A numerical study of the radiation forces on an absorbing particle using the ray optics approach

Abstract

Radial and axial force equations were developed using the ray optics approach for an absorbing particle. The results on the axial force indicated that the force was always positive regardless of the particle size, location, refractive index, and extinction coefficient. The magnitude of the force, however, depends strongly of these parameters. The results on the radial force showed that, for non-absorbing particles, the radial force was positive (in the direction toward the center of the radiation beam) and it became negative (in the direction away from the center of the radiation beam) only for perfectly reflecting particles or particles with refractive index less than 1. For highly absorbing particles (particles with extinction coefficient k2>0.1), the radial force was negative regardless of the particle size and refractive index. For weakly absorbing particles (k2<0.1), the direction of the force depended on both the particle size and the extinction coefficient. The calculated results for k2=0.005 showed that the radial force switched from the positive direction to negative direction as the particle size increased.