Analysis of local field enhancement including tip interaction for the application to nano-manipulation using FDTD calculations

Conventional optical tweezers rely on the field gradients near the focus of a laser beam which give rise to a trapping force towards the focus. However, the trapping volume of these tweezers is diffraction limited. Recently a promising method for nanomanipulation combining near-field optical tweezers and atomic force microscopy has been proposed. The light confinement in the vicinity of two nano-metric probes can produce strong gradient forces to trap and manipulate nano-scale particles at dimensions beyond the diffraction limit. In order to model this method, a finite difference time domain (FDTD) simulation enabling the calculation of the electromagnetic field in the vicinity of a metallic probe has been carried out for the first time in the context of near-field optical tweezers. For the case of nano-manipulating, a general model for the near-field simulation combining an actual optical fiber probe with a metallic probe under polarized laser irradiation is presented. The interaction between two near-field probes has been taken into account, and both near-field patterns of combined area and the dependence of field intensity have been analyzed. The simulations show that field enhancement and intensity profiles under the metallic probe strongly depend on the distance between the two probes, the incident angle of light, the polarization direction of incident light, the wavelength of the plane wave, and the permittivity of the probe material. The field intensity could be strong enough to manipulate nano-particles when the metallic probe is illuminated by an evanescent wave with high incident efficiency. Results of the FDTD calculations are found to be helpful for nano-manipulation.