Optical Trapping: A Versatile Technique for Biomanipulation

T he ability to manipulate biologically important molecules individually has made great impact in the ongoing biotechnological revolution. One of the most versatile and effective manipulation techniques is the optical trap, also known as laser tweezers. Tight focusing of high-intensity beams of single wavelength radiation (lasers) with high numerical aperture objectives allows for the generation of intense and localized electro-magnetic Ž elds or photon  uxes. Although photons have no mass, they can impart momentum. For example, at 1 W and assuming 10% of the laser light is re ected at the particle surface, a 1 mm bead can be accelerated at ;106 g.1 Through an appropriately shaped spatial distribution of light intensity, highly focused intense laser light can be exploited to exert force for manipulating small particles in room temperature solutions under biologically relevant conditions. This optical force, or radiation