Thomas Bieringer

Photoaddressable Polymers for Optical Data Storage

Photoaddressable polymers (PAPs) are side-chain copolymer systems functionalized with azobenzene chromophores and mesogenic side groups. They show very large birefringence values after illumination with polarized light. The molecular mechanisms involved are photoinduced isomerization cycles of the azo side groups, which induce strong molecular reorientations (cooperative motion of light-absorbing azobenzene chromophores and non-absorbing mesogenic groups). Due to the long-term and high-temperature stability of their light-induced birefringence, PAPs are very promising recording materials for optical data storage applications such as high-capacity DVDs and holographic memory.

Photoaddressable Alignment Layers for Fluorescent Polymers in Polarized Electroluminescence Devices

Liquid-crystalline (LC) polyfluorenes have been successfully aligned on photoaddressable polymers (PAPs). This is the first example of the alignment of a LC main chain polymer on a photoaligned layer. The degree of molecular alignment in the fluorescent polyfluorene layer on top of an ultra-thin PAP layer is shown to depend strongly on the chemical nature of the PAP. Good alignment with dichroic ratios of more than 10 was only achieved with PAPs containing liquid-crystalline side chains. Patterning with laterally structured alignment was realized in several ways, utilizing reorientation with orthogonally polarized light. Thin PAP layers have further been utilized as hole-conducting alignment layers in polymer light-emitting diodes (LEDs) with polarized emission. In order to facilitate hole transport through the alignment layer, different concentrations of a hole-transporting molecule (HTM) have been mixed into the PAP layer. These hole-conducting alignment layers retained their aligning abilities even at HTM concentrations of 20 wt.-%. LEDs with photometric polarization ratios in emission of up to 14 at a brightness of up to 200 cd/m2 and an efficiency of 0.3 cd/A could be realized.

Surface relief gratings generated by pulsed holography: A simple way to polymer nanostructures without isomerizing side-chains

Photoaddressable polymers based on azobenzene side-chains have recently drawn a lot of industrial attention due to their possible application as data storage media. From the scientific point of view an additional effect, the occurrence of light induced surface relief gratings, is nowadays extensively studied. This article describes the behavior of side-chain and guest–host polymers (with and without azobenzene groups) in holographic experiments with pulse lasers. We observe surface relief gratings also for polymers, which only contain an absorbing, but nonisomerizing chromophore. Relief formation in pulsed holographic experiments is shown to arise from thermal effects, but not from trans–cis isomerization.

Photoinduced orientations of azobenzene chromophores in two distinct holographic diffraction gratings as studied by polarized Raman confocal microspectrometry

Optically isotropic thin films (0.5 μm) of an amorphous side-chain copolymer containing 10% azobenzene moieties and 90% meso-azobenzene groups (K1-10 sample) were irradiated by two interfering orthogonal (±45°) or parallel (p + p) linearly polarized laser beams; permanent holographic diffraction gratings were thus inscribed. From atomic force microscopy (AFM) measurements, relatively weak amplitudes (about 50–60 nm) in the surface relief modulation (SRM) are detected but strong perturbations in the profiles and new substructures are evidenced at half-period positions. Using a confocal microspectrometric technique we have recorded various pre-resonance enhanced polarized Raman spectra from a large grating area and obtained spatially resolved Raman images of the intensity variations. This yields new insight into the photoinduced orientation effects, the angular distributions of the chromophore species, and finally the grating formation mechanisms. Different theoretical equations of the polarized Raman scattering intensities, taking account of the pump polarization directions and the high numerical aperture objective used, are derived. Then, an analysis of the experimental results allows us to extract values of the second 〈P2〉 and fourth 〈P4〉 coefficients in the chromophore orientation functions at several positions of the SRM, in particular at the top and bottom regions, and to calculate the corresponding normalized distribution functions ia information entropy theory. In both gratings, somewhat broader distributions are found in the bottom regions, regions of greater polymer removal. Thus, mass-transport phenomena have perturbed, to a weak extent in the (±45°) first case and to a larger extent in the (p + p) second case, the primary photoinduced orientations. So, two simultaneous and superimposed mechanisms appear to be responsible for the observed complex SRM in the highly birefringent K1-10 copolymer under study and significant second-order contributions are suggested. The grating surface profiles are thus reproduced by fitting the SRM amplitudes at the fundamental and doubled frequency and the corresponding phase constant parameters: a good agreement between the observed (AFM) and simulated profiles is obtained.

