functional materials such as surface relief gratings (SRGs). Herein, we develop and apply an efficient strategy to optimize the SRG inscription process by decoupling, for the first time, the important effects of the azo content and glass transition temperature (Tg). We prepare blends of a photoactive molecular glass functionalized with the azo Disperse Red 1 (gDR1) with a series of analogous photopassive molecular glasses. Blends with 10 and 40 mol % of gDR1 are completely miscible, present very similar optical properties, and cover a wide range of Tg from below to well above ambient temperature. SRG inscription experiments show that the diffraction efficiency (DE), residual DE, and initial inscription rate reach a maximum when Tg is 25-40 °C above ambient temperature for low to high azo content, respectively. Indeed, for a fixed 40 mol % azo content, choosing the optimal Tg enables doubling the SRG inscription rate and increasing DE 6-fold. Moreover, a higher azo content enables higher DE for a similar Tg. Spectroscopy measurements indicate that the photo-orientation of DR1 and its thermal stability are maximal with Tg around 70 °C, independent of the azo content. We conclude that the SRG potential of azomaterials depends on their capability to photo-orient but that the matrix rigidity eventually limits the inscription kinetics, leading to an optimal Tg that depends on the azo content. This study exposes clear material design guidelines to optimize the SRG inscription process and the photoactivity of azomaterials.

Influence of Supramolecular Interaction Type on Photoresponsive Azopolymer Complexes: A Surface Relief Grating Formation Study

The influence of the supramolecular interaction type—hydrogen bonding, proton transfer, and pure ionic bonding—on photoresponsive behavior, particularly surface relief grating (SRG) inscription, has been investigated using a homopolymer with an ambient temperature glass transition (Tg). To this end, poly(dimethylaminoethyl methacrylate) (PDMAEMA or PDM) of relatively high molecular weight (42K) was complexed at various azo/DM molar ratios with dimethylaminoazobenzene derivatives functionalized by hydroxyl (azoOH) or carboxylic acid (p-methyl red or azoCOOH) groups, and quaternized PDM (PDMQ) was complexed at equimolar ratio with a sulfonated analogue (methyl orange or azoSO3), and the structural and thermal properties of these complexes were determined. SRG inscription on spin-coated films was found to be possible because complexation increases the Tg above ambient. The efficiency of SRG inscription on spin-coated films, which increases with azo content, is highest for the purely ionic azoSO3/PDMQ complex...

Considerations on the surface relief grating formation mechanism in case of azo-polymers, using pulse laser irradiation method

Azobenzene-polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light, one of the most investigated being the capability to generate a surface relief gratings (SRG). Despite the effort to understand the SRG formation process, the mechanism remains unclear until now. The paper presents a study concerning the azo-polymer response to the pulse laser irradiation, in the context of the SRG inscription. We propose an inscription mechanism based on the material deformation in the solid state, probably induced by the azo-groups parallelization with the electric field vector. Aromatic polyimides containing azobenzene units were investigated and compared with other azo-polymers having a more flexible main chain, from the photochromic behavior’s perspective and the ability to generate SRG.

Surface relief gratings inscription in linear and crosslinked azo modified epoxy‐isocyanate polymers

ABSTRACTThis work analyses the SRG forming ability of a linear and a crosslinked polymer, both based on the same azo chromophore (Disperse Red 19, DR19) and diisocyanate (4,4'‐metilen‐bis(cyclohexyl isocyanate), DI) with similar glass transition temperature values. The effect of the experimental and geometrical parameters (such as beams polarization direction and interference pattern spacing) on the surface modulation process was examined as well. The absence of photodegradation or bleaching was corroborated by comparing Raman spectra inside and outside the SRG zone. Being a very important subject matter, the influence of crosslinking on the inscription rates and thermal stability of SRGs was investigated. Crosslinked films with an interesting modulation and good grating stability, even at high temperatures (160 °C), were obtained. Gratings shape was retained, and significant diffraction was still observed even well above the Tg of the bulk sample. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 587–594

Simple Two-Photon Inscription of Surface Relief Gratings with Azobenzene Functionalized Polymer

Two-photon fabrication of surface relief gratings on azobenzene functionalized polymer films was carried out using femtosecond laser pulses at 800 nm. The two-photon inscriptions were achieved by simply employing a relief grating as a phase mask. The relief patterns were successfully recorded with the phase mask and the near-infrared laser pulses. The period of the grating is the same as that of the mask. Polarization dependent formation of the grating was observed.

PHOTO-INDUCED DICHROISM AND SURFACE-RELIEF-GRATINGS OF HYPERBRANCHED AZO POLYMERS SYNTHESIZED BY AZO-COUPLING REACTION

A novel hyperbranched azo precursor polymer containing electron donor-acceptor type azobenzene chromophores was synthesized by step-growth polymerization of an AB_2 monomer,di[2-(N-ethylanilino)ethyl]-5-aminoisophthalate,through azo coupling reaction.The hyperbranched precursor polymer was then reacted with the diazonium salts of 4-amino benzonitril to introduce another kind of donor-acceptor azo chromophores in the peripheral groups.The structure and properties of the azo polymers were characterized by the spectroscopic methods and thermal analysis.The photo-induced dichroism of the hyperbranched polymer was studied and the orientation order parameter of the polymer was estimated to be 0.063.Thin films of the hyperbranched azo polymers were used to fabricate surface-relief-gratings(SRGs) by exposing the thin films to an interference pattern of Ar~+ laser beam at modest intensities(150 mW/cm~2).The azobenzene chromophore introduced by post-azo-coupling reaction in the para-positions of the terminal azobenzene units shows a significant effect on the SRG inscription rate.The hyperbranched azo polymers can potentially be used for applications such as reversible optical data storage,photoswitching,sensors,and other photo-driven devices.

