Ultrafast laser-induced photo-reduction on black copper

Abstract

Ultrafast laser micro- and nanostructured metal surfaces for blackbodies have attracted considerable attention. However, current research has predominantly concentrated on the unidirectional transformation of reflective metals into light-absorbing black surfaces, whereas in-situ, programmable modulation in the reverse direction—locally restoring reflectivity on an absorbing blackbody matrix—remains largely unexplored. Herein, we demonstrate a strategy for ultraviolet (343 nm) femtosecond laser-induced photoreduction and surface restructuring on chemically blackened copper. Benefiting from the single-photon energy of 3.62 eV overcoming the CuO bond dissociation energy (3.04 eV), laser irradiation drives the direct photoreduction of pristine divalent CuO nanosheets into low-valence Cu⁺ species (reaching 39.72%) alongside localized microstructural remelting and planarization. These combined chemical and morphological modifications are accompanied by an increase in visible-band diffuse reflectance from approximately 4.5% to approximately 8.0% (an approximately 80% relative enhancement), together with observable optical contrast between treated and untreated regions. Leveraging this spatial optical contrast, amplitude-type reflective Fresnel zone plates were directly patterned onto the light-absorbing copper foil, successfully demonstrating optical light-converging capability. This work elucidates the photophysical mechanisms of ultrafast laser interactions with metal-oxide nanostructures and establishes a viable route for in-situ writing planar micro-optical components and optical encoding patterns directly on monolithic light-absorbing metal substrates.