Size dependence of gradient and nongradient optical forces in silver nanoparticles

We examine the size dependence of gradient and nongradient contributions to the optically induced force on single, isolated nanometer-sized silver particles in water under plane-wave illumination. Using a recently developed method based on discrete-dipole approximation (DDA), we find that all contributions scale linearly with volume in this regime. This dependence can be rationalized by using semiempirical considerations based on the Mie-Debye theory. We also comment on a common approach to determining conservative and nonconservative force contributions on a single particle in an externally applied field. Our analysis suggests that the Mie-Debye theory cross sections cannot be used to evaluate conservative and nonconservative contributions to the force. Finally, we comment on aspects of the relationship between DDA and continuum-based treatments of optical force phenomena and find that inclusion of multiple scattering effects are essential to an understanding of the size dependence of the forces on mesoscopic particles.