AFM observation of force on a dielectric sphere in the evanescent field of totally reflected light

We present the first direct measurement of the radiation pressure force acting on a sphere in the evanescent field of a totally reflected light beam using the atomic force microscope (AFM). A dielectric sphere was attached to the AFM cantilever and placed into the evanescent light field of the Ar-laser beam illuminating a sapphire prism surface at an angle larger than the critical. A repulsive force due to the evanescent field was observed. The force decreases exponentially with the characteristic length of (45 ± 20) nm as the distance between the sphere and the total reflection surface increases. The measured magnitude of the force close to the surface is (3 ± 1.5) · 10 −10 N. Both the magnitude and the decay length are in good agreement with the calculated values.