Optical near-fields are localized to the source region of optical radiation or to the surfaces of materials interacting with free radiation. Due to the high field gradients of evanescent waves, strong forces are predicted in optical near-fields. In this paper, by solving Maxwell's equations with Finite difference Time Domain (FDTD) simulation based on the numerical software, the effect of changing the period of grating on the optical force around tapered tip with 90 degree laser beam incident angle was studied. The optimization results show that optimum value for period of circular gratings in order to achieve maximum optical force enhancement 208 nm was obtained. This maximum optical force is 1.4 × 10−14 N/W and occurs when laser wavelength is 655 nm.
