Measurement of rotation speed of birefringent material and optical torque from polarisation of transmitted light

A microscopic birefringent particle can be simultaneously trapped and rotated by a circularly polarised laser beam. A laser-trapped birefringent particle will act as a wave-plate (a calcite particle approximately 3 /spl mu/m thick is a /spl lambda//2 plate for 1064 nm light), and will alter the polarisation of incident light. If the particle is composed of a negative uniaxial birefringent material, such as calcite, the particle can also be stably trapped in the correct orientation by the same beam that is used to rotate it. As an elliptically polarised incident beam passes through the particle, the polarisation, and therefore the angular momentum carried by the beam is altered. The difference between the incident and outgoing angular momenta determines the torque acting on the particle to cause rotation. This torque can induce high rates of rotation, in excess of 350 Hz with very little heating. The maximum rotation rate results when the incident light is completely circularly polarised.