Polymer Blends with Azobenzene‐ Containing Block Copolymers as Stable Rewritable Volume Holographic Media

Abstract

Volume holography is a promising technology for high-capacity optical data storage. [1,2] When utilizing the entire volume of a medium instead of only its surface, capacities above one terabyte on a memory medium the size of a compact disc are theoretically possible. Recent developments in the field of holographic storage materials and advances in storage techniques and technology are summarized in review articles by Ashley et al. [3] and Hesselink et al. [4] Although there are still material challenges, write-once media seem to be fairly advanced. In search of rewritable media, many studies have been performed on photorefractive crystals and organic glasses. [1,4] Manufacturing single crystals of the required size is very expensive, and a major drawback of the organic photorefractive materials is the requirement of very high electric field strengths. With increasing material thickness, the voltage necessary for creating the field becomes difficult to handle. A different potential class of rewritable media are polymers containing photoaddressable azobenzene chromophores.[5] Many studies have been performed on azobenzene-containing polymer films with thicknesses in the range of 0.5-10 lm. [3,6] However, the preparation of the thick samples required for multiplexing with sufficiently low optical density remains difficult to achieve. Because the absorption coefficient depends on the wavelength, it is in principle possible to tune a laser with an adjustable wavelength (e.g., a dye laser) to the very edge of the absorption band. [7] If writing is to be performed at a wavelength where fixed-wavelength, solid-state lasers are available (515 or 532 nm), however, a concept that allows for diluting the chromophores is required. So far, the best results have been reported by Minabe et al. with a homogeneous polymer blend based on an azobenzene-containing polymer and a similar but nonabsorbing polymer. [8] In a sample of 250 lm thickness, a set of 20 data pages were successfully inscribed; yet, the individual holograms were not completely spatially overlapping. The important issues of stabilizing the inscribed data and rewritability were not addressed. We use a different concept of controlling the optical density, by using block copolymers that contain azobenzene side groups in one block. Block copolymers with less than 15 vol % of one component form spherical microphases with typical diameters in the range of 10-20 nm. Thus, we can macroscopically dilute the chromophores while, at the same time, keeping them in a local confinement. The latter is important for the stability of the inscribed information. In an earlier paper, we have already reported the formation of individual holographic gratings in 10 lm thick films of a material that contained azobenzene and nonabsorbing three-ring mesogenic side groups in the minority phase.[9] In the confined geometry, the presence of a cooperative effect of the side groups was demonstrated. The mesogenic side groups were reoriented together with the chromophores; furthermore, in materia...

Современное Состояние Разработки Светочувствительных Сред Для Голографии (Обзор)

“…Показана возможность использования слоев блочного полимера XLI, включающего полистирольную матрицу (77.5 вес.%), азобензольные (8.4 вес.%) и мезогенные цепи (12.3 вес.%), гидроксиэтилметакрилат (1.8 вес.%), для записи-стирания более 80 голограмм на одном участке регистрирующего слоя под разными угла-ми [149]. …”

Optical and Physical Applications of Photocontrollable Materials: Azobenzene-Containing and Liquid Crystalline Polymers

“…The mesoscopic morphologies created by polymer blends and/or block copolymers and photoalignment molecules are also used for anisotropic recording media. Synergetic effects were present in the holographic recording, leading to improved of diffraction efficiency with optical page data [102], large photo-induced birefringence and hologram stability [103], and achievement of high-density recording and the high-reproducibility in the cyclic recording and erasing process [104].…”

Supramolecular design principles for efficient photoresponsive polymer–azobenzene complexes

“…112 Second, the writing wavelength comparison for P4VP(7) and P4VP (8) showed that, in order to reach significant birefringence levels, it is important to use the wavelength that drives the photoisomerization reaction in both isomerization directions (trans-to-cis and cis-to-trans), even when that wavelength (488 nm) lies far from the absorption maximum (356 nm for P4VP(7) and 354 nm for P4VP (8)). 115 In general, the ability to record birefringence at wavelengths barely absorbed by the material may be beneficial for applications such as optical holography, 118,119 where low penetration depth of conventional azobenzene-containing polymers may become a limiting factor. One pertinent question provoked by these studies is the role of the strength of the hydrogen-bonding interaction.…”