FDTD analysis of optical forces on bowtie antennas for high-precision trapping of nanostructures

We theoretically investigate the optical forces generated by a high near-field resolution antenna system through finite difference time domain calculations along with the Maxwell stress tensor method. Our antenna choice is bowtie-shaped nanostructures with small gap regions, exploiting propagating waveguide modes as well as localized surface plasmons. Our analysis shows that the antenna system supports large optical forces at the resonance wavelength where the near-field intensities as well as their gradients are the largest within the gap region. We show that the system exhibits much larger optical forces when the incident light polarization is along the bowtie gap as the system can effectively leverage the gap effect, compared to the case when the system is under the polarization normal to the gap. We also investigate the forces on a dielectric bead in the vicinity of the antennas for different positions to show the optical force characteristics of the bowtieshaped antennas. Finally, the force analysis on different bead radiuses demonstrates the trapping efficiency of our antenna system.