Phase Dependent Ultrafast Light-Matter Interaction in Two-Dimensional Silver

Atomic-level control of two-dimensional materials provides a novel platform for nanoscale photonics and optoelectronics. Two-dimensional silver (2D-Ag) is stabilized via silver intercalation in epitaxial graphene on SiC. Two phases of 2D-Ag, termed Ag₍₁₎ and the Ag₍₂₎, have been distinguished through 1) in-plane phonon shear modes: Ag₍₁₎ at 17 cm^-1, and Ag₍₂₎ exhibiting two modes at 66 and 92 cm^-1 and 2) linear extinction: Ag₍₁₎ with two absorption peaks centered around 460 and 650 nm, and Ag₍₂₎ with a single band centered around 570 nm. Control of graphene chemistry enables the formation of single phase 2D-Ag, allowing us to explore the inherent differences between Ag₍₁₎ and Ag₍₂₎. The symmetry breaking at the SiC/2D-Ag interface creates an out-of-plane axial dipole, resulting in strong nonlinear optical (NLO) responses. Second harmonic generation microscopy is implemented to investigate the symmetry and second order susceptibility χ⁽²⁾ of Ag₍₁₎ and Ag₍₂₎. Both phases exhibit point group symmetry <i>3m</i>, verifying the epitaxial growth of the Ag layer on the modified SiC surface. The χ⁽²⁾ of Ag₍₂₎ is orders of magnitude greater than that of Ag₍₁₎. The carrier dynamics of 2D-Ag is studied with near-degenerate transient absorption (TA) spectroscopy. Both phases are excited near the resonance frequencies of 2D-Ag, setting pump wavelengths at 575 and 650 nm. Rapid decays (~2 ps) following the initial bleach are observed in the time-resolved TA spectra for both phases. The difference in coherent oscillations between Ag₍₁₎ and Ag₍₂₎ is attributed to electron-phonon coupling corresponding to the in-plane shear mode of each phase as confirmed with correlated Raman spectroscopy. The results suggest that we are able to tailor the NLO properties and carrier dynamics of 2D-Ag through phase engineering at the atomic level.