Linewidth Narrowing with Ultimate Confinement of an Alkali Multipole Plasmon by Modifying Surface Electronic Wave Functions with Two-Dimensional Materials

Ultrafast Plasmonic Response Ensured by Atomic Scale Confinement

“…23 The line narrowing of the MP band of the Cs layer via the decoupling from the bulk has been interpreted well by the time-dependent density-functional theory (TD-DFT) calculations with jellium models. 23 We have confirmed that the MP excitation possesses a strong charge transfer character in which the electron density fluctuations along the surface normal play a crucial role. Therefore, the MP band is active only for p-polarized light.…”

“…Upon the formation of the Cs top layer, a strong resonance band with a narrow line width (Figure 1b) appears in the reflectance spectrum, which is assigned to the MP. 23 We note that the strong MP band appears only after the dense Cs layer on top of Gr starts to grow, which is decoupled from the Ir substrate: neither the dilute Cs adatom gas nor the compact Cs layer intercalated at the Gr−Ir interface give the MP absorption.…”

“…These stem from a strong confinement of the induced charge density within the top alkali layer enabled by an electronic decoupling from the bulk substrate by Gr: the quantum effect in the multilayer structure. 23 Our focus in the current study is on the electronic decoupling of the top alkali layer which induces notable effects on the ultrafast dynamics of the MP of Cs/Gr/Cs/Ir. By using femtosecond pump−probe spectroscopy, a large modulation of the MP band occurs in which an ultrafast decay within several tens of femtoseconds dominates the signal.…”

“…With the help of a theoretical assignment of the phonon modes, a correlation between the phonon displacement and the MP band line shape is revealed, demonstrating that the nuclear motions confined within a single atomic layer can induce significant spectral modulation of the plasmon response. 27 ■ EXPERIMENTAL SECTION Experiments were carried out in an ultrahigh vacuum chamber 23,28,29 with a base pressure of <4 × 10 −8 Pa (Figure 1c). An Ir(111) single crystal was cleaned by cycles of Ar + sputtering, followed by annealing at 1470 K, showing a sharp (1 × 1) low energy electron diffraction (LEED) pattern.…”

“…30 The quality and uniformity of Gr were confirmed by a moirésuperstructure in LEED. 23 Cs atoms were deposited from a well-degassed source (SAES Getters) on the Ir(111) surface covered with Gr. The Cs deposition rate was estimated to be 0.3 ML/min, and the sample temperature during the deposition was 100−110 K, unless otherwise stated.…”