Azopolymer Films Research Articles

Cognitive ability process at the molecular level

In the standard SRG formation in azo-dye containing photoactive polymers, the photoactive molecules are excited by a coherent illumination pattern. The highly reactive molecules move in a non-uniform way, inducing a mass transport from the bright regions to the neighbouring dark regions. The maximum heights of the light induced SRG correspond to light intensity minima. We show that a well-defined surface relief grating is induced in an azo-polymer film by the combination of one low power coherent laser beam with another high power incoherent and unpolarised beam. The information brought by the coherent signal beam has been transmitted to peripheral incoherent regions by the molecular self-assembling process: i.e., the organised molecules communicate non-local information about photo-induced structural organisation to the non organised neighbouring ones. They communicate by exchanging light through surface relief variations. In this way, we see that a totally incoherent beam can provide the movement which is necessary to induce a well-defined SRG. We find in this way one of the simplest systems allowing to figure out the minimal requirements to organise disordered materials into well organised structures. We verify experimentally that random motion plus information exchange lead to self-organisation. Our experiment shows that complex behaviour can be experimented using simple systems: weak coherent light can serve as a seed to create information into a polymer film in such a way that molecules powered by incoherent light will build and transmit well defined complex structures.

Switching Device Based on a Thin Film of an Azo-Containing Polymer for Application in Memory Cells

Results obtained on a conductive multilevel device realized with a 100 nm azo-polymer film are reported. The capability to switch between three different conductance states makes the device versatile both for Write Once Read Many and Write Many Read Many memory cell application. The analysis of the - curves indicates that the hysteretic conductance change with the external voltage can be explained in terms of different hopping distances existing between the nearest neighbors intervening in the two conformational states of the molecular structure.

Optically controllable polarized luminescence from azopolymer films doped with a lanthanide complex

Two azopolymers (DACENO2 and DACEOCH3) with a special structure were designed and synthesized. The azopolymer films doped with the lanthanide complex Eu(TTA)3Phen were prepared by a casting method with a mixed solution of the azopolymer and lanthanide complex. UV-vis spectra show that the absorption peaks of the predesigned azopolymers DACENO2 and DACEOCH3 are separated from that of Eu(TTA)3Phen. Under this circumstance, the photo-induced anisotropy of the films could not be destroyed by excitation and emission during fluorescence measurements. Polarized luminescence of both oriented films was observed with polarization ratios of 3.0 for the DACENO2–Eu(TTA)3Phen film and 2.7 for the DACEOCH3–Eu(TTA)3Phen film at 613 nm, respectively. The luminescent properties of the film were found to be affected by the chemical structure of the azopolymer, the extent of the photo-induced alignment and the angle between the orientated direction of the film and the direction of the polarizer. All the results from this work revealed that the polarized luminescence from the azopolymer film doped with a lanthanide complex can be controlled by the extent of photo-induced alignment.

Surface relief gratings on poly(3-hexylthiophene) and fullerene blends for efficient organic solar cells

The use of periodic submicrometer structures as an efficient light-trapping scheme was investigated for high performance organic solar cells (OSCs) based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61. The gratings on an active layer are achieved by a soft lithographic approach using photoinduced surface-relief gratings (SRGs) on azo polymer films and poly(dimethylsiloxane) as a master and stamp, respectively. Incident photon to current conversion efficiency and the power conversion efficiency of OSC with gratings increased primarily due to enhanced short circuit current density, indicating that SRGs induce further photon absorption in active layers by increasing the optical path length and light trapping.

Fabrication of Gold Nano‐Structures with Azopolymer Templates†

Fabrication of gold nano‐patterns has been demonstrated employing surface relief structures created on films of an azobenzene‐functionalized polymer as templates. The surface relief templates were photoinscribed on the azopolymer films in one‐step with two laser beams. Thin layers of gold were over‐coated on the polymer templates by thermal evaporation. Gold lines of a few hundred nanometer width were successfully fabricated by pyrolyzing the azobenzene polymer. Sub‐micron gold dots were also created. The resulting gold structures exhibited the same periodicity as the polymer templates.

