SPECTROSCOPY, IMAGING, OTHER TECHNIQUES |
Detection of Forces and Displacements Along the Axial Direction in an Optical Trap
Christopher Deufel 1 and Michelle Wang 1*
1 Cornell University
* To whom correspondence should be addressed. E-mail: mdw17{at}cornell.edu.
Submitted on April 28, 2005
Revised on June 11, 2005
Accepted on 29 September 2005
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Abstract |
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We present measurements of the forces on, and displacements of, an optically trapped bead along the propagation direction of the trapping laser beam (the axial direction). In a typical experimental configuration, the bead is trapped in an aqueous solution using an oil-immersion, high numerical aperture objective. This refractive index mismatch complicates axial calibrations due to both a shift of the trap center along the axial direction and spherical aberrations. In this work, a known DNA template was unzipped along the axial direction and its characteristic unzipping force-extension data were used to determine (1) the location of the trap center along the axial direction, (2) the axial displacement of the bead from the trap center, and (3) the axial force exerted on the bead. These axial calibrations were obtained for trap center locations up to ~ 4 microns into the aqueous solution and with axial bead displacements up to ~ 600 nm from the trap center. In particular, the axial trap stiffness decreased substantially when the trap was located further into the aqueous solution. This approach, together with conventional lateral calibrations, results in a more versatile optical trapping instrument that is accurately calibrated in all three dimensions.
Key Words:
aberration, axial detection, force and displacement, optical trap, single molecule, unzipping DNA
