Morphology-modified contributions of electronic transitions to the optical response of plasmonic nanoporous gold metamaterial.

Nanoporous metals have emerged as promising functional architectures with tunable optical and electronic properties, high surface areas, and applicability in sensing, catalysis, and biomedicine. While their linear optical behavior and morphological properties have been extensively studied, the electronic properties, and in particular how they are affected by morphology, remain not fully understood. Here we combine experimental and theoretical studies of electronic excitation and relaxation in a nanoporous gold metamaterial. Optical pump-probe experiments show slower electron relaxation dynamics compared to the continuous film, consistent with a higher transient electronic temperature and stronger smearing of the Fermi-Dirac distribution, well reproduced by an extended two-temperature model. Furthermore, cathodoluminescence measurements reveal broadband localized plasmon resonances, and atomistic simulations disentangle intra- and interband effects, demonstrating that nanoscale porosity fundamentally reshapes the electronic response. These findings support nanoporosity as a key design parameter for controlling steady-state and ultrafast optical behavior in plasmonic materials.