FORCE EXERTED BY A LASER-BEAM ON A MICROSCOPIC SPHERE IN WATER - DESIGNING FOR MAXIMUM AXIAL FORCE

When light strikes a body, it exerts a force which arises from the mom entum of the photons. Here we use ray optics to predict the force on a sphere (much larger than the wavelength of light) exerted by a laser beam of low divergence (in contrast with high divergence beams used to form ''optical traps'') to determine if the magnitude of force attain able is large enough to dislodge microscopic particles adhered to surf aces. For simple dielectric microspheres (e.g. polystyrene or glass) t he maximum axial force of about 0.6 nN/W is exerted by an ideal unifor m beam, concentric with the sphere, when the beam radius is comparable to the sphere radius. Donut and Gaussian beams produce slightly small er maximum forces. Focusing the beam to a waist size much smaller than the sphere and aiming it at the edge of the sphere can produce axial forces of 5 nN/W, independent of beam shape. Unfortunately, this eccen tricity also generates a torque which would stress the adhesive nonuni formly. The absolute maximum force of 8.9 nN/W (2/v, where v is the sp eed of light through the surrounding medium) can be obtained without t orque using a concentric beam if the sphere is constructed of a highly reflective material. Our results suggest that radiation forces can be used to detach microscopic particles adhered to surfaces.