High performance photoinitiating systems for holography recording: need for a full control of primary processes.

Optimization of holography recording in photopolymers was studied from the point of view of a quite general process, that is, the photogeneration of radicals. On the basis of a dye/coinitiator photoinitiating system, the effect of primary events and their relative efficiency was investigated with respect to the final overall properties, such as the diffraction efficiency. Quenching of the dye excited states by the borate salts coinitiators exhibits important differences depending on the dye used (Rose Bengal or Safranine O). Keeping in mind that both singlet and triplet states of the dyes can react, and taking into account the viscosity of the matrix, a method to evaluate the overall quantum yield of radicals released is proposed. It is found that this quantum yield well correlates with the maximum rate of photopolymerization. More interestingly, the dose required to obtain a given diffraction efficiency was found to be also governed by the radical quantum yield, showing that the final property is directly governed by primary events. This shed some light on the efficiency of photochemical pathway to generate radicals for use in organic or polymer areas.

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 user's 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.

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.

Optically triggered Q-switched photonic crystal laser.

An optically triggered liquid crystal infiltrated Q-switched photonic crystal laser is demonstrated. A photonic crystal laser cavity was designed and fabricated to support two orthogonally polarized high-Q cavity modes after liquid crystal infiltration. By controlling the liquid crystal orientation via a layer of photoaddressable polymer and a writing laser, the photonic crystal lasing mode can be reversibly switched between the two modes which also switches the laser's emission polarization and wavelength. The creation of the Q-switched laser demonstrates the benefits of customizing photonic crystal cavities to maximally synergize with an infiltrated material and illustrates the potential of integrating semiconductor nanophotonics with optical materials.

Increased thermal stability of a poled electro-optic polymer using high-molar-mass fractions.

Polymer fractionation yields materials with higher average molar masses and thus substantially increased glass transition temperatures. We apply this technique to a photoaddressable polymer that exhibits high electro-optic coefficients, in order to enhance the thermal stability of the poled material. The degree of orientation of the dipolar chromophores is investigated with both pyroelectric and electro-optic measurements. The pyroelectric measurements at elevated temperatures show no significant contribution of beta relaxation as compared to alpha relaxation.

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.