No abstract

Photopolymer materials have significant advantages making them suitable for applications as holographic data storage media [I]. They provide phase holograms, which are very attractive since they can achieve very high diffraction efficiency [2]. It is often desirable to provide an insitu recording of holographic recording with different lasers sources. Diffractive optical elements (DOEs) [3-51 attract great interest and holographic photopolymer dispersed liquid crystals (H-PDLC) are emerging as very promising for these applications [6]. Recently, such materials have been sensitized to be sensitive in the near infrared region of light [7]. The characterization of these materials is an important challenge that can result in a better understanding of the holographic grating formation photochemical process. These steps are important in order to reach the fabrication of more efficient DOEs. Since, these materials have self-developing properties (via free radical photopolymerization), the growth curve of the diffraction efficiency can be monitored as it is being recorded. We are interested in (and we actually study) holographic recording characteristics of our photopolymers, which include diffraction efficiency (in transmission and reflection modes), modulation transfer h c t i o n , sensitivity, recording linearity, shrinkage, stability, etc. The corresponding details will be reported at the conference meeting. Materials and film preparationThe material is composed of a photoinitiator, a multifunctional acrylic monomer, a new liquid crystal and other organic additives [6]. The mixture was prepared at ambient temperature in the dark. Different chemical components were mixed and stirred carefully to obtain a homogeneous solution and then was injected between two glass slides of 25mmx25mm, which were separated by spacer (the thickness of the cell is defined by spacers, e.g. 30 pm thickness made of MylarTM film). The cell was filled by capillary effect and then was used for laser exposition. Holographic characteristics of materialsUp to day, the angular selectivity, polarization and wavelength dependence of gratings, recorded in our H-PDLC films, were investigated. The holographic recording was realized by a near infra red diode laser (834 nm), as well as by an Argon laser (514.5 nm) and He-Ne laser (632.8 nm). A symmetric incidence geometry with a beam ratio of 5050 was used. The thickness of the photosensitive layer was 30 pm, and the spatial frequency of the grating was about 1200 lines/". The diffraction efficiency demonstrates an increases with exposure time to achieve 70%-95% (depending upon the recording wavelength used). We have demonstrated that angular selectivity of the H-PDLC compound is not polarization dependent (during readout).

Several methods are available for collimating and circularizing beams for laser diodes. However, most of them make use of expensive equipment. In the present paper, a method involving a holographic polymer-dispersed liquid crystal DOE is used to circularize and collimate a beam emerging from a low cost infrared (850 nm) laser diode. Different characteristics of the recorded hologram are presented, such as angular and position tolerance. The material's shrinkage is also investigated in the present work.

We report the results of holographic and spectroscopic study of D2 dye doped PMMA and PS polymer compounds. The unusual non monotone relaxation dynamics observed in D2:PS compound shows the presence of unknown earlier excitation modes in this system. EXPERIMENTAL CONDITIONSAzo dye doped polymer guest-host systems attract very intensive studies for reversible optical storage. ' The understanding of photosensitivity mechanisms of these systems is thus an important challenge. We use, in the present work, dynamic holography and time resolved spectroscopy techniques to achieve this goal. New experimental results, concerning the photoexcitation and relaxation of azo dye doped PS and PMMA films, are reported. In particularly, we report, we believe for the first time, the observation of an unusual non monotone relaxation, which is observed in azo dye doped PS in the case of saturated holographic excitation. ''We have used Merck's D2 bisazo dye (containing two azo bonds) doped in mentioned above polymer films. The structural chemical formula of D2 is represented in the inset of Figure 1. We prepare several solutions for film preparation. The first one is the solution of D2 dye in THF (of concentration 0,44%) which is mixed for about 12 hours. The second one is of PS in THF (of concentration 15,98%) that is mixed for about 12 hours too. Then we take 2,4041g of PS solution and 0,0823g of D2 solution. The film preparation is conducted 1

Holographic polymer dispersed liquid crystal (H-PDLC) material was recently sensitized that is sensitive in the near infrared (800 nm to 855 nm). The compound was based on mixture of an acrylate monomer and liquid crystals (e.g., E7 and MBBA were investigated).In the present work, the angular sensitivity of gratings, recorded in these H-PDLC films, was investigated for various wavelengths. The holographic recording was realized by a diode laser (834 nm), which produced 1000 lines/mm spatial frequency of intensity grating. Significant broadening in the angular selectivity for p-polarization was observed for the formulation based on E7. The half-height width (HHW) of the angular selectivity was approximately 7 degrees (for 850 nm) and 10 degrees (for 670 nm). However, for s-polarization, the same sample has a HHW of about 2 degrees for both.We believe that this behavior is the indicator of a spatial modulation in the shape of the liquid crystals (LC) micro droplets. Droplet's shape should change spatially from elliptical to spherical correlated with (but not necessarily the same) spatial frequency of the light grating. For MBBA based samples, there is almost no polarization dependence and the HHW of the angular selectivity is about 2 degrees. Thus, the shape of the micro droplets is strongly influenced by the chemical composition of the compound used. It has been demonstrated that angular selectivity for H-PDLC compound is strongly polarization dependent. This indicates that the forms and sizes of micro droplets could be strongly modulated along the gradient of light interference patterns