Cornelia Denz

Volume hologram multiplexing using a deterministic phase encoding method

Abstract We present a novel phase encoding method for high capacity optical data storage in volume photorefractive materials. Using deterministic orthogonal phase codes in a reference-based multiplexing technique, we obtain a system that is able to retrieve multiple images with high diffraction efficiency without energy losses, adjustment problems or any time delay. The number of images which can be stored with that method is theoretically calculated and it is shown that image retrieval without any crosstalk is possible. Moreover, the method is experimentally demonstrated for a set of four orthogonal phase addresses. Actual limitations of the system are finally discussed.

Potentialities and limitations of hologram multiplexing by using the phase-encoding technique.

The advantages and limitations of data storage in holographic materials by implementing a pure phase-encoding method of the reference beam are studied. We show that if deterministic orthogonal binary phase addresses are used, such a system is theoretically able to store as many images as the usual angular multiplexing method. However, we demonstrate that imperfections of available optical components generate optical noise and limit the storage capacity. We propose an improved recording technique to overcome some of these limitations.

Airy beam induced optical routing

We present an all-optical routing scheme based simultaneously on optically induced photonic structures and the Airy beam family. The presented work utilizes these accelerating beams for the demonstration of an all-optical router with individually addressable output channels. In addition, we are able to activate multiple channels at the same time providing us with an optically induced splitter with configurable outputs. The experimental results are corroborated by corresponding numerical simulations.

Reconfigurable Optically Induced Quasicrystallographic Three‐Dimensional Complex Nonlinear Photonic Lattice Structures

2010 WILEY-VCH Verlag Gm Quasicrystals (QCs) are materials that possess a long-range order with defined diffraction patterns, but lack the characteristic translational periodicity of crystals. From the discovery of the non-crystallographic icosahedral quasiperiodic symmetry found in Al6Mn in 1984, [2] the distinct properties of quasicrystallographic structures attracted a great deal of interest in different realms of science in recent years. Another field of technological interest in the recent past is that of photonic crystals (PCs), the structured materials with a translational periodic modulation of the refractive index. Merging these two fields, a new class of material structures called photonic quasicrystals (PQCs) has drawn the attention of researchers stemming from a cumulative effect from both fields. This is mainly due to the fact that the higher rotational symmetry of QCs leads to more isotropic and complete photonic bandgaps (PBGs) even in materials with a low refractive index contrast. However, as in the case of PCs, the fabrication of 3D PQCs is much more involved in comparison to 2D PQCs and remains a real challenge today. Moreover, many of the conventional methods become technically either unsuitable or extremely complicated for the fabrication of 3D PQCs. Therefore, the fabrication and optimization of higher rotational symmetry 3D PQCs demand an approach that is flexible as well as reconfigurable. The purpose of the present Communication is dual fold. On the one hand, we demonstrate for the first time the generation of well-defined reconfigurable 3D quasi-crystallographic photorefractive nonlinear photonic structures with various rotational symmetries, which are experimentally realized in an externally biased cerium doped strontium barium niobate (SBN:Ce) photorefractive material as the nonlinear optical material of choice. These complex structures are envisaged to form a reconfigurable platform to investigate advanced nonlinear light–matter interaction in higher spatial dimensions with various rotational symmetries. On the other hand, we present a generalized versatile experimental approach for the fabrication of complex 3D axial PQCs with higher order rotational symmetry and variants of complex 3D structures similar to those having icosahedral symmetry, using a real-time reconfigurable holographic technique. It involves a programmable spatial light modulator (SLM)-assisted single step optical induction approach based on computer-engineered optical phase patterns. It is also important to note that the versatility of the experimental approach, we present, is not limited to photorefractive materials alone. It can be easily well adapted to various photosensitive materials as per the application requirement in the diverse fields of material science. Among various photosensitive materials, reconfigurable nonlinear photonic lattices can be easily generated by means of a so-called optical induction technique at very low power levels ( micro watts) in a photorefractive material, exploiting the wavelength sensitivity of these materials. The process of refractive index modulation, which leads to photonic lattice formation in such a medium is caused by a two-step process out of the incident light intensity distribution. Under the influence of an externally applied electric field, the incident light intensity distribution causes a charge carrier redistribution that results in a macroscopic space charge field in the photorefractive material. This, in turn, leads to a space-dependent refractive index modulation via the electro-optic effect thereby representing a nonlinear optical effect of third order that creates the refractive index modulation out of the incident intensity distribution. Apart from the possibility of permanent fixing of the generated structures in a photorefractive crystal, the recorded structure is reconfigurable: it can also be erased by the flush of white light so that new patterns could be again recorded in these materials. Therefore, photorefractive materials are ideal materials for reconfigurable PQC generation either to optimize the required photonic structure on the one hand or to be used as a reconfigurable platform to investigate novel nonlinear wave dynamics. From the optical properties point of view, the photonic lattices formed in SBN:Ce show both polarization as well as orientation anisotropy. In order to obtain refractive index modulated structures that mimic the intensity pattern, o-polarized writing beams are used causing a low modulation due to the appropriate electro-optic coefficient addressed. For the case of using e-polarized writing beams, as the relevant electrooptic coefficient is much higher, a strongly nonlinear refractive index modulation can be obtained for the fabricated lattices. Moreover, as maximum refractive index modulation is induced in the direction parallel to the crystal c axis, there exists also orientation anisotropy in SBN:Ce.