Self-Structured Surface Patterns on Molecular Azo Glass Films Induced by Laser Light Irradiation

In this work, formation of photoinduced self-structured surface pattern and its correlation with chromophoric structures were studied by using a series of star-shaped azo compounds, which exist as stable molecular glass at room temperature. For the synthesis, a star-shaped precursor (Tr-AN) was prepared by a ring-open reaction between 1, 3, 5-triglycidyl isocyanurate and N-methylaniline. The star-shaped azo compounds were then synthesized through azo-coupling reactions between the precursor and diazonium salts of 4-nitroaniline, 2-methyl-4-nitroaniline, and 4-aminobenzonitrile, respectively. Through these steps, three azo compounds were obtained to bear different donor-acceptor type azo chromophores at the peripheral positions. The formation of the photoinduced self-structured patterns was investigated by irradiating solid thin films of the molecular azo glass with a uniform laser beam (532 nm, 200 mW/cm(2)) at normal incidence. For comparison, formation of surface-relief-gratings (SRGs) was also investigated by exposing the thin films to an interference pattern of the laser beams (532 nm, 80 mW/cm(2)). The results show that SRGs can be inscribed on the films of all three star-shaped azo compounds, but self-structured surface patterns is only observed on film of the azo compound containing 4-cyanoazobenzene moieties (Tr-AZ-CN) under the same irradiation condition. The electron-withdrawing groups, which control the absorption band position, show a significant influence on both the self-structured pattern formation and SRG inscription rate. Under proper experimental conditions, both self-structured surface pattern and SRG can simultaneously be observed on the Tr-AZ-CN films. The observations can lead to a deeper understanding of the photoinduced effects, especially their correlation with molecular structures.

Phenomenology of Photoinduced Processes in the Ionic Sol−Gel-Based Azobenzene Materials

Very recently, supramolecular azobenzene-containing materials have been demonstrated to be effective for holographic inscription of surface relief gratings. High efficiency of grating formation in these materials is advantageously combined with easy material preparation because of the supramolecular principle of building blocs combination, availability of chemical components, and good film-forming properties. The materials show great promise for wide optical applications. Here, one type of material formulation based on ionic complexes of poly(aminosiloxane) is experimentally investigated concerning the induction of birefringence, efficiency of relief formation, and thermal stability of surface structures. Properties of the material were changed on the molecular level by varying the azobenzene content and on the macro level by varying chemical processing conditions. The results are discussed in terms of cross-linking and plasticizing effects caused by residual solvent and the low-molecular-weight azobenzene component. An improvement of both the efficiency of grating formation and their thermal stability by these means has been demonstrated. The results can be adapted to other azobenzene-containing materials. In addition, a drastic change of films solubility has been observed upon irradiation.

Microscopic Theory of Light-Induced Deformation in Amorphous Side-Chain Azobenzene Polymers

We propose a microscopic theory of light-induced deformation of side-chain azobenzene polymers taking into account the internal structure of polymer chains. Our theory is based on the fact that interaction of chromophores with the polarized light leads to the orientation anisotropy of azobenzene macromolecules which is accompanied by the appearance of mechanical stress. It is the first microscopic theory which provides the value of the light-induced stress larger than the yield stress. This result explains a possibility for the inscription of surface relief gratings in glassy side-chain azobenzene polymers. For some chemical architectures, elongation of a sample demonstrates a nonmonotonic behavior with the light intensity and can change its sign (a stretched sample starts to be uniaxially compressed), in agreement with experiments. Using a viscoplastic approach, we show that the irreversible strain of a sample, which remains after the light is switched off, decreases with increasing temperature and can disappear at certain temperature below the glass transition temperature. This theoretical prediction is also confirmed by recent experiments.

3D structures using surface relief gratings of azobenzene materials

We present here a method of step-by-step 3D structure fabrication using holographic formation of surface relief gratings (SRG) in azobenzene-containing layers alternated with spacer layers. The method possesses opportunities that could not be realized by other holographic approaches, especially the variation of grating periods or the shape of the SRG in different layers (hierarchical structures), full flexibility of the structure type and parameters. One of the main problems to be solved is the compatibility of active azobenzene-containing polymers and a material for the spacer layers. We resolved the problem and found active and spacer polymers with excellent compatibility. The materials allow the preparation of films with good optical quality and the inscription of SRG with modulation amplitudes up to 1 µm in reasonable time. The refractive index difference between active and spacer polymers (currently 0.15–0.2) results in appreciable diffraction efficiency of the SRG after the application of the spacer layer. The phase correlation technique between adjacent layers has been developed. The fabrication of multilayer structures has been demonstrated by examples of a combination of linear, tetragonal and hexagonal SRG with three active layers and hierarchical structures.

On the Inscription of Period and Half-Period Surface Relief Gratings in Azobenzene-Functionalized Polymers

Laser-light-induced surface relief grating inscription was carried out in the newly synthesized azobenzene-functionalized poly(amide-imide)s having the same main- and side-chain structures but different substituents in the azobenzene groups. The gratings were inscribed employing the two-wave mixing technique with linearly polarized laser beams. Three different polarization configurations were used: s-s, p-p, and s-p. The relatively deep surface relief gratings of period Lambda were formed for the case of s-s and p-p polarizations, whereas the s-p inscription resulted in the half-period grating (Lambda/2) with the weak surface modulation. The origin of the formation of Lambda/2 structure for s-p configuration results from the interference between zeroth- and first-order beams scattered on the polarization refractive index grating and having the same polarization. On the basis of this idea, we presented the simple kinetic model predicting and modeling the half-period grating formation with its temporal evolution. The proposed model is consistent with the experimental findings.