Photoinduced dichroism and surface-relief-gratings of hyperbranched azo polymers synthesized by azo-coupling reaction

This article reports the synthesis and photoresponsive properties of a hyperbranched azo polymer containing electron donor-acceptor type azo chromophores both in backbone and peripheral positions. For the synthesis, a hyperbranched azo precursor polymer was synthesized by step-growth polymerization of an AB2 monomer, bis[2-(N-ethylanilino)ethyl] 5-aminoisophthalate, through azocoupling reaction. The hyperbranched precursor polymer was then reacted with the diazonium salt from 4-cyanoaniline to introduce donor-acceptor azo chromophores in the peripheral groups. The resulting azo polymers were characterized by the spectroscopic methods and thermal analysis. The photo-induced dichroism of the hyperbranched polymer was studied and its orientation order parameter was estimated to be 0.063. Thin films of the hyperbranched azo polymers were used to fabricate surface-relief-gratings (SRGs) by exposing them to an interference pattern of Ar+ laser beam at modest intensities (150 mW/cm2). The azo chromophores introduced by post-azo-coupling reaction at the para-positions of the peripheral azobenzenes 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, photoswitch, sensors, and other photo-driven devices.

All-Optical Switching Based on Azo Polymer Material

Conventional all-optical switches based on azo polymer films and theall-optical switches based on the attenuated total reflection (ATR)geometry are investigated. A conventional switch system, including apump beam of 532 nm and a probe beam of 650 nm, is based on the photoinducedbirefringence effect of azo polymer. An ATR switch in aprism-multilayer configuration is achieved by changing the reflectanceof the probe beam with an external pump beam. The ATR method providesthe substantial improvement of the speed and the efficiency of themodulation over the conventional method. Although the azo polymerresponse still remains relatively slow, an enhanced nonlinear refractiveindex of the azo polymer film can effectively increase themodulation.

Experimental demonstration of photonic bandgaps in azopolymer resonant waveguide grating systems

Photonic bandgaps are demonstrated in one-dimensional corrugated waveguides in the infrared range. A coupling grating is superimposed with single and double Bragg gratings on an azopolymer film by a simple optical process, which allows easy control of the grating spacing. Light is coupled to the TE(0) resonant mode, and gaps in the dispersion curve are introduced by careful selection of the gratings. The analysis is carried out by measuring the transmission through the waveguide as a function of the wavelength and angle of incidence of a probe beam. This results in a direct measurement of the dispersion curves, which are in excellent agreement with theory.

Polyimide liquid crystal alignment layers prepared by soft-lithography

Elastomeric replicas of surface relief structures were prepared by molding poly(dimethylsiloxane) (PDMS) precursors against photo-inscribed surface-relief-gratings on azo polymer films. The PI solutions were micro-contact printed with the elastomeric PDMS replica on quartz slides. Good surface-relief-grating structures were formed on the quartz slides. The quartz slides covered with surface-relief-grating polymide (PI) films were then assembled into liquid crystal (LC) cells. The transmittance passing through the cell between crossed polarizers changed periodically with a regular 90° separation of the rotational angle. The pretilt angle of the cell was found to be 2.8°. Results showed that the PI films with surface-relief-grating structures by this micro-contact printing process have good liquid crystal alignment ability. This preparation method of alignment layers can be considered a potentially useful technique in the LC display (LCD) industry in the future.

Synthesis and Electrooptical Properties of Metal-Containing Azopolymers

We propose new structures of polymeric composites with changeable optical characteristics (transmission and/or reflection of polarized light) under the influence of magnetic and/or electric fields. The new monomers 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-azobenzene, 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-2-nitroazobenzene,and 4-methacroyloxy-(4′-carboxy-3′-hydroxy)-2-diethylamino-azobenzene, their complexes with Co, and polymers based on their base are synthesized. The influence of the electric field on the absorption spectra of films of the new metal-containing azopolymers is investigated. The effect of an electric field on the transmission of a linearly polarized light through the films is revealed. This effect is determined by the re-orientation of the dipole moments of azobenzene groups which is induced by a polarized light under the action of an electric field. The increase of the dipole moments of azobenzene groups due to the introduction of donor and acceptor substituents reduces the influence of Co ions on the electrooptical properties of polymeric films.

