M. Reicherter

Optical particle trapping with computer-generated holograms written on a liquid-crystal display.

Computer-generated holograms written on a liquid-crystal display can be used to generate dynamic light fields of arbitrary shape. This method was used to simultaneously trap polystyrene particles laterally and to displace them independently of one another.

Multi-functional optical tweezers using computer-generated holograms

Optical tweezers are capable of trapping microscopic particles by photon momentum transfer. The use of dynamic computer-generated holograms for beam shaping allows a high flexibility in terms of trap characteristics and features. We use a liquid crystal display (LCD) to display the holograms. Efficiency losses caused by the periodic electrode structure of the LCD have been clearly reduced by use of an optically addressed spatial light modulator. We realized multiple traps, which can hold and move at least seven silica spheres independently in real time. We also demonstrate the controllability of trapped particles in three dimensions without the need for mechanical elements in the setup.

Fast digital hologram generation and adaptive force measurement in liquid-crystal-display-based holographic tweezers.

Computer-generated holograms in conjunction with spatial light modulators (SLMs) offer a way to dynamically generate holograms that are adapted to specific tasks. To use the full dynamic capability of the SLM, the hologram computation should be very fast. We present a method that uses the highly parallel architecture of a consumer graphics board to compute analytical holograms in video real time. A precice characterization of the SLM (Holoeye LC-R-2500) and the adaption of its settings to our near-infrared application is necessary to guarantee an efficient hologram reconstruction. The benefits of a fast computation of adapted holograms and the application of an efficient SLM are demonstrated by measuring the trapping forces of holographic tweezers.

Using graphics boards to compute holograms

Todays,consumer graphics boards incorporate highly integrated, parallel-working graphics processing units (GPUs) with transistor counts and performance that exceed those of CPUs. In addition to playing the latest 3D video game, you can use the graphics boards power to solve computational problems in science or engineering work. Current GPUs are programmable and flexible enough to transfer computational problems from the CPU to the GPU.Why shift computational work to the GPU? The short answer is processing power. As we demonstrate, using a standard graphics board can considerably speed up the overall performance of tasks such as computing Fourier holograms in real time.

Dynamic correction of aberrations in microscopic imaging systems using an artificial point source

In biological micromanipulation image aberrations are introduced not only by the optical system, but also by the immersion liquid. Whereas optical system aberrations are constant and it is relatively easy to measure and correct for them, the immersion caused aberrations are variable in time and space. In this paper a method using a spherical microparticle as an artificial point source for aberration control is presented. The particle is positioned by optical tweezers at the location of the biological sample. In the experiment holographic tweezers are used. They are based on computer generated holograms, written into spatial light modulators, which create light traps for the microparticle in the object plane. The light traps can be moved without any mechanically moving parts, just by changing the hologram. The particle strongly focuses the light, therefore an artificial point source in the object space is created. The illumination light is filtered, so that only the signal corresponding to a spherical wave is analyzed by the wavefront detection system. The information about the wavefront distortion is used to dynamically correct for it. This can be done by using spatial light modulators. The method is suitable for biophotonic imaging systems, where refractive index variations in the sample plane are significant. The integration with holographic tweezers is advantageous since it offers flexibility in positioning and imaging the particles.

Application of SLMs for optical metrology

For wavefront sensing, wavefront shaping, and optical filtering, spatial light modulators can be very useful. With the availability of high resolution liquid crystals (LC) spatial phase modulators and micromechanical systems (MEMS) containing large arrays of micromirrors, new applications in optical metrology become possible. For wavefront analysis and correction, dynamic CGHs are used. A correction hologram for the aberrated system is computed from which the lens shape can be derived. For Hartmann sensors, usually static microlenses are used. It was found advantageous to generate dynamic microlenses in order to correct for local wavefront aberrations. Optically addressed spatial light modulators can be applied very effectively for the characterisation and defect analysis of primarily periodic structures such as microchips or microlens arrays. For triangulation based methods, better results can be obtained by adapting the projected fringes to the object in terms of shape and brightness. Examples and experimental results are discussed.

Fast hologram computation and aberration control for holographic tweezers

Holographic tweezers offer a very versatile tool in many trapping applications. Compared to tweezers working with acousto optical modulators or using the generalized phase contrast, holographic tweezers so far were relatively slow. The computation time for a hologram was much longer than the modulation frequency of the modulator. To overcome this drawback we present a method using modified algorithms which run on state of the art graphics boards and not on the CPU. This gives the potential for a fast manipulation of many traps, for cell sorting for example, as well as for a real-time aberration control. The control of aberrations which can vary spatially or temporally is relevant to many real world applications. This can be accomplished by applying an iterative approach based on image processing.

Advantages of holographic optical tweezers

In the last decade optical tweezers became an important tool in microbiology. However, the setup becomes very complex if more than one trap needs to be moved. Holographic tweezers offer a very simple and cost efficient way of manipulating several traps independently in all three dimensions with an accuracy of less 100 nm. No mechanically moving parts are used therefore making them less vulnerable to vibration. They use computer-generated holograms (CGHs) written into a spatial light modulator (SLM) to control the position of each trap in space and to manipulate their shape. The ability to change the shape of the optical trap makes it possible to adapt the light field to a specific particle shape or in the case of force measurements to adjust the trapping potential. Furthermore the SLM can be used to correct for aberrations within the optical setup.

Fast hologram computation for holographic tweezers

We have shown that it is possible to accelerate considerably the computation of phase-only Fourier holograms by using a consumer graphics board (MSI 6800GT) instead of the ordinary CPU. Our fastest CPU solution (Pentium 4 @ 3.0 GHz) — using handcoded assembly code together with the multimedia extensions SSE — was outperformed by a factor of more than thirty resulting in an average performance of 14.1 GFlops for 100 doughnuts. The presented algorithm can be used for the computation of holograms for an arbitrary (up to 250) number of traps located at different positions in three dimensions and having independent trapping potentials.

Dynamic holography and its application in measurement systems

Spatial light modulators are of growing interest not only for optical correlators but also for new optical measurement and processing methods. We present different applications of dynamic phase holograms based on liquid crystal elements in the field of optical measurement and manipulation. Within digital holography, modern modulators can be used in order to test the geometry as well as the behavior of objects under external load. A direct comparison between the test objects and a master object at different locations around the world is possible. Holographic tweezers are used in order to position small particles in three dimensions and to measure very small forces. We also present results of novel methods for testing aspheric surfaces and the application of dynamic hologram reconstructions for the ablation of complex patterns on the microscopic scale.