Optics and Quantum Optics

Small birefringent particles held in optical tweezers rotate under the spin angular momentum transfer from polarized light. In contrast to windmill shape particles which rotate in a light beam due to (linear) momentum transfer, the rotation of these small crystals can be switched from clockwise to counter clockwise by changing the polarization of the incident light. The torque on the platelets is not constant during one revolution, but varies with the platelet orientation, the retardation, and the ellipticity of the incident light. The films show rotating small particles of highly birefringent materials in our laser tweezer. If driven by elliptically polarised light a wobbling angular velocity can be observed, whereas it switches to a fast rotation at constant velocity in circularly polarised light.