A thesis submitted June 29, 2015 for the degree of Doctor of Philosophy and defended August 27, 2015.
The PhD School of Science
Faculty of Science
Niels Bohr Institute, Condensed Matter Theory, University of Copenhagen
Lene Broeng Oddershede
Photodiode based detection for multiple trap optical tweezers
This thesis is concerned with the position tracking of microscopic, optically trapped particles and the quantification of the forces acting on them. A new detection method for simultaneous, three-dimensional tracking of multiple particles is presented, its performance is evaluated, and its usefulness is illustrated in specific application examples.
Optical traps enable contact-less, all-optical manipulation of microscopic objects. Over the last decades, this laser-based micro-manipulation tool has facilitated numerous exciting discoveries within biology and physics, and it is today regarded as one of the workhorses of biophysical research. There exists a variety of implementations of optical traps, from simple single traps to complex multiple traps with engineered three-dimensional light fields. In comparison to single beam optical traps, multiple beam optical traps offer more freedom in terms of the design of experiments due to the extended manipulation capabilities. However, this advantage typically comes at the price of a more challenging detection of the trapped particles’ positions and the forces they experience. To alleviate these problems, we have developed a new detection technique that is simple, effective and easy-to-implement in existing optical setups. The technique relies on spatial filtering and is compatible with the prevalent photodiode based detection method, i.e., back focal plane interferometry, both in terms of the technical implementation and the subsequent data analysis. The proposed method is demonstrated experimentally, and it is confirmed that hallmarks of photodiode based detection remain valid. Its efficiency is experimentally quantified and the results are supported by simulations.
Two important application cases are explored in more detail: Crosstalk elimination in dual-beam optical traps and the three-dimensional tracking of multiple trapped particles in parallel in holographically generated traps. Standard dual-beam optical traps rely on polarization states – not only to create the two individual traps, but also to separate the signals from the respective optical traps that contain information about the movement of the trapped particles.
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