Measurement and Control of Optically Induced Rotation of Anisotropic Shaped Particles

When a particle of anisotropic shape is trapped by a laser beam, it generally begins to rotate around itself. This self-rotation is caused by the torque resulting from total forces acting on the particle surface. We have measured the rotation rate rapidly and automatically from temporal variation of light scattered from the rotating particle. The rotation rate, which could be measured up to 2200 rpm, has been shown to be linearly dependent on the power of the trapping beam. We controlled the rotation rate by a feedback loop connecting the measured rotation rate and the trapping beam power.