Thomas Fäcke

From the Surface to Volume: Concepts for the Next Generation of Optical–Holographic Data‐Storage Materials

Optical data storage has had a major impact on daily life since its introduction to the market in 1982. Compact discs (CDs), digital versatile discs (DVDs), and Blu-ray discs (BDs) are universal data-storage formats with the advantage that the reading and writing of the digital data does not require contact and is therefore wear-free. These formats allow convenient and fast data access, high transfer rates, and electricity-free data storage with low overall archiving costs. The driving force for development in this area is the constant need for increased data-storage capacity and transfer rate. The use of holographic principles for optical data storage is an elegant way to increase the storage capacity and the transfer rate, because by this technique the data can be stored in the volume of the storage material and, moreover, it can be optically processed in parallel. This Review describes the fundamental requirements for holographic data-storage materials and compares the general concepts for the materials used. An overview of the performance of current read-write devices shows how far holographic data storage has already been developed.

Comparison of a new self developing photopolymer with AA/PVA based photopolymer utilizing the NPDD model.

The development of suitable recording media for applications such as holographic optical elements and holographic data storage are of significant research and commercial interest. In this paper, a photopolymer material developed by Bayer MaterialScience is examined using various optical techniques and then characterised using the Non-local Photo-polymerization Driven Diffusion model. This material demonstrates the capabilities of a new class of photopolymer offering high index modulation, full colour recording, high light sensitivity and environmental stability. One key result of this study is the materials high spatial frequency resolution, indicating a very low non-local effect, thus qualifying it as a very good storage medium.

Holographic recording aspects of high-resolution Bayfol HX photopolymer

We have been developing a new class of recording materials for volume holography, offering the advantages of full color recording and depth tuning without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two-chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network gives the necessary mechanical stability to the material prior to recording. In this paper we describe several aspects of holographic recording into Bayfol® HX which are beneficial for its effective use and discuss them within a more elaborate reaction-diffusion model. Inhibition phenomena and the influence of precure are studied within this model and are investigated experimentally for single hologram recording and angular multiplexed hologram recordings. Also the dark reaction after the exposure period and the minimum allowable waiting time for full hologram formation are addressed. The proper understanding of these phenomena is important for the optimal usage of these new materials, in for example step-and-repeat mass production of holograms.

Reaction-diffusion model applied to high resolution Bayfol HX photopolymer

We have been developing a new class of recording materials for volume holography, offering the advantages for full color recording and depth tuning without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network gives the necessary mechanical stability to the material prior to recording. In this paper we show that the recording process in these materials can be successfully described within a reactiondiffusion model. For the first time the combination of plane-wave recording data in transmission and reflection geometry was used to extract the model parameters. This was achieved via a master curve construction of the respective power density response functions of the photopolymer at saturation recording conditions. Within that model, power density response, spatial frequency response, non-locality effects, beam ratio effects and even dosage response can be predicted and explained for a wide range of CW recording conditions which are important for various holographic applications of these new materials.

Materials in optical data storage

Abstract Optical Data Storage has become the mainstream technology for distributing audio, video and computer software content as well as recording and archiving personal data during the past two and a half decades. The continuous demand for ever higher storage capacities and faster data transfer rates has led to the development of three disc format families Compact Discs, Digital Versatile Discs and Blu-ray Disc, which have become established in the market. Research and development for future high density formats exploits the already well established “bit-wise” recording and volume storage approaches like holographic and “page-wise” recording. Each existing and future technology imposes specific requirements on materials that can facilitate functioning and reliable products for the consumer. In this review several aspects of the interrelation between technological requirements and materials are highlighted.

New recording materials for the holographic industry

This paper describes a new class of recording materials for volume holographic applications suitable to meet commercial manufacturing needs. These next-generation holographic photopolymers have the ability to satisfy the unmet demand for color and depth tuning that is only possible with volume holograms. Unlike earlier holographic photopolymers, these new materials offer the advantages of no chemical or thermal processing combined with low shrinkage and detuning. Furthermore, these materials exhibit high transparency, a high resolution of more than 5000 lines/mm and are environmentally robust. Bayer MaterialScience plans to commercialize these materials, which combine excellent holographic characteristics with compatibility to mass-production processes. In this paper, we will briefly discuss the potential markets and applications for a new photopolymer, describe the attributes of this new class of photopolymers, relate their ease of use in holographic recording, and discuss potential applications of such materials..

Time-sequential autostereoscopic 3-D display with a novel directional backlight system based on volume-holographic optical elements.

