Experimental determination of trapping efficiency of optical tweezers

Abstract

We report an experimental determination of trapping efficiency of optical fields acting on spherical particles diluted in viscous fluids. This parameter was derived from experimentally calculated trap stiffness for micron-sized silica beads with particle radii of 1.5 and 4.8 microns, respectively. In order to obtain the trap stiffness, we used the equipartition theorem, and introduced an experimental procedure to calibrate the optical tweezers set-up. The method consists of the calculation of the trapping efficiency by using the curves of the trap stiffness vs. the incident power. Using this approach we measured the efficiency for different values of displacements of the beads in the optical trap, and compared these results with numerical and theoretical calculations reported in the literature, and an excellent agreement was found. We have shown that the ratio of efficiencies is very close to the ratio of the particle radii.

Keywords:

• lasers
• biophysics
• biological applications
• cellular structures
• medical applications
• microscopy

Acknowledgements

This work was performed in the framework of the Associateship Programme of the ‘Abdus Salam International Centre for Theoretical Physics (ICTP)’, and the bilateral cooperation agreement with the Optical Manipulation Lab, Istituto Officina dei Materiali CNR-IOM, Trieste, Italy. The work was also sponsored by the Instituto Venezolano de Investigaciones Científicas (IVIC), Venezuela.