Analysis of Surface Relief Gratings on Azo Polymer Films

Photoinduced surface relief gratings (SRGs) formed on Azo polymer films by using a two-beam interference technique were observed and analyzed. The heights of the SRGs at various polarization combinations of two writing beams were measured and compared with the force distributions from various theoretical models. As a result, we could suggest a combined model of the optical gradient force model and the pressure gradient force model by which the measured data could be explained well.

Optical phase-front inscription over optical fiber end for flexible control of beam propagation and beam pattern in free space

We report a novel compact all-fiber technique to control the beam propagation and beam pattern in free space. By coating an azo-polymer thin film layer on the optical fiber end surface, various surface relief grating (SRG) structures were inscribed over the film using a single step direct exposure of interference patterns to form a novel organic/inorganic composite fiber optics beam shape converter. The linear and concentric SRGs were experimental characterized along with numerical simulations.

Confinement of surface patterning in azo-polymer thin films

Azobenzene polymer thin films are known to spontaneously generate surface patterns in response to incident light gradients. This peculiar process is investigated in terms of the dynamics of the various azobenzene photomotions, which occur on different length scales. In particular, the formation and thermal erasure of surface relief gratings are measured as a function of film thickness and by using combinatorial samples with thickness gradients. The thermal erasure of gratings in this system provides a direct measure of the glass-transition temperature, which is found to deviate substantially from the bulk value. Thin azo films exhibit a glass transition up to 50 K higher than the bulk. These dynamical measurements allow the authors to probe the length scale of mass transport, which is found to be approximately 150 nm. Furthermore, surface mass transport is completely arrested in thin films<40 nm. According to these results, mass transport involves the coordinated motion of many polymer chains in the depth of the sample, rather than surface diffusion of individual chains.

The anisotropic nanomovement of azo-polymers

Nanoscale polymer movement is induced by a tightly focused laser beam in an azo-polymer film just at the diffraction limit of light. The deformation pattern that is produced by photoisomerization of the azo dye is strongly dependent on the incident laser polarization and the longitudinal focus position of the laser beam along the optical axis. The anisotropic photo-fluidity of the polymer film and the optical gradient force played important roles in the light induced polymer movement. We also explored the limits of the size of the photo-induced deformation, and we found that the deformation depends on the laser intensity and the exposure time. The smallest deformation size achieved was 200 nm in full width of half maximum; a value which is nearly equal to the size of the diffraction limited laser spot.

Photoinduced Immobilization of Biomolecules on the Surface of Azopolymer Films and Its Dependence on the Concentration and Type of the Azobenzene Moiety

We report on the photoinduced immobilization of Immunoglobulin G (IgG) on the surface of azobenzene-bearing acrylate copolymers (azopolymers). Two different types of azopolymers were synthesized that incorporated either a 4-aminoazobenzene moiety (H-azopolymer) or a 4-aminocyanoazobenzene moiety (CN-azopolymer), using different concentrations of the respective moieties. IgG was immobilized on the surfaces of the azopolymer films by exposure to visible light, and each of the films was then treated with an aqueous solution of an antigen. Antigen−antibody reactions were confirmed on the surfaces of the films, indicating that immobilized IgG generated by a photoirradiation can retain its activity. The amount of the immobilized antibodies on the azopolymer surfaces increased with azobenzene content up to 30 wt % and saturated over 30 wt % when measured under the same conditions. The efficiency of the immobilization process was found to correlate with the depth of deformations on the surfaces of the azopolymers...

