Direct measurement of torque in the optical trap

Principal Investigator: Pál Ormos
This project is part of the EU-FP6 cooperation ATOM-3D

If a flat object is grabbed in an optical trap formed by linearly polarized light, it will be oriented due to the anisotropic scattering of polarized light. This gives additional control in the optical manipulation of microscopic particles. We studied the details of the effect experimentally and interpreted the results. By photopolymerisation we prepared cross shaped test objects that assume a stable position in the trap. The test object as designed and built:



Click here to download a movie of an experiment demonstrating how a flat particle can be oriented and rotated by spinning a half-wave plate in the path of the trapping laserbeam.

We determined the function of the restoring torque upon the deflection angle experimentally and interpreted the data by model calculations. The restoring torque vs. angle difference between the plane of particle and polarisation:



Using this phenomenon, with the help of particles of appropriate shape we can apply and measure torque – a most useful tool in biology.

Reference:
Péter Galajda and Pál Ormos, Orientation of flat particles in optical tweezers by linearly polarized light, Opt. Express, 11 5 446-451 (2003)