Probe optimization for nano-manipulation in metal probe-based near-field optical tweezers based on FDTD simulation

Metal probe-based near-field tweezers can provide optical trapping and alignment of dielectric particles at nanometer scale. A finite difference time domain (FDTD) numerical method of solution is applied to optimize the metal probe geometry for better nano-manipulation. Calculations for copper probes of truncated, infinite and finite rectangular pyramid are considered. The calculations show that sharper probes would generate stronger fields, and probes whose geometry size matched to the excitation frequency could lead to higher field enhancement, in addition, finite probes might generate strong field enhancement due to resonance with the excitation source. Consequently, FDTD calculations were made to optimize metal probes in the near-infrared regime, the enhancements for cone probes are found to be higher than for similar length pyramidal probes. The cone probes designed with FDTD are particularly well suited for use in metal probe-based near-field tweezers.