Shape-induced force fields in optical trapping


Advances in optical tweezers, coupled with the proliferation of two-photon polymerization systems, mean that it is now becoming routine to fabricate and trap non-spherical particles. The shaping of both light beams and particles allows fine control over the flow of momentum from the optical to mechanical regimes. However, understanding and predicting the behaviour of such systems is highly complex in comparison with the traditional optically trapped microsphere. In this Article, we present a conceptually new and simple approach based on the nature of the optical force density. We illustrate the method through the design and fabrication of a shaped particle capable of acting as a passive force clamp, and we demonstrate its use as an optically trapped probe for imaging surface topography. Further applications of the design rules highlighted here may lead to new sensors for probing biomolecule mechanics, as well as to the development of optically actuated micromachines.

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Figure 1: T-matrix calculations showing the effect of particle shape on optical force in a Gaussian beam.
Figure 2: Experimentally measured force–displacement profiles of horizontally trapped cylinders and tapers.
Figure 3: Using a shaped particle as a scanning probe to image surface topography.


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This work was funded through the Engineering and Physical Sciences Research Council, UK (grant nos. EP/I010785/1 and EP/I007822/1). It was carried out with the support of the Bristol Centre for Nanoscience and Quantum Information and using the computational facilities of the Advanced Computing Research Centre, University of Bristol ( The authors thank J. Rarity for the use of the Nanoscribe Photonic Professional direct write laser lithography system for structure fabrication, and Sarah Dolan for support with illustrations. M.J.M. and M.J.P. acknowledge Royal Society Wolfson Merit Awards.

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D.B.P. fabricated the structures and designed and performed all experiments and analysis. S.H.S. formulated the concept of relating shape to optical force and performed all theoretical calculations, with support from S.H. Y.-L.D.H. assisted with structure fabrication. M.J.P. provided support to the optical tweezers apparatus. M.J.M., M.J.P. and D.M.C. developed the concept of the scanning probe application. S.H.S., D.B.P. and S.H. wrote the paper, and all other authors provided editorial input.

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Correspondence to D. B. Phillips.

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Phillips, D., Padgett, M., Hanna, S. et al. Shape-induced force fields in optical trapping. Nature Photon 8, 400–405 (2014).

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