Measurements of viscoelasticity in the microscopic regime are of interest in polymer solutions as well as in
microscopic structures such as cells. Viscoelasticity can be studied using a localized microrheometer based
on optical tweezers. We rotate a birefringent micron-sized calcium carbonate sphere crystallized in a vaterite
structure. By applying a time-dependent torque or using the time-dependent thermal torque, viscoelasticity can
be measured. The torque can be measured purely optically, by measuring the polarization state of the trapping
beam after passing through the particle. We control the torque by controlling the relative amplitudes of two
orthogonally circularly polarized components of the trapping beam with two acousto-optic modulators. This
allows a wide range of oscillation frequencies to be used. We demonstrate applications of the methods on several
Optical forces and torques acting on microscopic objects trapped in focussed laser beams promise flexible methods of driving micromachines through a microscope cover slip or even a cell wall.
We are endeavouring to engineer special purpose micro-objects for a range of tasks. Colloidal self assembly of calcium carbonate provides birefringent spheres which can exert considerable torque, while two photon polymerisation allows us to fabricate objects of arbitrary shape that can be designed to exchange both spin and orbital angular momentum. Numerical calculations of forces and torques can allow an optimal design, and optical measurements provide us with certain knowledge of the forces and torques which are actually exerted.