Hubert Audorff

Polarization Dependence of the Formation of Surface Relief Gratings in Azobenzene-Containing Molecular Glasses

This paper presents a comprehensive study of the formation of surface relief gratings in a series of photoresponsive molecular glasses. Holographic experiments were performed on films of the azobenzene-containing molecular glasses. Seven relevant polarization configurations of the writing beams were systematically applied, and simultaneously the diffraction efficiency was monitored during the process of inscription. The temporal evolution of the diffraction efficiency can be precisely simulated with a model which takes both the surface relief and the phase grating in the volume into account. From the measured diffraction efficiencies, the modulation heights can be directly calculated and they were independently confirmed by atomic force microscopy. We found that all experimental results can be explained with the gradient force model, and we suggest that the grating heights generated with different writing polarizations can be ascribed to the varying strengths of the gradient force. For materials with different substituents at the azobenzene chromophore, the optical susceptibility at the writing laser wavelength and, therefore, the gradient force varies. By applying the most efficient polarization configuration in combination with the best material, we were able to reach modulation heights of up to 600 nm, which is a factor of 2 higher than modulations usually reported for azobenzene-containing polymers.

Holographic Gratings and Data Storage in Azobenzene-Containing Block Copolymers and Molecular Glasses

This review covers synthesis, materials development, and photophysics of azobenzene-containing block copolymers as potential media for reversible volume holographic data storage. For high-density holographic data storage, volume gratings must be inscribed in millimeter-thick samples to achieve efficient angle multiplexing. It is demonstrated that block copolymers with azobenzene side-groups in the minority block develop no detrimental surface relief structures and exhibit superior performance regarding volume gratings, compared to homopolymers and statistical copolymers. Several material concepts for optimizing the refractive index modulation and the stability of volume gratings are presented. Stabilities of more than 2 years were achieved. Most important is the development of polymer blends comprising the azobenzene-containing block copolymer and an optically transparent homopolymer. This enables the preparation of millimeter-thick samples with the required optical density of ∼ 0. 7 at the writing wavelength by conventional injection molding techniques. The inscription of up to 200 holograms at the same lateral position was demonstrated. In addition, more than 1,000 write/erase cycles can be performed. This is the first time that the inscription and erasure of the long-term stable angle-multiplexed volume gratings in a rewritable polymeric medium have been achieved by purely optical means. A second important application for azobenzene-containing materials is the controlled preparation of surface relief structures. It is demonstrated that azobenzene-containing molecular glasses are an ideal class for efficient formation of surface relief gratings (SRGs) with amplitude heights of more than 600 nm. Clear relationships can be established between the chemical structure of the molecules and the behavior of SRG formation. All results are in agreement with the gradient force model by Kumar et al. The surface patterns are stable enough to be transferred to a polymer surface via replica molding.

Holographic Investigations of Azobenzene‐Containing Low‐Molecular‐Weight Compounds in Pure Materials and Binary Blends with Polystyrene

This paper reports on the synthesis and the thermal and optical properties of photochromic low-molecular-weight compounds, especially with respect to the formation of holographic volume gratings in the pure materials and in binary blends with polystyrene. Its aim is to provide a basic understanding of the holographic response with regard to the molecular structure, and thus to show a way to obtain suitable rewritable materials with high sensitivity for holographic data storage. The photoactive low-molecular-weight compounds consist of a central core with three or four azobenzene-based arms attached through esterification. Four different cores were investigated that influence the glass transition temperature and the glass-forming properties. Additional structural variations were introduced by the polar terminal substituent at the azobenzene chromophore to fine-tune the optical properties and the holographic response. Films of the neat compounds were investigated in holographic experiments, especially with regard to the material sensitivity. In binary blends of the low-molecular-weight compounds with polystyrene, the influence of a polymer matrix on the behavior in holographic experiments was studied. The most promising material combination was also investigated at elevated temperatures, at which the holographic recording sensitivity is even higher.

