Radiation forces on a dielectric sphere in the Rayleigh scattering regime

Theoretical expressions of the radiation pressure force for a dielectric sphere in the Rayleigh regime of light scattering under illumination of a Gaussian laser beam with the fundamental mode are derived in explicit form as a function of measurable quantities of the beam parameter in MKS units. Correctness of the derived expressions and validity of the size range of the Rayleigh approximation for the radiation forces as a sum of the scattering force and the gradient force are investigated by a graphical comparison of the calculated forces in longitudinal and transverse components with those obtained from the generalized Lorenz-Mie theory. Fairly good agreement in both components is found within ordinary particle-size ranges of the Rayleigh scattering theory. Furthermore, the good agreement in the transverse component, where the gradient force is dominant, is found to be satisfactory beyond the existing criterion in particle size of the Rayleigh scattering theory until the particle size becomes comparable with the spot size of the illuminating laser beam.