Dynamic and Reversible Organization of Zeolite L Crystals Induced by Holographic Optical Tweezers

Organization and patterning of zeolite L crystals with their unique properties such as their one-dimensional nano channel system is of highest topical interest with various applications in many areas of science. We demonstrate full three-dimensional optical control of single zeolite L crystals and for the first time fully reversible, dynamic organization of a multitude of individually controlled zeolite L crystals.

Self-bending of photorefractive solitons

Self-bending of photorefractive solitons is caused by diffusion in photorefractive crystals and becomes an important effect when the beam size is in the range of the charge carriers diffusion length. In this paper we present an experimental and numerical examination of the beam bending dependence on relevant parameters such as the applied electric field and the beam intensity. We demonstrate that the bending dependence on the electric field in the low saturation regime has the form of a square function at low values of the field and becomes linear for higher values. For stronger saturation the curve gets the form of a square root function. The bending dependence on the beam intensity has a maximum at defined intensity. The experimental data are compared with numerical simulations, giving a good qualitative agreement.

Nonlinear lattice structures based on families of complex nondiffracting beams

We present a new concept for the generation of optical lattice waves. For all four families of nondiffracting beams, we are able to realize corresponding nondiffracting intensity patterns in a single setup. The potential of our approach is shown by demonstrating the optical induction of complex photonic discrete, Bessel, Mathieu and Weber lattices in a nonlinear photorefractive medium. However, our technique itself is very general and can be transferred to optical lattices in other fields such as atom optics or cold gases in order to add such complex optical potentials as a new concept to these areas as well.

Full 3D translational and rotational optical control of multiple rod‐shaped bacteria

The class of rod-shaped bacteria is an important example of non-spherical objects where defined alignment is desired for the observation of intracellular processes or studies of the flagella. However, all available methods for orientational control of rod-shaped bacteria are either limited with respect to the accessible rotational axes or feasible angles or restricted to one single bacterium. In this paper we demonstrate a scheme to orientate rod-shaped bacteria with holographic optical tweezers (HOT) in any direction. While these bacteria have a strong preference to align along the direction of the incident laser beam, our scheme provides for the first time full rotational control of multiple bacteria with respect to any arbitrary axis. In combination with the translational control HOT inherently provide, this enables full control of all three translational and the two important rotational degrees of freedom of multiple rod-shaped bacteria and allows one to arrange them in any desired configuration.

Guiding and dividing waves with photorefractive solitons

Abstract In this paper we show the guidance of a HeNe probe beam in photorefractive (2+1)D-solitons created by a beam of a frequency-doubled Nd:YAG laser in SBN. In the first part the development of a single soliton is shown in a time-resolved manner while the guided probe beam is found to follow exactly the movement of the soliton and even copies its shape. In the second part the guidance of a probe beam in interacting solitons is performed. When two (2+1)D-solitons propagate simultaneously different interaction scenarios can be observed, including the mutual exchange of energy. Using this effect, a guided probe beam can be divided effectively in two parts. Thus we present to our knowledge the first realization of a 1-to-2 waveguide divider using (2+1)D-solitons.

Reduced-Symmetry Two-Dimensional Solitons in Photonic Lattices

We demonstrate theoretically and experimentally a novel type of localized beam supported by the combined effects of total internal and Bragg reflection in nonlinear two-dimensional square periodic structures. Such localized states exhibit strong anisotropy in their mobility properties, being highly mobile in one direction and trapped in the other, making them promising candidates for optical routing in nonlinear lattices.