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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 13, Iss. 15 — Jul. 25, 2005
  • pp: 5798–5806

Three-dimensional tracking of Brownian motion of a particle trapped in optical tweezers with a pair of orthogonal tracking beams and the determination of the associated optical force constants

Ming-Tzo Wei and Arthur Chiou  »View Author Affiliations

Optics Express, Vol. 13, Issue 15, pp. 5798-5806 (2005)

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We report the first experimental results on quantitative mapping of three-dimensional optical force field on a silica micro-particle and on a Chinese hamster ovary cell trapped in optical tweezers by using a pair of orthogonal laser beams in conjunction with two quadrant photo-diodes to track the particle’s (or the cell’s) trajectory, analyze its Brownian motion, and calculate the optical force constants in a three-dimensional parabolic potential model. For optical tweezers with a 60x objective lens (NA = 0.85), a trapping beam wavelength λ = 532nm, and a trapping optical power of 75mW, the optical force constants along the axial and the transverse directions (of the trapping beam) were measured to be approximately 1.1×10-8N/m and 1.3×10-7N/m, respectively, for a silica particle (diameter = 2.58μm), and 3.1×10-8 N/m and 2.3×10-7 N/m, respectively, for a Chinese hamster ovary cell (diameter ~ 10 μm to 15 μm). The set of force constants (Kx, Ky, and Kz) completely defines the optical force field E(x, y, z) = [Kx x2 + Ky y2 + Kz z2]/2 (in the parabolic potential approximation) on the trapped particle. Practical advantages and limitations of using a pair of orthogonal tracking beams are discussed.

© 2005 Optical Society of America

OCIS Codes
(140.7010) Lasers and laser optics : Laser trapping
(170.1420) Medical optics and biotechnology : Biology
(170.4520) Medical optics and biotechnology : Optical confinement and manipulation

ToC Category:
Research Papers

Original Manuscript: June 3, 2005
Revised Manuscript: July 9, 2005
Published: July 25, 2005

Ming-Tzo Wei and Arthur Chiou, "Three-dimensional tracking of Brownian motion of a particle trapped in optical tweezers with a pair of orthogonal tracking beams and the determination of the associated optical force constants," Opt. Express 13, 5798-5806 (2005)

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