Optical Measurement Of Microscopic Torque By Using He-ne Laser

In recent years there has been an explosive development of interest in the measurementrnof forces at the microscopic level, such as within living cells [1,2,3], as well asrnthe properties of fluids and suspensions on this scale, using optically trapped particlesrnas probes. The next step would be to measure torques and associated rotationalrnmotion[4]. We demonstrate an optical system that can apply and accurately measurernthe torque exerted by the trapping beam on a rotating birefringent particle (calcite).rnHere we used a He-Ne laser beam of wave length of 633nm and power of 35mW. Byrntaking this laser beam we get an optical torque of 9.983pN.μm and the frequencyrnof the rotating particle is 112.16Hz and 136.94Hz for RCP and LCP beam. Thernlaser-induced torque acting on an optically trapped microscopic birefringent particlerncan be used for these measurements. Here we present a method for simple, robust,rnaccurate, simultaneous measurement of the rotation speed of a laser trapped birefringentrnparticle, and the optical torque acting on it, by measuring the change in angularrnmomentum of the light passing through the particle. This method does not dependrnon the size or shape of the particle or the laser beam geometry, nor does it depend onrnthe properties of the surrounding medium. This could allow accurate measurementrnof viscosity on a microscopic scale

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