  Optical vortex beams carry orbital angular momentum and thus exert torque on
illuminated objects. A dielectric microtool - a microbarbell - is used in a
two-laser optical tweezers to measure the torque of a focused optical vortex.
The tool was either freely rotating due to the applied torque or set into
oscillations by the counteracting force. Four different trapping configurations
provided different ways of sensing the torque and gave consistent results. The
value of torque was determined by confronting the experimental results with
numerical and analytical models.
