Conical Refraction Bottle Beams for Entrapment of Absorbing Droplets.

Conical refraction (CR) optical bottle beams for photophoretic trapping of airborne absorbing droplets are introduced and experimentally demonstrated. CR describes the circular split-up of unpolarised light propagating along an optical axis in a biaxial crystal. The diverging and converging cones lend themselves to the construction of optical bottle beams with flexible entry points. The interaction of single inkjet droplets with an open or partly open bottle beam is shown implementing high-speed video microscopy in a dual-view configuration. Perpendicular image planes are visualized on a single camera chip to characterize the integral three-dimensional movement dynamics of droplets. We demonstrate how a partly opened optical bottle transversely confines liquid objects. Furthermore we observe and analyse transverse oscillations of absorbing droplets as they hit the inner walls and simultaneously measure both transverse and axial velocity components.

Multiplexing and switching of virtual electrodes in optoelectronic tweezers based on lithium niobate.

We introduce a method for trapping and arranging microparticles in arbitrary two-dimensional patterns with high flexibility. For this purpose, optoelectronic tweezers based on lithium niobate as photoconductor are used to create virtual electrodes through modulated illumination. The evolving field gradients arrange microparticles due to dielectrophoretic (DEP) forces and enable an all-optical approach for DEP. In order to increase flexibility further, we investigate multiplexed electrode structures for in situ reconfiguration of particle arrangements. Using the all-optical erasure of previously written virtual electrodes, we demonstrate electrode switching and sequential particle trapping in a microchannel for microfluidic applications.

Multimodal biophotonic workstation for live cell analysis.

A reliable description and quantification of the complex physiology and reactions of living cells requires a multimodal analysis with various measurement techniques. We have investigated the integration of different techniques into a biophotonic workstation that can provide biological researchers with these capabilities. The combination of a micromanipulation tool with three different imaging principles is accomplished in a single inverted microscope which makes the results from all the techniques directly comparable. Chinese Hamster Ovary (CHO) cells were manipulated by optical tweezers while the feedback was directly analyzed by fluorescence lifetime imaging, digital holographic microscopy and dynamic phase-contrast microscopy.

Two-dimensional dielectrophoretic particle trapping in a hybrid crystal/PDMS-system.

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.