Photoaddressable Polymers for Rewritable Optical Disc Systems

We present results of the feasibility study on photoaddressable polymers (PAPs) applied to optical disc systems. PAPs represent a new class of organic rewritable materials that exhibit a huge signal birefringence (Δn) at the readout wavelength. By optimizing the disc structure and the readout optics, the signal amplitude is proved to be sufficient for the optical disc systems. A low noise profile and sharp recording profile led to a maximum carrier-to-noise ratio (C/N) of 58 dB and a clear eye-pattern. They also showed a potential for multi-level recording, since the recording was dominated by a pure photon mode.

Comparison of the birefringence in an azobenzene-side-chain copolymer induced by pulsed and continuous-wave irradiation

A comparative study of photoinduced birefringence in azobenzene-side-chain copolymers under continuous-wave (cw) and pulsed irradiation is presented. Absolute refractive index changes were determined by the attenuated total reflection spectroscopy. We clearly demonstrated that pulsed, similarly to cw, irradiation induces a three-dimensional reorientation of azobenzene chromophores. For the same light dose, pulsed irradiation using ns pulses of appropriate pulse energy led to higher values of birefringence, which can be attributed to a cooperative motion of azobenzene side chains and thermal effects induced by the laser pulses.

Length-scale dependence of surface relief gratings in azobenzene side-chain polymers

Polarized irradiation of azobenzene side-chain polymers yields intriguing surface relief structures. Up to now, the physical mechanism of this process is unclear. We report on a study aimed at clarifying the intrinsic length scales of the substantial mass transport which evolves during laser irradiation. We vary the period of an incident intensity grating by more than an order of magnitude, and investigate the resulting relief amplitudes with atomic force microscopy.

An Investigation of the Photoinduced Changes of Absorption of High-Performance Photoaddressable Polymers

Recently, a novel class of photoaddressable polymers (PAPs) has been reported, which allows for reversible photorecording with values of the photoinduced birefringence in excess of 0.2. These polymers are copolymers of methacrylate-type monomers with a mesogenic and a nonmesogEnic photoactive azobenzene in the side chain. This report presents a detailed analysis of the three-dimensional photoinduced reorientation of side chains within thin layers of PAP materials, utilizing polarized UV/Visible spectroscopy, infrared, and photothermal deflection spectroscopy (PDS). The three-dimensional orientation distribution before and after illumination with 514 nm polarized light was investigated by angle-dependent UV/Vis absorption spectroscopy. As-prepared samples possess an uniaxial orientation with the photoactive units being preferentially aligned within the plane of the substrate layer. These films become biaxial upon photoalignment with polarized light incident normal to the substrate plane. For PAP materials containing mesogenic side chains, the out-of-plane absorption stays nearly unchanged, proving that the photoinduced reorientation occurs mainly within the layer plane. For a homopolymer bearing only a nonmesogenic azobenzene in the side chain, photobleaching is observed, associated with a disease in absorption in all three principle directions. This finding can be understood in terms of an orientational coupling between the photodegradation product and the active azobenzene units. Photodegradation was well established by infrared spectroscopy. Photothermal deflection spectroscopy has been performed to manifest photoinduced changes in the transparent wavelength region of the PAP material, below the absorption edge. In contrast to the pronounced optical changes in the UV/Vis and IR spectra, the absorption in this range remains nearly unchanged. This result provides evidence that the polymers can be well applied for holographic storage in thick-layer devices.

Chromophore orientation and the formation of surface reliefs in azobenzene side-chain polymers

The occurrence of surface relief gratings in azobenzene polymers under illumination with two superimposed laser beams is a puzzling effect which was discovered only few years ago. The origin of the large mass transport which occurs far below the glass transition temperature is not yet understood. We have performed experiments with cw holography. Due to low laser fluence, thermal effects can mostly be neglected. We have studied the relation between the light induced orientation of the chromophores in the bulk of the material and the observed surface relief gratings. We have performed a number of studies: Surface relief gratings were characterized under different polarization geometries of the writing beams, furthermore the grating period was varied. We could show that there is a competing behavior between the diffraction efficiencies by the volume grating and the surface relief, respectively. In order to analyze the orientational distribution function of the azobenzene chromophores, we have performed confocal Raman microspectroscopy in different regions of the surface relief grating. Our results are not in agreement with the current theories for the formation of surface reliefs.