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Please use this identifier to cite or link to this item: http://hdl.handle.net/123456789/2373

Authors: Tamirat, Abebe Desta
Advisors: Dr. Araya Asfaw
Keywords: physics
Copyright: 25-May-2009
Date Added: 3-May-2012
Publisher: AAU
Abstract: In recent years there has been an explosive development of interest in the measure- ment of forces at the microscopic level, such as within living cells [1,2,3], as well as the properties of fluids and suspensions on this scale, using optically trapped parti- cles as probes. The next step would be to measure torques and associated rotational motion[4]. We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent particle (calcite). Here we used a He-Ne laser beam of wave length of 633nm and power of 35mW . By taking this laser beam we get an optical torque of 9.983pN.μm and the frequency of the rotating particle is 112.16Hz and 136.94Hz for RCP and LCP beam. The laser-induced torque acting on an optically trapped microscopic birefringent particle can be used for these measurements. Here we present a method for simple, robust, accurate, simultaneous measurement of the rotation speed of a laser trapped birefrin- gent particle, and the optical torque acting on it, by measuring the change in angular momentum of the light passing through the particle. This method does not depend on the size or shape of the particle or the laser beam geometry, nor does it depend on the properties of the surrounding medium. This could allow accurate measurement of viscosity on a microscopic scale.
URI: http://hdl.handle.net/123456789/2373
Appears in:Thesis - Physics

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