We have constructed a laser optical force trap ("laser tweezers") by coupling an Nd:YAG laser to an optical microscope with a high numerical aperture objective. The laser beam (approximately 0.1 W power) is focused to a diffraction-limited spot at the specimen plane of the objective: the wavelength chosen (1,064 nrn) is not strongly absorbed by most biological materials and is thus not ablative. Because the intensity of the laser beam increases towards the center of the focal spot, small particles brought near the spot will be attracted to the center and held there. Movement of the laser beam will tend to move any trapped particles with it. The laser tweezers can permit precise, nondestructive repositioning of small structures inside a living cell, without recourse to rnicromanipulators. Initial work has involved the use of laser tweezers on cells of Paramecium tetraurelia held by a rotocornpressor. We have been able to trap and reposition small organelles, especially the highly refractile structures known as crystals. Using a trapped crystal as a "tool," we have been able to push micronuclei and other structures for many micrometers to virtually any desired location in a cell. In spite of extended exposure of specific structures and of individual cells to the laser beam, no damage has been detectible. Exposed cells, which were removed from the rotocornpressor and cultured, showed complete viabilty. The laser tweezers technique shows tremendous potential for applications to the study of many fundamental cellular and developmental phenomena in paramecia and other ciliates. For example, we intend to use this technique to investigate temporal and spatial characteristics of nuclear determining regions during sexual reorganization in Paramecium. 0 1992 Wiley-Llss, Inc ulation of neutral atoms and molecules in vacuum [for review, see Chu, 19911. However, the technique was found to apply also to small particles in aqueous solutions, including bacteria and structures in eukaryotic cells [Ashkin and Dziedzic, 19871. Several intriguing projects have made use of the unexcelled ability of laser tweezers to capture and reposition biological structures without damage. Although the mechanical forces generated by the tweezers are extremely small, further exploration of potential applications of this technique to biological questions is amply justified.The principle of the laser tweezers depends on the radiation pressure generated by focused laser illumination [Chu, 19911. In practice, a laser beam is directed into a light microscope objective of high numerical aperture [Ashkin et al., 1986 Ashkin and Dziedzic, 19871. The beam is focused into a diffraction-limited spot in the field of the objective. Small particles near this spot will tend to be attracted into its center and held there. Physically, the process is analogous to the attraction between a speck of dust and a comb carrying a high electrostatic charge. The large electric field gradient near the comb produces a force on a n adjacent neutral object (the dust part...
