Formation Of Surface Relief Gratings Research Articles

Optical characteristics and holographic gratings recording on As30Se70 thin films

We have systematically studied the optical properties, the effect of laser illumination, and the effect of light induced surface deformation through direct fabrication of surface relief gratings (SRGs) on As30Se70 thin films. The transmittance, T, for as deposited and illuminated films show a red shift in the transmittance edge which associated with the decrease in energy gap and this confirms the photo-darkening effect. The variations in linear and non-linear optical parameters values have been discussed in the view of illumination effect. Giant stable surface relief gratings have been obtained by one-step holographic recording. The surface relief gratings formation in our film is investigated by atomic force microscopy (AFM). Results have been analysed in terms of available theories for these phenomena.

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 formation in azo-polyelectrolyte layers with a protective polymer coating

Azobenzene-modified photosensitive polymers offer the possibility to reshape flat films upon UV-Vis irradiation. The photochemical isomerization between trans-cis configurations is the key mechanism for surface relief grating (SRG) formation on the free surface of thin azo-polymeric films illuminated by structured illumination. In the present work, we consider a sandwich configuration wherein an azopolymeric film (PAZO) is coated by a plasma-deposited polyacrylic acid (PPAA). It is demonstrated that the PPAA coating protects the PAZO against dissolution in aqueous environment. More interestingly, light-induced SRG can still be formed in the PAZO film, and the resulting modulation is transferred to the PPAA coating above. Results obtained open the possibility of exploiting water-soluble azopolymers in bio-oriented applications.

All-Optical Surface Micropatterning by Electric Field Intensity Gradient

In this report an all-optical photo-induced formation of surface relief gratings is shown. For the surface patterning of As2S3 and As4S1.5Se4.5 films a direct holographic recording setup with a 532 nm wavelength Nd:YAG CW laser light was used. Our investigations have shown that the light-induced mass transfer process strongly depends on the material itself and on the polarization of the light. It has been shown that an electric field intensity gradient has to be obtained to achieve a direct patterning. The evolution of a surface relief in relation to recording parameters and thickness of the sample has been investigated in detail.

Making polymer brush photosensitive with azobenzene containing surfactants

We report on rendering polyelectrolyte brushes photosensitive by loading them with azobenzene-containing cationic surfactants. Planar poly(methacrylic acid) (PMAA) brushes are synthesized using the “grafting from” free-radical polymerization scheme followed by exposure to a solution of photosensitive surfactants consisting of positively-charged head groups and hydrophobic tails into which azobenzene moieties are inserted. In this study the length of the hydrophobic methylene spacer connecting the azobenzene and the charged head group ranges from 4 to 10 CH2 groups. Under irradiation with UV light, the photo-isomerization of azobenzene integrated into a surfactant results in a change in size, geometry, dipole moment and free volume of the whole molecule. When the brush loaded with photosensitive surfactants is exposed to irradiation with UV interference patterns, the topography of the brush deforms following the distribution of the light intensity, exhibiting surface relief gratings (SRG). Since SRG formation is accompanied by a local rupturing of polymer chains in areas from which the polymer material is receding, most of the polymer material is removed from the surface during treatment with good solvent, leaving behind characteristic patterns of lines or dots. The azobenzene molecules still integrated within the polymer film can be removed by washing the brush with water. The remaining nano-structured brush can then be re-used for further functionalization. Although the opto-mechanically induced rupturing occurs for all surfactants, larger species do not penetrate deep enough into the brush such that after rupturing a leftover layer of polymer material remains on the substrate. This indicates that rupturing occurs predominantly in regions of high surfactant density.

Thermal, optical and photoinduced properties of a series of homo and co-polyimides with two kinds of covalently bonded azo-dyes and their supramolecular counterparts

The paper describes the synthesis and characterization of new aromatic polyimides with one or two different moieties of the azo-dyes covalently attached to the polymer backbone and their supramolecular analogues. Azo-functionalized polyimides were prepared using post-polymerization method including the introduction of Disperse Red 13 and/or 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine to homo and co-polyimides containing hydroxyl groups via Mitsunobu reaction. The degree of functionalization of polymers with chromophores was estimated by UV–Vis spectroscopy. Polyimides containing hydroxyl groups were applied as matrixes to create supramolecular systems based on hydrogen bonds. Hydrogen-bond interactions in azosystems were studied by FTIR spectroscopy. The polymers were characterized by 1H NMR, FTIR, X-ray, UV–Vis, DSC and TGA methods. The photoisomerization process was investigated in supramolecular systems. The light-induced anisotropy was studied in a holographic gratings recording experiment and by photoinduced birefringence measurements. The polymer films were investigated by atomic force microscopy (AFM) after the diffraction grating recording to confirm formation of surface relief gratings (SRGs). To the best of our knowledge, that the first time photoinduced anisotropy has been studied by birefringence measurements in polyimides containing two different azo-dyes.

