Analysis of trapping force and torque for fiber-optical trap by the finite-differential time-domain (FDTD) method

The FDTD method is used to calculate the light field distribution in an uncoated fiber probe with small dielectric particles. By integrating Maxwell's stress tensor in the FDTD calculation, the time-averaged force exerted on the dielectric particles with different sizes and at different positions is obtained. The calculation results are also verified experimentally. The force near the axis in the vicinity of the tip is about 10^-14 N for the incident 10 mW laser of 980 nm. Furthermore, the torque exerting on an ellipsoidal dielectric particle by focusing light field with two fiber probes is calculated and the dependence of this torque on both the declination angle of the ellipsoidal particle and the incident laser power ratio of the two probes is obtained. The detailed intensity map within the particles is also given.