Dielectrophoretic forces originating from highly modulated electric fields can be used to trap particles on surfaces. An all-optical way to induce such fields is the use of a photorefractive material, where the fields that modulate the refractive index are present at the surface. We present a method for two-dimensional particle alignment on an optically structured photorefractive lithium niobate crystal. The structuring is done using an amplitude-modulating spatial light modulator and laser illumination. We demonstrate trapping of uncharged graphite particles in periodic and arbitrary patterns and provide a discussion of the limitations and the necessary boundary conditions for maximum trapping efficiency. The photorefractive crystal is utilized as bottom part of a PDMS channel in order to demonstrate two-dimensional dielectrophoretic trapping in a microfluidic system.

No abstract

Lithium tantalate crystals of compositions ranging from 48.3 mol % to 50.0 mol % lithium oxide are fabricated by vapor transport equilibration. Light-induced refractive index changes of the crystals are investigated with holographic methods at usual cw-laser intensities ͑Ϸ10 5 W/m 2 ͒ and with a single focused laser beam at high light intensities up to 2 ϫ 10 7 W/m 2. In stoichiometric crystals the index changes are reduced by more than two orders of magnitude when compared with congruently melting ones. Simultaneously, the normalized photoconductivity ph / I, where I is the light intensity, increases by nearly two orders of magnitude. Therefore, stoichiometric lithium tantalate is an attractive material for applications such as frequency conversion via quasi-phase matching.

No abstract

We quantitatively investigate the axial imaging properties of dynamic phase-contrast microscopy, with a special focus on typical combinations of tracer particles and magnifications that are used for velocimetry analysis. We show, for the first time, that a dynamic phase-contrast microscope, which is the integration of an all-optical novelty filter in a commercially available inverted microscope, can visualize threedimensional velocity fields with a significantly reduced optical sectioning depth. The depth of field for dynamic phase-contrast microscopy is extracted from the three-dimensional response function and compared with the respective values for incoherent bright-field illumination. These results are then used to perform a depth-resolved particle image velocimetry analysis of Hagen-Poiseuille as well as electro-osmotically actuated flows in a microchannel.