OPTICAL GRADIENT FORCES OF STRONGLY LOCALIZED FIELDS

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