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18 October 2004 Design and characterization of all-optical constant-force laser tweezers
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Optical tweezers are widely used to manipulate individual biomolecules. Conventionally, they hold a molecule at a constant extension while the restoring force of the molecule is measured. Many applications, however, require that the molecule be held under constant force while its extension is measured. We developed two all-optical trapping techniques that provide such constant-force conditions together with a fast extension measurement scheme. Our techniques are based on a scanning-line optical trap, which provides a one-dimensional flat optical potential well. A position-independent lateral force is generated either by a synchronous modulation of the laser intensity during the scan or the use of an asymmetrically shaped beam profile in the back focal plane of the microscope objective. The position of the trapped particle as a measure of the extension of an attached molecule is measured by monitoring the diffraction pattern of the forward-scattered laser light. The performance of these techniques, along with consideration for the design and calibration of the instrument is discussed.
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Rajalakshmi Nambiar, Ben Liesfeld, Arivalagan Gajraj, and Jens-Christian D. Meiners "Design and characterization of all-optical constant-force laser tweezers", Proc. SPIE 5514, Optical Trapping and Optical Micromanipulation, (18 October 2004);

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