Quantum plasmonics in sub-atom-thick optical slots

We show using time-dependent density functional theory (TDDFT) that light can be confined into slot waveguide modes residing between individual atomic layers of coinage metals such as gold. As the top atomic monolayer lifts a few Å off the underlying bulk Au (111), ab initio electronic structure calculations show that for gaps > 1.5Å visible light squeezes inside the empty slot in-between giving optical field distributions 2Å thick, below the atomic diameter. Paradoxically classical electromagnetic models are also able to reproduce the resulting dispersion for these sub-atomic slot modes, where light reaches in-plane wavevectors ~2nm-1, and slows to