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Unformatted text preview: A Practical Guide to Optical Trapping Joshua W. Shaevitz [email protected] August 22, 2006 1 Introduction to optical trapping In the last few decades, novel microscopy techniques have been developed to monitor the activity of single enzymes as they perform their biological functions in vitro. Motor proteins such as kinesin, myosin, F 1 F o ATPase, and RNA polymerase have been mercilessly subjected to magnetic, elastic, and optical forces [14, 40, 48, 16, 18]. In 1986, Ashkin and colleagues reported the first observation of a stable three- dimensional optical trap, or optical tweezers, created using radiation pressure from a single laser beam . Only a few years later, Block and colleagues had used an optical trap to manipulate and apply forces to E. coli flagella  and single kinesin motors . Optical traps use light to manipulate microscopic objects as small as 10 nm using the radiation pressure from a focused laser beam. In addition, measurement of the light deflection yields information about the position of the object in the laser focus. Many excellent reviews have been written about optical trapping, its uses, and designs, see e.g. [2, 6, 22, 27, 37, 38, 43]. In particular, Lang and Block  is a thorough review of the optical trapping literature. This manuscript is meant to be a practical guide to understanding optical traps, and not an in depth review. When possible, simple examples and explanations are used to give the reader an intuitive feel for how these systems work and how they are implemented. I hope that this document will continue to improve, and welcome any comments. The picoNewton and nanometer ranges of force and distance accessible to optical traps make them particularly useful for studying biological systems (Fig. 1) . Optical forces have been used to investigate structural properties of biological polymers such as DNA [10, 46], membranes , whole cells  and microtubules . Microrheological properties of these objects can be probed through the application of forces either to the object itself, or to a small dielectric sphere, or bead, to which the object is attached. Molecular motors represent the most used application of optical traps in the biological sciences. A great deal has been learned about kinesin [1, 9, 11, 12, 19, 20, 44], dynein [26, 25], myosin [28, 32, 33, 41, 42], and RNA polymerase [13, 15, 30, 36, 45] using optical forces. 2 How optical traps work In the focus of a laser beam a dielectric particle, such as a glass or polystyrene bead, experiences a force, called the gradient force, that tends to bring push towards the laser focus where the light intensity is highest. This force arises from the momentum imparted to the bead as it scatters the laser light....
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- Fall '10
- Physics, Keplerian telescope, optical trap, Steven M. Block, AODs, optical traps