Accurate measurement of microscopic forces and torques using optical tweezers

  • Melanie McLaren National Laser Centre, CSIR
  • Elias Sidderas-Haddad School of Physics, University of the Witwatersrand
  • Andrew Forbes National Laser Centre, CSIR
Keywords: optical tweezing, angular momentum of light, vortex beams, optical forces, optical trapping


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.

Author Biography

Andrew Forbes, National Laser Centre, CSIR
School of Physics, University of KwaZulu-Natal


1. Kuyper CL, Chui DT. Optical trapping: A versatile technique for biomanipulation. Appl Spectrosc. 2002;56:300A–312A. doi:10.1366/00037020260377652

2. Arai Y, Yasuda R, Akashi K, et al. Tying a molecular knot with optical tweezers. Nature. 1999;399:446–448. doi:10.1038/20894, PMid:10365955

3. Ehrlicher A, Betz T, Stuhrmann B, et al. Guiding neuronal growth with light. Proc Natl Acad Sci USA. 2002;99:16024–16028. doi:10.1073/pnas.252631899, PMid:12456879, PMid:138558

4. Carnegie DJ, Stevenson DJ, Mazilu M, et al. Guided neuronal growth using optical line traps. Opt Express. 2008;16:10507–10517. doi:10.1364/OE.16.010507, PMid:18607464

5. Squires TM, Quake SR. Microfluidics: Fluid physics at the nanoliter scale. Rev Mod Phys. 2005;77:977–1026. doi:10.1103/RevModPhys.77.977

6. Eriksson E, Enger J, Nordlander B, et al. A microfluidic system in combination with optical tweezers for analyzing rapid and reversible cytological alterations in single cells upon environmental changes. Lab Chip. 2007;7:71–76. doi:10.1039/B613650H

7. Hopkins RJ, Mitchem L, Ward AD, Reid JP. Control and characterisation of a single aerosol droplet in a single-beam gradient force optical trap. Phys Chem Chem Phys. 2004;6:4924–4927. doi:10.1039/b414459g

8. Ashkin A, Dziedzic JM, Bjorkholm JE, Chu S. Observation of a single-beam gradient force optical trap for dielectric particles. Opt Lett. 1986;11:288–290. doi:10.1364/OL.11.000288, PMid:19730608

9. Ashkin A. Acceleration and trapping of particles by radiation pressure. Phys Rev Lett. 1970;24:156–159. doi:10.1103/PhysRevLett.24.156

10. Rohrbach A, Stelzer EHK. Trapping forces, force constants, and potential depths for dielectric spheres in the presence of spherical aberrations. Appl Opt. 2002;41:2494–2507. doi:10.1364/AO.41.002494, PMid:12009161

11. Smith S, Bhalotra S, Brody A, Brown B, Boyda E, Prentiss M. Inexpensive optical tweezers for undergraduate laboratories. Am J Phys. 1998;67:26–35. 2002;41:2494–2507. doi:10.1119/1.19187

12. Ashkin A. Forces of a single-beam gradient laser trap on a dielectric sphere in the ray optics regime. Biophys J. 1992;61(2):569–582.

13. Beth RA. Mechanical detection and measurement of the angular momentum of light. Phys Rev. 1936;50:115–125. doi:10.1103/PhysRev.50.115

14. Poynting JH. The wave motion of a revolving shaft, and a suggestion as to the angular momentum in a beam of circularly polarised light. Proc R Soc Lond A. 1909;82:560–567. doi:10.1098/rspa.1909.0060

15. Friese MEJ, Nieminen TA, Heckenberg NR, Rubinsztein-Dunlop H. Optical alignment and spinning of laser-trapped microscopic particles. Nature. 1998;394:348–351. doi:10.1038/28566

16. Simpson NB, Dholakia K, Allen L, Padgett MJ. Mechanical equivalence of spin and orbital angular momentum of light: An optical spanner. Opt Lett. 1997;22:52–54. doi:10.1364/OL.22.000052, PMid:18183100

17. Allen L, Beijersbergen MW, Spreeuw RJC, Woerdman JP. Orbital angular momentum of light and the transformation of Laguerre–Gaussian laser modes. Phys Rev A. 1992;45:8185–8189. doi:10.1103/PhysRevA.45.8185, PMid:9906912

18. He H, Friese MEJ, Heckenberg NR, Rubinsztein-Dunlop H. Direct observation of transfer of angular momentum to absorptive particles from a laser beam with a phase singularity. Phys Rev Lett. 1995;75:826–829. doi:10.1103/PhysRevLett.75.826, PMid:10060128

19. Fischer P, Little H, Smith RL, et al. Wavelength dependent propagation and reconstruction of white light bessel beams. J Opt A: Pure Appl Opt. 2006;8:477–482. doi:10.1088/1464-4258/8/5/018

20. Neuman K, Block S. Optical trapping. Rev Sci Instrum. 2004;75:2787–2809. doi:10.1063/1.1785844, PMid:16878180, PMid:1523313

21. Dholakia K, Reece P, Gu M. Optical micromanipulation. Chem Soc Rev. 2007;37:42–55. doi:10.1039/b512471a, PMid:18197332

22. Bechhoefer J, Wilson S. Faster, cheaper, safer optical tweezers for the undergraduate laboratory. Am J Phys. 2001;70:393–400. doi:10.1119/1.1445403

23. Leach J, Mushfique H, Di Leonardo R, Padgett M, Cooper J. An optically driven pump for microfluidics. Lab Chip. 2006;6:735–739.

24. Neale SL, MacDonald MP, Dholakia K, Krauss TF. All-optical control of microfluidic components using form birefringence. Nat Mater. 2005;4:530–533. doi:10.1038/nmat1411, PMid:15965480