We have recently demonstrated how holographic optical tweezers can be used to build and dynamically manipulate extended 3-D structures. Although successful trapping can be maintained even when a large number of traps are simultaneously manipulated, in general a gradual degradation of trap quality is observed as the number of traps increased. This degradation is partly attributed to the increased 3-D size of the structures. To build and control such large structures the high numerical aperture focusing objective lens has to operate away from its design conjugate for most of the traps, and therefore aberrations will be significant even for high quality objective lenses. A second effect is the decreasing efficiency of the liquid crystal spatial light modulators as they are required to display holograms that contain high spatial frequencies. However these factors do not appear to account fully for the observed weakening of the traps, and it is likely that a reduction of contrast in the trapping optical field also plays an important role. We examine the effects individual optical traps have on each other when they are in close proximity. Techniques that may be used to mitigate the reduced contrast will also be discussed.
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.
We use holographic optical tweezers to trap multiple micron-sized objects and manipulate them in 3-dimensions. Trapping multiple objects allow us to create 3-dimensional structures, examples of which include; simple cubes which can be rotated or scaled, complex crystal structures like the diamond lattice or interactive 3-dimensional control of trapped particles anywhere in the sample volume.
The micromanipulation of objects into 2-dimensional and 3-dimensional geometries within holographic optical tweezers is carried out using a modified Gerchberg-Saxton algorithm. The modified algorithm calculates phase hologram sequences, used to reconfigure the geometries of optical traps in several planes simultaneously. The hologram sequences are calculated automatically from the initial, intermediate and final trap positions. Manipulation of multiple objects in this way is semi-automated, once the traps in their initial positions are loaded.
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.
When a collimated beam of light is reflected by an approximately flat, mirror polished object and a screen is placed in the reflected beam some distance away for the object, a 'mirror image' or Makyoh topogram of the object is formed on the screen. For objects with surface height variations, the topogram will not have a uniform intensity distribution, but even small height variations will show up strongly amplified as dark or bright patches/lines. Makyoh topography has now been used for a number of years as a sensitive tool for the inspection of mirror polished surfaces, and in particular, semiconductor wafer surfaces. The main drawbacks of conventional Makyoh topography are: 1) Ambiguity of interpretation because almost identical Makyoh topograms can result from an object with some given surface height profile and constant reflectivity, an object with constant surface height and a given non-uniform reflectivity profile, or an object with both height variations and a non- uniform reflectivity profile. 2) Lack of quantitative interpretation, for example surface height values cannot be obtained from the contrast in conventional topograms.
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