Optimization of probe-laser focal offsets for single-particle tracking

Ai Tang Chang*, Yi Ren Chang, Sien Chi, Long Hsu

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


In optical tweezers applications, tracking a trapped particle is essential for force measurement. One of the most popular techniques for single-particle tracking is achieved by analyzing the forward and backward light pattern, scattered by the target particle trapped by a trap laser beam, of an additional probelaser beam with different wavelength whose focus is slightly apart from the trapping center. However, the optimized focal offset has never been discussed. In this paper, we investigate the tracking range and sensitivity as a function of the focal offset between the trapping and the probe-laser beams. As a result, the optimized focal offsets are a 3.3-fold radius ahead and a 2.0-fold radius behind the trapping laser focus in the forward tracking and the backward tracking, respectively. The experimental result agrees well with a theoretical prediction using the Mie scattering theory.

Original languageEnglish
Pages (from-to)5643-5648
Number of pages6
JournalApplied Optics
Issue number23
StatePublished - 10 Aug 2012


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