Azopolymer Thin Films Research Articles

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.

Spectroscopic Linear and Nonlinear Optical Characterizations of Azopolymer Gratings Inscribed on p(DR1M) Thin Films

ABSTRACT In this study we have investigated the electrical poling effects in a p(DR1M) azo-polymer thin film and in related diffraction gratings inscribed on other films. Their optical properties were determined using polarization measurements in UV-visible spectroscopy, in confocal Raman micro-spectrometry and in second harmonic generation (SHG) in the transmission mode. Near-field scanning optical microscopy (NSOM) experiments together with SHG responses in reflection were also performed in order to retrieve simultaneously the topography and the SHG contrasts of the surface gratings. This multitechnique approach led to the determination of the first even and odd parity Legendre polynomials, from ⟨P 1⟩ up to ⟨P 4⟩, allowing the establishment of precise chromophore orientational distribution functions. In particular, from SHG near-field scanning optical microscopy (NSOM) experiments we have evidenced that poled gratings with large diffraction efficiencies and χ(2) nonlinear susceptibilities exhibit new sub-structures with puzzling differences from their topographical AFM images. A combination of all the linear and nonlinear results allows us, for the first time, to explain the peculiar orientational and mechanical properties in the peak areas of the surface relief modulations, at near each half-period grating (Λ/2 ≈ 0.7μm). An efficient poling of the azobenzene dye groups in such gratings thus leads to new interesting properties.

Biaxial Orientation Induced in a Photoaddressable Azopolymer Thin Film As Evidenced by Polarized UV−Visible, Infrared, and Raman Spectra

The main goal of this spectroscopic study was to obtain relevant information about the symmetry properties for a thin film of a new class of photoaddressable azopolymers, which are known to display a huge birefringence (Δn exceeding −0.45 at 632.8 nm) and a high stability and are attractive materials for rewritable recording media and optical memories. First, various real-time birefringence experiments are performed using a linearly polarized irradiation at 532.0 nm and probing the thin film sample at 675.0 nm to confirm the largest Δn values ever reported for such amorphous polymers. Then, polarized transmission measurements in the UV−visible and mid-infrared spectral ranges are compared for an isotropic and anisotropic sample. They provide strong evidences for cooperative orientational effects and for the establishment of a biaxial symmetry; the two average orientation factors 〈F2,0〉 and 〈F2,2〉 are thus extracted from infrared data for several vibrational modes. The remaining and related fourth rank factors, namely 〈F4,0〉, 〈F4,2〉 and 〈F4,4〉, are then determined from the confocal micro-Raman spectra recorded under various polarization configurations, more precisely from the two Raman intensity ratios, R1 = IXY/IXX and R2 = IYX/IYY. Reasonable values of the different orientation order parameters are thus obtained allowing to determine the overall orientation distribution function of the chromophores, in particular for the ω(CC) ring (8a) vibrational mode at 1599 cm-1. Definitive conclusions are thus drawn about the effective biaxial symmetry in this copolymer sample.

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

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

Molecular Photonics and Electronics

Molecular Photonics and Electronics

Properties of birefringence and surface relief gratings inscribed on azopolymer thin films using various interference patterns

Polarization properties of transmitted and diffracted orders from holographic diffraction gratings inscribed on azopolymer thin films have been analysed for selected polarization configurations of the interfering beams. Using the Jones matrix formalism, the polarization states of transmitted and diffracted orders are predicted considering either a pure polarization grating or a combination of a polarization and a surface relief grating for co-and contra-circularly polarized beams, linearly orthogonal polarized beams (p + s) and parallel in-plane polarized beams (p + p). In the (p + s) setup, the formation of a twice-frequency surface modulation, whose origin comes from an additional interference between two first diffracted orders, is reported. In the case of the (p + p) polarization configuration, the optical erasure process with a single beam is investigated at various levels of diffraction efficiency with the aim of finding optimum conditions to duplicate surface relief gratings using a polarized single beam exposure.

Photoinduced linear and/or circular birefringences from light propagation through amorphous or smectic azopolymer films

The self-induced rotation of the azimuth of elliptically polarized light passing through birefringent azopolymer thin films is investigated. The experiments were carried out on thin films of the amorphous p(DR1M-co-MMA) and p(DR1M) azopolymer samples and of the p(6MAN) derivative in its glassy and liquid-crystalline phases. In fact, using various controlled input light ellipticities, linear birefringence (LB) and/or circular birefringence (CB) measurements were performed separately and in conjunction with polarization analyses of the transmitted pump beam. According to a general theoretical analysis based on Jones’ matrix formalism, it is thus shown that the induced rotation angle through the films depends mainly on the ellipticity of the input light, on the generated LB level and, to a lesser extent, on the CB photoinduced in the liquid-crystalline phase. In the latter case, it is concluded that irradiation with circularly polarized light does induce a chiral arrangement in the polymer film, although the photosensitive chromophores do not contain any optically active group.

Inscription of holographic gratings using circularly polarized light: influence of the optical set-up on the birefringence and surface relief grating properties

The polarization properties of the optical set-up used for holographic recording of diffraction gratings on azopolymer thin films are analyzed. The state of polarization of circularly polarized light is fully analyzed after reflection on a mirror at various incidences (Lloyd-mirror set-up). The Stokes analysis is performed using a photopolarimeter and the phase shift, the ellipticity and the azimuth orientation are compared with those calculated from Fresnel formulae. At large angles of incidence, an initially right circularly polarized (RCP) beam becomes elliptically polarized with an azimuth of nearly +45°. From these results, holographic diffraction gratings are recorded on an azobenzene-containing polymer thin film using (i) co- and contra-circularly polarized beams and (ii) a right circularly polarized beam interfering with a +45° linearly polarized light beam. Using Jones-matrix formalism, the polarization states of the diffracted orders from the birefringence (Δn) and the surface-relief (2Δd) gratings are derived and compared with experimental measurements. Finally, the induced local birefringences and surface-relief amplitudes are discussed in connection with atomic force microscopy measurements. The diffraction efficiencies obtained under the (+45°+RCP) and (LCP + RCP) (where LCP = left circularly polarized) configurations are thus compared and discussed.

A Detailed Investigation of the Polarization-Dependent Surface-Relief-Grating Formation Process on Azo Polymer Films

We have carried out two-beam interference experiments to form surface-relief gratings (SRGs) on azo polymer films under different polarization combinations. The mechanism of SRG formation on azobenzene chromophore functionalized polymer films has been shown to depend on both the spatial variation of the intensity as well as the polarization state of the incident field pattern. The treatment presented in this paper clearly defines the intensity and polarization distribution across the two-beam interference region for different polarization configurations of the interfering beams. The efficient photoisomerization process due to the spatial variation of the optical field renders the material anisotropic and the surface mobile. The simultaneous presence of a component of electric field gradient in the grating vector direction results in large-modulation-depth SRGs. We have carried out this investigation in one class of prototypical amorphous epoxy-based azo polymer thin films. Our results confirm that efficient SRG formation on azo functionalized polymer film requires photoinduced anisotropy and a component of optical field gradient in the direction of the grating vector.