Facile fabrication of close-packed microlens arrays using photoinduced surface relief structures as templates

We demonstrate the cost-effective and facile method of fabricating close-packed microlens arrays using photoinduced two-dimensional (2-D) surface relief structures as original templates. 2-D surface relief structures are produced by successive inscription of two beams interference patterns with different grating vectors on azopolymer films. The employed exposure dose of 1st inscription stage and 2nd inscription stage are optimized to obtain symmetrical modulation heights. These photoinduced 2-D surface relief structures on azopolymer films are used directly to mold PDMS, and PDMS molds were then transferred onto photopolymer to imprint microlens arrays. Using this method, tetragonally and hexagonally close-packed microlens arrays are successfully fabricated in rapid and cost-effective way.

Inscription of local surface relief gratings with a Scanning Near-field Optical Microscope on an azo-polymer film

A Scanning Near-field Optical Microscope (SNOM) was used to inscribe locally limited Surface Relief Gratings (SRGs) on pDR1M, which is an azo-polymer film. The influence of parameters such as the line scanning time and the line spacing on the amplitude and the period of the gratings is discussed. SRGs have been used to orient Liquid Crystals (LCs), where the LC alignment takes place due to the elastic distortions of the nematic medium close to the surface. In addition, the photoalignment of the azo-chromophores was also used to induce LC alignment. In this work we investigate the use of local SGRs together with the photoalignment to get a local control of the LC alignment properties. The advantage of this method is that it is reversible and the gratings can easily be erased.

Photomechanical Surface Patterning in Azo-Polymer Materials

Azobenzene thin films undergo an unexplained spontaneous surface patterning when exposed to light intensity and/or polarization gradients. The elastic modulus of an azobenzene−polymer film is measured before and during laser irradiation using AFM indentation experiments. It is found that there is no significant change in elastic modulus with laser illumination, indicating that photosoftening can be neglected in these systems. In particular, this eliminates mechanisms that require photosoftening as candidate explanations for azo surface patterning. AFM measurements of patterning in azo-polymer thin films, irradiated at various temperatures, are compared to recent neutron reflectometry measurements of photomechanical effects in the same material. The magnitude and sign of the patterning exactly match the literature trend for photomechanical effects. This represents the first report of measuring both photoexpanded and photocontracted surface patterns in the same material, at different temperatures. These results are interpreted to mean that the unexplained surface mass transport phenomenon observed in the azobenzene system is in fact due to this newly identified photomechanical effect. Previous patterning results are discussed in terms of this explanation, and it is shown that the photomechanical effect can explain the vast majority of the literature results to date.

Transitions in the orientational order of liquid crystals induced by periodic patterned substrates

The orientational order of liquid crystals (LCs) induced by periodic patterned substrates has been investigated with cells coated by azopolymer films that could be photoaligned in a controlled way. Two regimes were observed depending on the period of the patterns: (i) above 3.0 microm the LC follows the direction imposed by the patterned substrate since the energy stored in the surface potential minimizes the elastic energy of the LC medium. (ii) For periods smaller than 1.0 microm a homogeneous in-plane state was induced and the LC did not follow the orientation imposed by the surface. This in-plane transition could be explained qualitatively by a theoretical model based on the competition between the Frank-Oseen elastic energy and the phenomenological surface potential. The results also suggest an out-of-plane transition for the LC director as the period was reduced. These results agree with data in the literature for patterned substrates with completely distinct architectures. This indicates that for a particular LC sample the overall behavior depends basically on the texture period instead of the texture architecture. The textures were characterized with a scanning near-field optical microscope (SNOM), which allowed simultaneous morphological and optical images in the submicrometer range.

Surface relief gratings on azo polymer films through reversible photoisomerization by the irradiation of a monochromatic light

AbstractSurface relief gratings were induced on azobenzene‐functionalized polymer films by exposure to s‐polarized and circularly polarized Ar+‐laser (488‐nm) irradiation through a photomask. The surface relief structures were investigated with atomic force microscopy. A very regularly spaced surface grating with a modulation depth of about 400 nm was obtained after 30 min of circularly polarized Ar+‐laser irradiation. The diffraction efficiency was measured after irradiation by an Ar+ laser, and an efficiency of about 2% was achieved. The polymer films also showed a very high period of recording. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3123–3126, 2006