High-Q/V Plasmonic Metasurfaces Enable Ultrafast Probing of Long-Lived Carrier Relaxation in Potassium

Alkali metals are promising materials for low-loss plasmonics, because of their intrinsically weak optical damping and favorable free-electron response. However, their high chemical reactivity and weak optical absorption make their ultrafast carrier dynamics difficult to access conventionally. Here, we investigate the long-lived carrier relaxation in potassium using a mode-engineered concave metasurface as a resonantly enhanced optical readout. A strongly confined hybrid mode with a high quality-factor-to-mode-volume ratio (Q/V) greatly enhances the sensitivity to pump-induced dielectric perturbations. Transient-reflectance spectroscopy resolves two characteristic relaxation times of ∼0.5 ps and ∼8 ps due to the electronic redistribution and electron–phonon-mediated energy relaxation, respectively. Two-temperature-model analysis yields a record-low electron–phonon coupling factor, consistent with weak electron-to-lattice energy transfer in potassium. These results demonstrate high-Q/V mode engineering as an effective strategy for accessing weak nonequilibrium carrier dynamics in low-loss plasmonic materials and for exploring light–matter interactions in minimally dissipative plasmonic environments.