Measurement of the effective focal shift in an optical trap

The focus of an oil-immersion microscope objective is shifted because of the refractive-index mismatch between the cover glass and the aqueous sample. We present a procedure with which to determine the focal shift by use of an inverted microscope equipped with optical tweezers. As the position of th...

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Bibliographic Details
Published in:Optics letters 2005-06, Vol.30 (11), p.1318-1320
Main Authors: NEUMAN, Keir C, ABBONDANZIERI, Elio A, BLOCK, Steven M
Format: Article
Language:English
Subjects:
Biological and medical sciences
Conventional optical microscopes
Exact sciences and technology
Fundamental and applied biological sciences. Psychology
General aspects
Instrumentation. Materials. Reagents. Research laboratory organization
Instruments, apparatus, components and techniques common to several branches of physics and astronomy
Optical instruments, equipment and techniques
Optical trapping
Physics
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Summary:The focus of an oil-immersion microscope objective is shifted because of the refractive-index mismatch between the cover glass and the aqueous sample. We present a procedure with which to determine the focal shift by use of an inverted microscope equipped with optical tweezers. As the position of the sample chamber is scanned vertically, we measure the axial displacement of an optically trapped bead; the relative motion of the bead with respect to the surface supplies the effective focal shift. Measurements of this quantity deviate from electromagnetic calculations of the focal shift, a discrepancy attributable to the depth-dependent decrease in axial trap stiffness that arises from spherical aberration.
ISSN:0146-9592
1539-4794
DOI:10.1364/ol.30.001318