Xinning Yu, Yikun Jiang, Huajin Chen, Shiyang Liu, and Zhifang Lin
One of the theoretical challenges in studying optical trapping is the decomposition of the optical force into the gradient force (conservative component) and scattering force (nonconservative component), which can be achieved either for Raleigh particles or for very large particles in the regime of ray optics. However, for the moderate particles in between these two limits, the scenario is still a mystery. In this paper we present a theoretical approach to bridge this gap and fully split the optical force acting on a spherical particle immersed in a generic monochromatic free-space optical field into two such essentially different components, which is efficient even for large particles with the exact consideration of light polarization, thus offering a benchmark for examining the effective range for application of ray optics. Our approach models general optical fields by a series of homogeneous plane waves. The analytical expressions for the gradient and scattering parts of the optical force exerted on a spherical particle of arbitrary size illuminated by multiple interferential plane waves are then derived. As examples of applications, we investigate the gradient and scattering forces acting on a dielectric particle immersed in the Bessel beam. Our results are in excellent agreement with those obtained based on ray optics methods when the illuminated particle is large enough, while exhibiting effects of Mie resonance that are totally missing in the ray optics for moderate particle sizes. Finally, we study the effect of particle size on the gradient force acting on a spherical particle sitting in multiple interferential plane waves. Our extensively numerical results, up to a size as large as 2000 illuminating wavelengths, suggest an overall decreasing tendency in the ratio of the magnitude of the gradient force to that of the total force as the particle size increases.
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