Three-dimensional orientation of microscopic objects using combined elliptical and point optical tweezers

We report the development of a simple approach for controlled three-dimensional orientation of microscopic objects. The approach makes use of the fact that whereas in an elliptical trap, an object lacking spherical symmetry orients with its long axis along the major axis of the elliptical tweezers, in point tweezers it orients with its long axis along the trapping beam. Therefore, a combined use of point and elliptical tweezers and a control over the relative power of the two trapping beams can be used for three-dimensional orientation of the object. The use of this approach for three-dimensional orientation of biological objects varying in size from 2 to 20 μm is discussed. PACS 07.60.-j; 87.80.Cc