Photorefractive materials exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and noncentrosymmetric structure. Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a nonlocal response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm(-1) in the centrosymmetric glasses, and a gain of 444 cm(-1) in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process to induce a permanent arrangement of azo-dye chromophores.

We provide a basis for automated single-cell sorting based on optical trapping and manipulation using human peripheral blood as a model system. A counterpropagating dual-beam optical-trapping configuration is shown theoretically and experimentally to be preferred due to a greater ability to manipulate cells in three dimensions. Theoretical analysis performed by simulating the propagation of rays through the region containing an erythrocyte (red blood cell) divided into numerous elements confirms experimental results showing that a trapped erythrocyte orients with its longest axis in the direction of propagation of the beam. The single-cell sorting system includes an image-processing system using thresholding, background subtraction, and edge-enhancement algorithms, which allows for the identification of single cells. Erythrocytes have been identified and manipulated into designated volumes using the automated dual-beam trap. Potential applications of automated single-cell sorting, including the incorporation of molecular biology techniques, are discussed.

Ultrasensitive bioanalytical assays are of great value for early detection of human diseases and pathogens. The sensitivities of immunoassays and DNA probing can be enhanced by multilabeling the biorecognition partner used for affinity-based assays. However, the bioreactivity of biomolecules is affected by a high degree of multilabeling at multiple functional sites. It is proposed that dendritic scaffoldings be used to link multiple signal-generating units to a single site with potentially minimum impact on the bioaffinity. A prototype label, a zeroth-generation dendron, bearing three [Ru(bpy)3]2+ units for electrochemiluminescence (ECL) assays was synthesized and characterized preliminarily by spectroscopic, electrochemical, and ECL methods. No evidence of interaction between the neighboring [Ru(bpy)3]2+ units in the label molecule was found from these characterizations. Both the photoluminescence and ECL of the prototype label have features very similar to those of mononuclear [Ru(bpy)3]2+ compounds. Labeling a model protein with a triad of [Ru(bpy)3]2+ at one NH2 position was demonstrated. The results reported here provide support to applying the proposed multilabeling strategy to affinity-based bioanalytical assays.

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A sophisticated modeling program was used recently to predict the trapping and the manipulation properties of elongated cylindrical objects in the focal region of a high-intensity laser beam. On the basis of the model, the cylinders should align their longest diagonal dimension with the propagation axis of the laser beam and follow the beam when it is displaced transverse to the cylinder's central axis. Experimental confirmation of the cylinder's behavior is presented and confirms the suitability of the enhanced ray-optics approach to modeling micrometer-scale objects in optical-trap environments.