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 >1.5 Å, visible light squeezes inside the empty slot underneath, giving optical field distributions 2 Å thick, less than the atomic diameter. Paradoxically classical electromagnetic models are also able to reproduce the resulting dispersion for these subatomic slot modes, where light reaches in-plane wavevectors ∼2 nm-1 and slows to <10-2c. We explain the success of these classical dispersion models for gaps ≥1.5 Å due to a quantum-well state forming in the lifted monolayer in the vicinity of the Fermi level. This extreme trapping of light may explain transient "flare" emission from plasmonic cavities where Raman scattering of metal electrons is greatly enhanced when subatomic slot confinement occurs. Such atomic restructuring of Au under illumination is relevant to many fields, from photocatalysis and molecular electronics to plasmonics and quantum optics.

Impact of the energy dispersion anisotropy on the plasmonic structure in a two-dimensional electron system.

The effect of the band structure anisotropy (triangular, square, and hexagonal wrapping) on the electronic collective excitations (plasmons) in a two-dimensional electron gas (2DEG) is studied in the framework of the random-phase approximation. We show that the dynamical dielectric response in these systems strongly depends on the direction of the in-plane momentum transfer q. The effect is so pronounced that it results in a different number of electronic collective excitations in some q regions, both with - and ∼q-like energy dispersions. This finding is in striking contrast to the conventional 2DEG case with isotropic energy band dispersion where only a single plasmon with dispersion can exist. Our prediction of acoustic modes (with the ∼q dispersion) in a one-energy-band electron system expands the previous knowledge that such kind of plasmon can be realized only in two-component systems.

Microcavity phonon polaritons from the weak to the ultrastrong phonon-photon coupling regime.

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.