Photoinduced mass transport in low molecular organic glasses and its practical application in holography

In this paper we present the synthesis and optical properties of amorphous low molecular compound 4-((4-(bis(5,5,5-Triphenylpentyl)amino)phenyl)diazenyl)benzoic acid. The surface relief gratings formation by polarization holography method on these thin films (thicknesses are 200–1700nm) was shown. The dependence of the surface relief modulation on various parameters: recording exposure dose, the polarization state of the recording beams, the grating period and the film thickness of the material was demonstrated. Values of surface tension at the air-film before and after irradiation of the substance by polarized radiation were measured.The efficiency of surface relief formation in different medium was compared. The possibility of practical application of 4-((4-(bis(5,5,5-Triphenylpentyl)amino)phenyl)diazenyl)benzoic acid in the holography as a non-etching photoresist was showed.

Properties of some azo-copolyimide thin films used in the formation of photoinduced surface relief gratings

The patterning of the free standing azo-copolyimide films under different irradiation conditions led to the uniform surface relief gratings.

High-Modulation-Depth Surface Relief Gratings Using s–s Polarization Configuration in Supramolecular Polymer–Azobenzene Complexes

The formation of the surface relief gratings (SRG) in azo-polymers strongly depends on the polarization configuration of the writing beams. So far the s–s polarization configuration has been considered as ineffective in terms of SRG formation. Here, we report that very high-amplitude SRGs can be recorded when using the s–s inscription geometry in supramolecular polymer–azobenzene complexes and that the efficiency of the process strongly depends on the molecular weight of the polymer. Furthermore, a single holographic irradiation leads to the formation of the surface relief grating (primary grating) and, unexpectedly, to the formation of the secondary fine-structure grating with a considerable modulation depth in the direction perpendicular to the primary grating. The detailed analysis of holographic recording has been performed on the basis of the proposed phenomenological model. A new aspect of the phenomenon of the SRG formation has been indicated by demonstrating that the SRG can be inscribed for the s–s polarization configuration. Since the SRG formation is considered as an important new tool in micro/nanofabrication technologies, both the formation of the SRG for s–s geometry and the formation of the fine-structure grating open new possibilities in photonic applications.

Photosensitive response of azobenzene containing films towards pure intensity or polarization interference patterns

In this paper, we report on differences in the response of photosensitive azobenzene containing films upon irradiation with the intensity or polarization interference patterns. Two materials are studied differing in the molecular weight: an azobenzene-containing polymer and a molecular glass formed from a much smaller molecule consisting of three connected azobenzene units. Topography changes occurring along with the changes in irradiation conditions are recorded using a homemade set-up combining an optical part for generation and shaping of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. In this way, we could reveal the unique behavior of photosensitive materials during the first few minutes of irradiation: the change in topography is initially driven by an increase in the azobenzene free volume along with the trans-cis isomerization, followed by the mass transport finally resulting in the surface relief grating. This study demonstrates the great potential of our setup to experimentally highlight puzzling processes governing the formation of surface relief gratings.

Probing opto-mechanical stresses within azobenzene-containing photosensitive polymer films by a thin metal film placed above.

Azo-modified photosensitive polymers offer the interesting possibility to reshape bulk polymers and thin films by UV-irradiation while being in the solid glassy state. The polymer undergoes considerable mass transport under irradiation with a light interference pattern resulting in the formation of surface relief grating (SRG). The forces inscribing this SRG pattern into a thin film are hard to assess experimentally directly. In the current study, we are proposing a method to probe opto-mechanical stresses within polymer films by characterizing the mechanical response of thin metal films (10 nm) deposited on the photosensitive polymer. During irradiation, the metal film not only deforms along with the SRG formation but ruptures in a regular and complex manner. The morphology of the cracks differs strongly depending on the electrical field distribution in the interference pattern, even when the magnitude and the kinetics of the strain are kept constant. This implies a complex local distribution of the opto-mechanical stress along the topography grating. In addition, the neutron reflectivity measurements of the metal/polymer interface indicate the penetration of a metal layer within the polymer, resulting in a formation of a bonding layer that confirms the transduction of light-induced stresses in the polymer layer to a metal film.

