Calculation of axial optical forces exerted on medium-sized particles by optical trap

Optical trapping has become an efficient technique of trapping and manipulating micrometer and sub-micrometer dimension particles. Particles in the range between the applicable regime of ray optics theory (ROT) and the Rayleigh regime (so-called medium-sized particles) are focused on. By using ROT and the generalized Lorenz–Mie theory (GLMT), axial optical forces and their dependences on particle sizes and beam waists are presented. Furthermore, by comparing the numerical results of these two theories, the applicability of the GLMT to particles of arbitrary size and the limit of ROT in the region of small particles are analyzed. A new criterion of the applicable region of ROT is obtained, i.e. the relative particle size β=2πa/λ⩾20β=2πa/λ⩾20, where a is the particle radius and λ is the wavelength of light. The theoretical results will be of great help to the design and optimization of the most efficient optical trap.