Physics

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Optical Measurement of Microscopic Torque by using He-Ne Laser
(Addis Ababa University, 2009-06) Abebe Tamirat; Asfaw Araya (PhD)
In recent years there has been an explosive development of interest in the measurement 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 particles 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 birefringent 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