A novel directional backlight system based on volume-holographic optical elements (VHOEs) is demonstrated for time-sequential autostereoscopic three-dimensional (3-D) flat-panel displays. Here, VHOEs are employed to control the direction of light for a time-multiplexed display for each of the left and the right view. Those VHOEs are fabricated by recording interference patterns between collimated reference beams and diverging object beams for each of the left and right eyes on the volume holographic recording material. For this, self-developing photopolymer films (Bayfol® HX) were used, since those simplify the manufacturing process of VHOEs substantially. Here, the directional lights are similar to the collimated reference beams that were used to record the VHOEs and create two diffracted beams similar to the object beams used for recording the VHOEs. Then, those diffracted beams read the left and right images alternately shown on the LCD panel and form two converging viewing zones in front of the users eyes. By this he can perceive the 3-D image. Theoretical predictions and experimental results are presented and the performance of the developed prototype is shown.

Photopolymeric films with highly tunable refractive index modulation for high precision diffractive optics

The design and manufacturing of high efficiency and reliable volume phase holographic optical elements require photosensitive material where it is possible to finely control the refractive index modulation. Bayfol HX photopolymers show this feature together with other interesting advantages, in particular the self-developing and the large refractive index modulation. In this paper, the design of Volume Phase Holographic Gratings (VPHGs) is reported underlying the relationship of gratings’ performances with the refractive index modulation. The trend of this property with the change of the laser power density and the ratio of the two writing beams is shown. Based on these results, VPHGs for astronomical instrumentation have been designed and manufactured.

Detailed investigation of the propagation rate of urethane acrylates

Temperature dependent propagation rate coefficients, kp, are determined for four acrylate monomers containing a carbamate moiety via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) technique. Therefore, the Mark–Houwink–Kuhn–Sakurada coefficients K and a of the respective polymers were additionally determined via triple-detection SEC. The monomers under investigation were synthesized from hydroxyethyl acrylate, hydroxyl(iso)propyl acrylate as well as phenyl isocyanate and hexyl isocyanate, respectively, in all four possible combinations. For 2-(phenylcarbamoyloxy)ethyl acrylate (PhCEA) an activation energy of 14.3 kJ mol−1 and a frequency factor of A = 1.2 × 107 L·mol−1 s−1 are obtained for kp. The MHKS parameters for poly(PhCEA) are K = 8.3 × 10−5 dL g−1 and a = 0.677. For 2-(phenylcarbamoyloxy)isopropyl acrylate (PhCPA) an activation energy of 14.2 kJ mol−1 and a frequency factor of A = 4.9 × 106 L mol−1 s−1 are found for kp and the MHKS parameters for poly(PhCPA) read K = 10.3 × 10−5 dL g−1 and a = 0.657. The activation parameters of kp of 2-(hexylcarbamoyloxy)ethyl acrylate (HCEA) are EA = 13.3 kJ mol−1 and A = 6.6 × 106 L mol−1 s−1 with K = 36.0 × 10−5 dL g−1 and a = 0.552 for poly(HCEA). For 2-(hexylcarbamoyloxy)isopropyl acrylate (HCPA) EA is 14.1 kJ mol−1 and A = 6.6 × 106 L mol−1 s−1 with K = 26.0 × 10−5 dL g−1 and a = 0.587 for poly(HCPA). All rate measurements were performed in 1 M solutions in butyl acetate. The fast propagating nature and reduced activation energy of the monomers may be understood on the basis of the increased nucleophilicity that is induced by the carbamate functionality present in all monomers. Rate-increasing effects from solvent polarity and/or from H-bonding can, however, not be excluded and might also contribute to the observed high propagation rates.

Holographic recordings with high beam ratios on improved Bayfol® HX photopolymer

Bayfol® HX film is a new class of recording materials for volume holography. It was commercialized in 2010 and is offering the advantages for full-color recording and moisture resistance without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two-chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network provides the necessary mechanical stability to the material prior to recording. In addition to the well-known security and imaging applications, Bayfol® HX film also offers a new opportunity for the manufacturing of volume Holographic Optical Elements (vHOEs) in new optical and optoelectronic applications. For the implementation of holographic recording layouts and associated exposure schedules for these HOEs detailed understanding of the photopolymer material properties and the knowledge how to achieve the optical requirements of dedicated holographic applications are necessary. In this paper we extend the application of our simulation method for the writing mechanism for the Bayfol® HX photopolymer film. Different photopolymer product variations, including development of photopolymer grades with improved bleaching properties and increased dynamical range, which enable simultaneous multi-color recording, while maintaining a high diffraction efficiency of the recorded holograms are covered. The model is investigated experimentally by recording and evaluation of specifically designed directional diffuser vHOEs as they would be used e.g. for light shaping or light management purposes. One important observation is the capability of Bayfol® HX film type photopolymers to form highly efficient diffraction gratings even at very high intensity ratios of the reference beam versus the object beam.