Frank Holtmann

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.

Holographic phase contrast for dynamic multiple-beam optical tweezers

We propose and demonstrate holographic phase contrast (HPC) as a new method to transfer a spatial phase distribution of arbitrary shape into a corresponding intensity pattern. A powerful application of HPC is the use in optical tweezers to dynamically control multiple traps like arrays or even more complex trapping geometries. Due to the image plane nature of HPC no hologram calculation is required and hence real-time control of complex tweezers configurations is possible. The inherent optical amplification by HPC can improve the fundamental limit in trapping power in optical tweezers that are based on common spatial light modulators.

Full-field particle velocimetry with a photorefractive optical novelty filter

We utilize the finite time constant of a photorefractive optical novelty filter microscope to access full-field velocity information of fluid flows on microscopic scales. In contrast to conventional methods such as particle image velocimetry and particle tracking velocimetry, not only image acquisition of the tracer particle field but also evaluation of tracer particle velocities is done all-optically by the novelty filter. We investigate the velocity dependent parameters of two-beam coupling based optical novelty filters and demonstrate calibration and application of a photorefractive velocimetry system. Theoretical and practical limits to the range of accessible velocities are discussed.

Depth-resolved velocimetry of Hagen-Poiseuille and electro-osmotic flow using dynamic phase-contrast microscopy

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 three-dimensional 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.

Nonlinear Dynamic Phase Contrast Microscopy for Microflow Analysis

For the investigation and control of microfluidic systems innovative microscopy techniques are needed which can comply the requirements regarding to sensitivity and spatial as well as temporal resolution. A promising approach for this challenge is nonlinear dynamic phase contrast microscopy. It is an alternative full field approach that allows to detect motion as well as phase changes of unstained micro-objects in real-time without contact and non destructive, i.e. fully biocompatible. In this contribution we will present the dynamic phase contrast technique and its applications in micro flow velocimetry and micro-mixing analysis.

Nonlinear dynamic phase contrast microscopy for microfluidic and microbiological applications

In live sciences, the observation and analysis of moving living cells, molecular motors or motion of micro- and nano-objects is a current field of research. At the same time, microfluidic innovations are needed for biological and medical applications on a micro- and nano-scale. Conventional microscopy techniques are reaching considerable limits with respect to these issues. A promising approach for this challenge is nonlinear dynamic phase contrast microscopy. It is an alternative full field approach that allows to detect motion as well as phase changes of living unstained micro-objects in real-time, thereby being marker free, without contact and non destructive, i.e. fully biocompatible. The generality of this system allows it to be combined with several other microscope techniques such as conventional bright field or fluorescence microscopy. In this article we will present the dynamic phase contrast technique and its applications in analysis of micro organismic dynamics, micro flow velocimetry and micro-mixing analysis.