Zsolt John Laczik

Interactive approach to optical tweezers control

We have developed software with an interactive user interface that can be used to generate phase holograms for use with spatial light modulators. The program utilizes different hologram design techniques, allowing the user to select an appropriate algorithm. The program can be used to generate multiple beams and can be used for beam steering. We see a major application of the program to be in optical tweezers to control the position, number, and type of optical traps.

3D manipulation of particles into crystal structures using holographic optical tweezers

We have developed holographic optical tweezers that can manipulate many particles simultaneously in three dimensions in order to create micro-crystal structures that extend over many tens of microns. The technique uses specific hologram-design algorithms to create structures that can be dynamically scaled or rotated about arbitrary axes. We believe the generation and control of pre-determined crystal-like structures have significant potential in fields as diverse as photonic-crystal construction, seeding of biological tissue growth and creation of metrological standards within nanotechnology.

Assembly of 3-dimensional structures using programmable holographic optical tweezers

The micromanipulation of objects into 3-dimensional geometries within holographic optical tweezers is carried out using modified Gerchberg-Saxton (GS) and direct binary search (DBS) algorithms to produce the hologram designs. The algorithms calculate sequences of phase holograms, which are implemented using a spatial light modulator, to reconfigure the geometries of optical traps in many planes simultaneously. The GS algorithm is able to calculate holograms quickly from the initial, intermediate and final trap positions. In contrast, the DBS algorithm is slower and therefore used to pre-calculate the holograms, which are then displayed in sequence. Assembly of objects in a variety of 3-D configurations is semi-automated, once the traps in their initial positions are loaded.

Interactive application in holographic optical tweezers of a multi-plane Gerchberg-Saxton algorithm for three-dimensional light shaping.

Phase-hologram patterns that can shape the intensity distribution of a light beam in several planes simultaneously can be calculated with an iterative Gerchberg-Saxton algorithm [T. Haist et al., Opt. Commun. 140, 299 (1997)]. We apply this algorithm in holographic optical tweezers. This allows us to simultaneously trap several objects in individually controllable arbitrary 3-dimensional positions. We demonstrate the interactive use of our approach by trapping microscopic spheres and moving them into an arbitrary 3-dimensional configuration.

Effect of half-stop lateral misalignment on imaging of dark-field and stereoscopic confocal microscopes

There are several ways to realize dark-field imaging in confocal microscopy. In a recent paper [J. Microsc. 181 260-268 (1996)] we suggested a simple modification of a commercial confocal microscope to incorporate dark-field imaging. This modification involved an aperture stop covering half of the entrance pupil of the objective lens. Now we investigate the lateral misalignment of the aperture stop for dark-field and stereoscopic confocal microscopes. We show the effect of lateral alignment of the half-stop on the point-spread and transfer functions and also examine the detected signal from a sloping plane reflector. Lateral and axial resolution values are given from theoretical data.

3D beam shaping using diffractive optical elements

Conventionally laser beam shaping problems are defined by the required intensity and/or phase distribution in a single 2-D output plane, although recently there have also been examples for beam shaping solutions where the system output had to satisfy constraints in a small number of axially separated planes. For a number of application areas it is beneficial to be able to work with beams that have a particular intensity distribution that is specified in a 3-D volume. Laser material processing, optical microscopy and laser trapping (optical tweezers) are a few examples for these. We will discuss how diffractive optical elements can be used to generate beams with prescribed 3-D intensity profiles, with particular emphasis on techniques for the design of such diffractive optics. Practical examples will be given for the implementation of the diffractive optical elements using programmable spatial light modulators and for the application of the 3-D beams.

Discrete-dipole-approximation-based light-scattering calculations for particles with a real refractive index smaller than unity

To assess the efficiency and accuracy of light-scattering calculations based on the discrete dipole approximation (DDA) for particles with a real relative refractive index smaller than unity, differential scattering cross sections and scattering efficiency factors were calculated for spherical particles. We performed the calculations for oxide particles and voids embedded in glass and silicon, using the exact scattering theory (Mie scattering) and the DDA. A comparison of the results shows that the DDA is applicable in the above refractive-index regime, and the conditions under which DDA-based calculations can provide scattering data with good accuracy are discussed.

An interactive approach to optical tweezer control

We have developed an interactive user-interface that can be used to generate phase holograms for use with spatial light modulators. The program utilises different hologram design techniques allowing the user to select an appropriate algorithm. The program can be used to generate multiple beams, interference patterns and can be used for beam steering. We therefore see a major application of the program to be within optical tweezers to control the position, number and type of optical traps.