  In recent years there has been an explosive development of interest in the
measurement of forces at the microscopic level, such as within living cells, as
well as the properties of fluids and suspensions on this scale, using optically
trapped particles as probes. The next step would be to measure torques and
associated rotational motion. This would allow measurement on very small scales
since no translational motion is needed. It could also provide an absolute
measurement of the forces holding a stationary non-rotating particle in place.
The laser-induced torque acting on an optically trapped microscopic
birefringent particle can be used for these measurements. Here we present a new
method for simple, robust, accurate, simultaneous measurement of the rotation
speed of a laser trapped birefringent particle, and the optical torque acting
on it, by measuring the change in angular momentum of the light from passing
through the particle. This method does not depend on the size or shape of the
particle or the laser beam geometry, nor does it depend on the properties of
the surrounding medium. This could allow accurate measurement of viscosity on a
microscopic scale.
