Optical gradient forces of strongly localized fields

We present a new approach for determining optical gradient forces applied by strongly focused laser beams on dielectric particles. We show that when the electromagnetic field is focused to a diffraction limited spot a dipole approximation is valid for ally particle size. We derive intuitive predictions for force-displacement curves, maximal trapping forces, and force constants. The theory fits well with recent measurements of particles trapped by laser tweezers. We also discuss effects of radiation pressure and gravity. [S0031-9007(98)06833-5].