Optical tweezers use the gradient force created by tightly focused single laser beam to trap dielectric microparticles. While this technique has been used for over 20 years to manipulate particles without mechanical contact, it is only recently htat accurate and quantitative photonic forces measurements have been considered. Moreover of great relevance has resulted the monitoring of Brownian motion of particles confined in optical traps since that provides precious information on local visco-elastic properties of the surrounding fluid. In this work we consider the still open question concerning the calibration of an optical tweezers which represents the key point for any absolute measurements. In particular, we discuss a novel method to calibrate a quadrant photodiode used as sensor position in the forward scattering scheme. The voltage signals provided by the qudrant photodiode are converted in length units by comparing them with the absolute bead positions measured by means of a calibrated CCD camera. Finally we briefly discuss how calibrated optical tweezers, combined with thermal analysis of the Brownian motion, are potentially of great relevance for microrheological studies of complex fluids.

Date Published: 22 April 2005
PDF: 11 pages
Proc. SPIE 5830, 13th International School on Quantum Electronics: Laser Physics and Applications, (22 April 2005); doi: 10.1117/12.618434

Published in SPIE Proceedings Vol. 5830:
13th International School on Quantum Electronics: Laser Physics and Applications
Peter A. Atanasov; Sanka V. Gateva; Lachezar A. Avramov; Alexander A. Serafetinides, Editor(s)