Research Articles

Accurate measurement of microscopic forces and torques using optical tweezers

Melanie McLaren, Elias Sidderas-Haddad, Andrew Forbes
South African Journal of Science | Vol 107, No 9/10 | a579 | DOI: https://doi.org/10.4102/sajs.v107i9/10.579 | © 2011 Melanie McLaren, Elias Sidderas-Haddad, Andrew Forbes | This work is licensed under CC Attribution 4.0
Submitted: 03 January 2011 | Published: 19 September 2011

About the author(s)

Melanie McLaren, National Laser Centre, CSIR, South Africa
Elias Sidderas-Haddad, School of Physics, University of the Witwatersrand, South Africa
Andrew Forbes, National Laser Centre, CSIR, South Africa


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Abstract

It is now well known that matter may be trapped by optical fields with high intensity gradients. Once trapped, it is then possible to manipulate microscopic particles using such optical fields, in so-called optical tweezers. Such optical trapping and tweezing systems have found widespread application across diverse fields in science, from applied biology to fundamental physics. In this article we outline the design and construction of an optical trapping and tweezing system, and show how the resulting interaction of the laser light with microscopic particles may be understood in terms of the transfer of linear and angular momentum of light. We demonstrate experimentally the use of our optical tweezing configuration for the measurement of microscopic forces and torques. In particular, we make use of digital holography to create so-called vortex laser beams, capable of transferring orbital angular momentum to particles. The use of such novel laser beams in an optical trapping and tweezing set-up allows for the control of biological species at the single-cell level.

Keywords

optical tweezing; angular momentum of light; vortex beams; optical forces; optical trapping

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