Photo-induced molecular alignment of trisazobenzene derivatives

Optically active small-molecular trisazobenzene derivatives are explored that allow facile photo-induced fabrication of holographic volume gratings which are unusually stable over time compared to structures based on other small-molecular organic compounds. The origin of this favorable characteristic of such architectures is investigated with three compounds that structurally differ only in the length of an alkyl spacer positioned between the molecular core and the active azobenzene chromophores. Species comprising spacers of sufficient length, and that exhibit a latent liquid-crystalline phase, feature efficient formation of stable, ordered domains in which the three side groups orient perpendicular to the polarization of the inscribing light beam. It is demonstrated that molecular order in these domains can be significantly improved by annealing at temperatures between the glass transition temperature and the clearing point of the specific compounds. This phenomenon allows combining the highly desirable processing characteristic of relatively short holographic writing times with small molecules in the fabrication of stable volume gratings, the latter feature so far having been reserved predominantly for polymeric species.

Holographic Studies of Azobenzene-Containing Low-Molecular-Weight Organic Glasses

The formation of phase and surface relief gratings in low-molecular-weight organic glasses containing azobenzene moieties has been studied with holographic methods. Advantages of this class of materials are the simple synthesis, the perfectly amorphous phase, and the possibility of blending them with polymers. Surface relief gratings are formed very efficiently in molecular glasses, and this process can be explained by the gradient force model. Heights up to 610 nm were measured; the temporal evolution of the diffraction efficiency could be reproduced in computer simulations. For technical applications, the surface relief gratings can easily be duplicated by replica molding. Since surface gratings are detrimental to holographic data storage at high densities, it is also possible to suppress their formation by using proper polarizations of the writing beams. Reorientation of the azobenzene groups in the bulk of the glasses and angular multiplexing was demonstrated and the thermal stability of the corresponding phase gratings was studied. Different combinations of molecular cores and substituents at the azobenzene moieties were investigated to find the best systems which yield a high sensitivity and fast grating build-up.

Blends of azobenzene-containing polymers and molecular glasses as stable rewritable holographic storage materials

In recent years the developments in computer technology have drastically accelerated. In order to handle the ever growing amount of data to be stored, an increase in the storage capacities of mass storage media is necessary. A promising approach is holography, where the storage capacity is greatly increased by using the entire volume instead of only the surface of the medium. Write-once media, which are mainly based on photopolymer systems, seem to be fairly advanced. For a rewriteable media, only few systems can meet the strict requirements for holographic data storage materials. Photoaddressable azobenzene-based polymer systems are the most promising candidates, however the photo-physical sensitivity of these materials has to be further increased. Low molecular-weight organic glasses with azobenzene moieties can also be used for reversible inscription of holographic volume gratings. They exhibit a faster response time than a comparable photoaddressable polymer due to a lack of chain entanglements. A new photochromic molecular glass with optimized physical and photo-physical properties is investigated as blending materials to improve the photochromic response of photoaddressable polymers. By doping this photochromic molecular glass into photoaddressable polymers we are able to combine the high stability of polymer systems with the fast response of molecular glasses, thus creating a system which has the advantages of both material classes.

Blends of azobenzene-containing diblock copolymers and molecular glasses for holographic data storage

Azobenzene-containing diblock copolymers are a promising material class for holographic data storage. They have many advantages, yet, a problem are their long writing times. Low-molecular-weight glasses containing azobenzene moieties are also photo-addressable and develop holographic gratings much faster. By blending molecular glass formers with diblock copolymers, materials for holographic data storage with distinctly improved properties can be obtained. The writing times of holographic gratings in the blends decrease with increasing content of the molecular glass former; nevertheless, the gratings are still stable.

Optimization of the photochromic response of photoaddressable polymers with azobenzene-containing molecular glasses

This paper reports on a new series of photochromic azobenzene-containing molecular glasses which are investigated as blending materials to improve the photochromic response of photoaddressable polymers. In order to identify potential candidates for blending, a variety of molecular glasses are synthesized and screened with respect to their photochromic response. Various end groups at the azobenzene chromophore and different core compounds are employed to obtain high thermal stability and good glass-forming properties as well as high sensitivity and refractive-index modulation. The best combination of structural variations is chosen to tailor a photoaddressable material with optimized physical and photophysical properties. By doping this photochromic molecular glass former into a photoaddressable polymer we are able to combine the high stability of polymer systems with the faster response of molecular glasses, thus creating a system which has the advantages of both material classes. With this approach we achieve an increase of the low photochromic sensitivity of the polymer, which is the biggest problem of these materials. In order to obtain a better understanding of the influence of the photochromic molecular glass former on the photochromic response in a polymer system we conduct holographic experiments on a concentration series of the glass former in an inert polymer matrix. By inscribing the holographic gratings at elevated temperatures we are able to increase the sensitivity of our molecular glass further.