Surface relief grating formation in amorphous As40S15Se45 and As2S3 films under 0.532 μm wavelength illumination

In this research we have studied the influence of λ = 0.532 μm laser light illumination on the formation and properties of surface relief gratings in amorphous As2S3 and As40Se15Se45 films, as well as a bulk As2S3 sample. The surface relief grating formation was studied for a wide range of grating periods (∼1 – ∼80 μm) for the variable thicknesses of the samples. The examination of the relief depth, Δh, and its dependence on the grating period at constant exposures shows that the impact of sample thickness could manifest itself as a shift in the optimal grating period at holographic surface relief formation (As2S3) and affect the process itself.

Photoresponsive Behaviors of Azobenzene Functionalized 2-Arm, 3-Arm, and 4-Arm Telomers: A Comparative Study

Azobenzene functionalized 2-arm, 3-arm, and 4-arm telomers were synthesized according to the synthetic process described earlier. Here we report their photoresponsive behaviors to elucidate the effects of number of arms of star azotelomers on their photoresponsive behaviors. Photoisomerization behaviors of star azotelomer films were investigated. Surface relief gratings were inscribed on spin-coated azotelomer films by irradiating interference pattern of Ar+ laser at 488 nm. Diffraction efficiencies of azotelomers were found to increase with an increase in number of arms. The formation of surface relief gratings (SRG) was observed with atomic force microscopy (AFM) and optical polarized microscopy.

Comparative studies of polyimides with covalently bonded azo-dyes with their supramolecular analoges: Thermo-optical and photoinduced properties

The paper describes the synthesis and characterization of aromatic polyimides with azopyridine derivatives covalently attached as a side-chain and their supramolecular analoges based on the hydrogen bonds. Moreover, azopolymers differ in the chemical structure of the polymer backbone and the content of the chromophore. Azo-functionalized polymers were obtained by a two-step synthetic approach. This includes the preparation of a precursor with pendant hydroxyl groups followed by the covalent attachment of the chromophore, that is, 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine to the polyimide backbone. The degree of functionalization of the polymers was estimated by the UV–Vis spectroscopy. Supramolecular films were built on the basis of the hydrogen bonds between the hydroxyl groups of the polymer and azopyridine derivatives, that is, 4-(4-hydroxyphenylazo)pyridine and 4-[4-(6-hydroxyhexyloxy)phenylazo]pyridine. The polymers were characterized and evaluated by FTIR, 1H NMR, X-ray, UV–Vis, DSC and TGA methods.The synthesized precursor polymers revealed the glass transition temperature (Tg) in the range of 254–366°C, whereas azopolymers and supramolecular systems exhibited lower Tg within the ranges 129–153°C and 53–171°C, respectively. The polymers were easily soluble in common organic solvents enabling to obtain films of a good optical quality. The light-induced optical anisotropy was studied with the help of the holographic recording. The polymer films were investigated by an atomic force microscopy before and after the diffraction grating recording in order to confirm the formation of surface relief gratings. To the best of our knowledge, for the first time the photoinduced anisotropy in polyimides functionalized with azopyridine units has been compared with their supramolecular systems analoges.

Disperse and disordered: a mexylaminotriazine-substituted azobenzene derivative with superior glass and surface relief grating formation

Materials containing azobenzene chromophores exhibit photomechanical behaviors, including the formation of surface relief gratings (SRG) caused by irradiation with two interfering laser beams. While azo-functionalized polymers were extensively studied, small molecules offer the advantage of being monodisperse species, which translates into easier synthesis and purification, as well as more uniform behavior. A drawback is that they tend to crystallize and do not always form high-quality thin films. Glass-forming compounds incorporating azobenzene were previously synthesized in several synthetic steps and in low yield. Herein, a Disperse Red 1 (DR1) functionalized with a mexylaminotriazine group is synthesized in 94% yield using a simple and straightforward procedure. It shows both the ability to form extremely stable glassy phases, and the ability to form SRG in the solid state with growth rates and grating heights closely similar to DR1-functionalized polymers.

Photoinduced Orientation of Photoresponsive Polymers with N-Benzylideneaniline Derivative Side Groups

Axis-selective photoreaction, photoinduced orientation, and formation of surface relief (SR) gratings of liquid crystalline polymethacrylates comprised of N-benzylideneaniline (NBA) derivative side groups are explored. Irradiating with linearly polarized (LP) 313 nm light generates molecular reorientation of the NBA side groups based on the axis-selective trans–cis–trans photoisomerization and photocleavage of the NBA groups. The in-plane order parameter (S) and birefringence (Δn) are ∼0.53 and ∼0.16, respectively. Furthermore, thermally stimulated amplification of the photoinduced optical anisotropy occurs upon exposing films to LP 313 nm light and subsequently annealing in the liquid crystalline temperature range of the material. The amplified S and Δn values are ∼0.71 and ∼0.21, respectively. Finally, polarization holography using 325 nm He–Cd laser beams with various interferometric polarization conditions demonstrates the formation of SR gratings with a molecularly oriented structure based on the per...

