Understanding Quantum Plasmonic Enhancement in Nanorod Dimers from Time-Dependent Orbital-Free Density Functional Theory

Localized surface plasmon resonances could yield extreme enhancement of local electric fields at surfaces of plasmonic nanostructures. Herein, we have performed quantum mechanical simulations to systematically study plasmonic resonances in sodium (Na) nanorod dimers based on time-dependent orbital-free density functional theory. Several representative geometries, including end-to-end, side-to-side, and right-angle Tand L-shaped dimer arrangements are explored in detail. The optical spectra, tunneling electric current, and electric field enhancement (hot spots) are examined as a function of the size of the nanorods, their relative arrangement, and their gap distance (<= 2 nm). Two plasmon resonant modes are identified to be responsible for the observed electric field enhancement. One of them is of quantum nature, arising from quantum tunneling across the gap of the two nanorods. The other mode is of electrostatic nature, originating from the dipolar interaction between the plasmonic oscillations of each nanorod. Among the examined geometries, the end-to-end dimer exhibits the strongest field enhancement, which increases with the aspect ratio and the gap distance. The interplay between electron tunneling across the gap and the spill-out of electrons at the nanorod surfaces is revealed to dominate the modulation of plasmonic resonances and field enhancement in the nanorod dimers.