Mostafa A. Ellabban

Neutron optical beam splitter from holographically structured nanoparticle-polymer composites.

We report a breakthrough in the search for versatile diffractive elements for cold neutrons. Nanoparticles are spatially arranged by holographical means in a photopolymer. These grating structures show remarkably efficient diffraction of cold neutrons up to about 50% for effective thicknesses of only 200   μm. They open up a profound perspective for next generation neutron-optical devices with the capability to tune or modulate the neutron diffraction efficiency.

Holographic scattering in photopolymer-dispersed liquid crystals

Strong polarization-conserving holographic scattering was observed in a photopolymer-dispersed liquid crystal film fabricated from the UV curable mixture of commercially available constituents. During the photopolymerization process a bright corona of diffracted light evolves around the pump beam. The intensity of the rotationally symmetric light distribution increases upon exposure. By rotating the sample, two characteristic diffraction rings appear which can be explained by the Ewald sphere construction. Our results demonstrate that the associated parasitic holograms are very pronounced. Hence, their presence must be accounted for whenever preparing and utilizing holographic polymer-dispersed liquid crystals in any application.

Holographic scattering as a technique to determine the activation energy for thermal fixing in photorefractive materials

We introduce holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals. After recording a parasitic hologram at ambient temperature, we measured the time dependence of the transmitted intensity at the fixing temperature, to determine the time constant. The temperature dependence of the latter allowed us to evaluate the thermal activation energy. For comparison, we performed an equivalent experiment employing the standard two-wave mixing method. The values obtained using the two techniques agree very well.

Out-of-phase mixed holographic gratings : a quantative analysis

We show, that by performing a simultaneous analysis of the angular dependencies of the +/- first and the zeroth diffraction orders of mixed holographic gratings, each of the relevant parameters can be obtained: the strength of the phase grating and the amplitude grating, respectively, as well as a potential phase between them. Experiments on a pure lithium niobate crystal are used to demonstrate the applicability of the analysis.

Diffraction gratings for neutrons from polymers and holographic polymer-dispersed liquid crystals

We discuss the applicability of holographically recorded gratings in photopolymers and holographic polymer-dispersed liquid crystals as neutron optical elements. An experimental investigation of their properties for light and neutrons with different grating spacings and grating thicknesses is performed. The angular dependences of the diffraction efficiencies for those gratings are interpreted in terms of a rigorous coupled wave analysis. Starting from the obtained results we work out the lines for the production of an optimized neutron optical diffraction grating, i.e. high diffraction efficiency in the Bragg diffraction regime with moderate angular selectivity.

Angular and wavelength selectivity of parasitic holograms in cerium doped strontium barium niobate

The angular and wavelength selectivity of noise gratings in doped strontium barium niobate crystals for samples with two different thicknesses are investigated. We found that an offset in the optimum reconstruction angle with respect to the recording geometry occurs as a function of the thickness. This is a consequence of the unidirectional amplification. Thus the scattering intensity for the thinner sample is considerably lowered at the very recording conditions. We analyze the experimental results in terms of the kinematic theory of diffraction.

Out-of-phase mixed holographic gratings : a quantative analysis: erratum

The two principal equations in [Opt. Express 16, 6528–6536 (2008)] which both contain an error are corrected in this erratum.

Neutron-optical gratings from nanoparticle-polymer composites

Abstract The preparation of neutron-optical phase gratings with light-optical holography is reviewed. We compare the relevant concepts of: (i) Kogelniks theory for Bragg diffraction of light by thick volume gratings, which can be used to analyze holographic gratings with both light and neutrons, and (ii) the dynamical theory of neutron diffraction. Without going into mathematical detail, we intend to illuminate their correspondence. The findings are illustrated by analyzing data obtained from reconstruction of nanoparticle holographic gratings with both light and neutrons.

Light-induced phase and amplitude gratings in centrosymmetric Gadolinium Gallium garnet doped with Calcium

The photosensitive properties of a centrosymmetric gadolinium gallium garnet crystal doped with calcium are investigated at room temperature. Elementary holograms can be recorded over a wide range of wavelengths in the visible spectral range. The photosensitive properties are studied experimentally using beam coupling and angular response experiments. Mixed absorption and refractive-index gratings are observed and their amplitudes and relative phases determined. Moreover, the candidate centers that are responsible for the photorefractive effect are discussed.

Peculiar behaviour of optical polarization gratings in light-sensitive liquid crystalline elastomers

The angular dependence of the diffraction efficiency of volume-type holographic gratings recorded in a single-domain light-sensitive liquid crystalline elastomer was investigated. Usually this dependence is expected to be very similar for intensity gratings and for polarization gratings. However, our measurements resolved a profound difference between the two types of the gratings: a typical Bragg peak of the diffraction efficiency is observed only for intensity gratings, while polarization gratings exhibit a profound dip at the Bragg angle. The appearance of this dip is explained by strongly anisotropic optical absorption of the actinic light during the recording process.