Understanding Quantum Plasmonics from Time-Dependent Orbital-Free Density Functional Theory

Using time-dependent orbital-free density functional theory, we perform quantum mechanical simulations to understand plasmonic responses in sodium nanoparticle dimers and trimers. The electronic structure, optical absorption, electric field enhancement, and photoinduced tunneling current are examined. For dimers, the maximum field enhancement is reached at a gap distance of 6 Å, below which a conductive channel is formed. The tunneling current peaks at the resonant energy of the charge-transfer plasmon (CTP). One such CTP is formed in symmetric linear trimers, and it splits into two in asymmetric linear trimers—one corresponds to a “global” and the other a “local” oscillation as a result of constructive and destructive interferences between the CTPs of the corresponding dimers. The interference leads to “nanofocusing” where the intensity of the “global” mode reaches its maximum while the “local” mode is quenched completely. Similarly, the electric field can be tuned to be localized at one of the two gaps and vanish at the other.