Dielectrophoretic particle manipulation

Dielectrophoresis is an electronic effect that can be neglected in homogeneous or weakly inhomogeneous electric fi elds, but becomes dominant if the fi eld is strongly modulated. In this case, the fi eld acts upon locally induced dipoles in the object to be controlled. This results in a net force, either attracting or repulsing matter from regions of high fi eld intensity, depending on the electric properties of the object and the surrounding medium. The magnitude and sign of this force, which is described by the Clausius-Mosotti factor, is highly dependent on the frequency of the applied fi eld, and can thus be used not only for the mere arrangement of particles but also for sorting and analytic purposes. Conventional dielectrophoretic setups consist of fi xed electrodes on a substrate, which, on the downside, require a laborious  manufacturing process including lithography.

Assembly on nonlinear crystals

In contrast, we use light for the creation of the electrodes: Nonlinear optics can store a fi eld structure inside a photorefractive material, which can then exert dielectrophoretic forces upon matter. Due to the fact that induced patterns can also be erased using homogeneous illumination, we established all-optical switching of these so-called virtual electrodes. The time constants necessary for a change of the electrode pattern in the photoconductive crystal can vary from several minutes to fractions of a second, depending on the material and the laser power available. This opens a variety of possible trapping scenarios, from a permanent arrangement on lithium niobate which is stable over several days, to real-time manipulation on the surface of bismuth silicon oxide (BSO). Additionally, the electrooptic properties of the materials allow the direct visualization and measurement of the refractive index change, and hence of the internal electric fi eld which are directly connected via the Pockels effect.

Furthermore, their crystalline structure make photorefractive materials chemically inert and thus suitable for the use with almost all solvents. The high quality of the surface facilitates the application of microchannels made from polydimethylsiloxane (PDMS), which is self-adhesive, provided the pressure inside the channel is not too strong. Therefore, photorefractive materials are well-suited for integration into microfl uidic applications.