Opto-Mechanical Scission of Polymer Chains in Photosensitive Diblock-Copolymer Brushes

In this paper we report on an opto-mechanical scission of polymer chains within photosensitive diblock-copolymer brushes grafted to flat solid substrates. We employ surface-initiated polymerization of methylmethacrylate (MMA) and t-butyl methacrylate (tBMA) to grow diblock-copolymer brushes of poly(methylmethacrylate-b-t-butyl methacrylate) following the atom transfer polymerization (ATRP) scheme. After the synthesis, deprotection of the PtBMA block yields poly(methacrylic acid) (PMAA). To render PMMA-b-PMAA copolymers photosensitive, cationic azobenzene containing surfactants are attached to the negatively charged outer PMAA block. During irradiation with an ultraviolet (UV) interference pattern, the extent of photoisomerization of the azobenzene groups varies spatially and results in a topography change of the brush, i.e., formation of surface relief gratings (SRG). The SRG formation is accompanied by local rupturing of the polymer chains in areas from which the polymer material recedes. This opto-mechanically induced scission of the polymer chains takes place at the interfaces of the two blocks and depends strongly on the UV irradiation intensity. Our results indicate that this process may be explained by employing classical continuum fracture mechanics, which might be important for tailoring the phenomenon for applying it to poststructuring of polymer brushes.

Soft Matter Beats Hard Matter: Rupturing of Thin Metallic Films Induced by Mass Transport in Photosensitive Polymer Films

The interface between thin films of metal and polymer materials play a significant role in modern flexible microelectronics viz., metal contacts on polymer substrates, printed electronics and prosthetic devices. The major emphasis in metal-polymer interface is on studying how the externally applied stress in the polymer substrate leads to the deformation and cracks in metal film and vice versa. Usually, the deformation process involves strains varying over large lateral dimensions because of excessive stress at local imperfections. Here we show that the seemingly random phenomena at macroscopic scales can be rendered rather controllable at submicrometer length scales. Recently, we have created a metal-polymer interface system with strains varying over periods of several hundred nanometers. This was achieved by exploiting the formation of surface relief grating (SRG) within the azobenzene containing photosensitive polymer film upon irradiation with light interference pattern. Up to a thickness of 60 nm, the adsorbed metal film adapts neatly to the forming relief, until it ultimately ruptures into an array of stripes by formation of highly regular and uniform cracks along the maxima and minima of the polymer topography. This surprising phenomenon has far-reaching implications. This is the first time a direct probe is available to estimate the forces emerging in SRG formation in glassy polymers. Furthermore, crack formation in thin metal films can be studied literally in slow motion, which could lead to substantial improvements in the design process of flexible electronics. Finally, cracks are produced uniformly and at high density, contrary to common sense. This could offer new strategies for precise nanofabrication procedures mechanical in character.

Surface relief grating formation in luminescent dye doped photochromic polymer containing azobenzene side groups

Abstract We study the formation of a surface relief grating and photoluminescence in a thin layers of a photochromic polymer doped with the luminescent dyes 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole and Rhodamine 6G. Surface topography measurements via Atomic Force Microscopy confirmed the existence of a surface relief grating with amplitudes as high as 650 nm both for doped and undoped photochromic polymers. Spectroscopic measurements carried out for polymers containing luminescent dyes have shown efficient photoluminescence and amplified spontaneous emission which is characteristic for gain media.

How does the isomerization rate affect the photoisomerization-induced transport properties of a doped molecular glass-former?

We investigate the effect of the isomerization rate f on the microscopic mechanisms at the origin of the massive mass transport found in glass-formers doped with isomerizing azobenzene molecules that result in surface relief gratings formation. To this end we simulate the isomerization of dispersed probe molecules embedded into a molecular host glass-former. The host diffusion coefficient first increases linearly with f and then saturates. The saturated value of the diffusion coefficient and of the viscosity does not depend on f but increases with temperature while the linear response for these transport coefficients depends only slightly on the temperature. We interpret this saturation as arising from the appearance of increasingly soft regions around the probes for high isomerization rates, a result in qualitative agreement with experiments. These two different physical behaviors, linear response and saturation, are reminiscent of the two different unexplained mass transport mechanisms observed for small or large light intensities (for small intensities the molecules move towards the dark regions while for large intensities they move towards the illuminated regions).