Reflective microcavity confinement reshapes ultrafast energy localization across water–copper interfaces

The evolution of ultrafast energy localization as a femtosecond laser focus across a reflective liquid–metal interface remains incompletely understood, particularly within confined geometries. Here, we investigate the interface-crossing energy localization of femtosecond breakdown in water-filled copper microcavities using in situ emission spectroscopy. Axial focal scanning reveals three distinct localization modes, corresponding to direct focusing breakdown, localization collapse near the interface, and reflection-mediated relocalization, respectively. In planar reflective geometries, the breakdown emission rapidly vanishes after the focus enters the metal substrate, followed by reflection-mediated relocalization after millimeter-scale propagation. In contrast, reflective microcavity confinement maintains continuous emission enhancement across the interface region and compresses the reflective relocalization distance to approximately 200 μm. These results demonstrate that reflective confinement reshapes ultrafast energy-localization during liquid–metal interface crossing. The bottom and sidewall reflections redirect the post-interface field back into the confined liquid, suppressing the localization-collapse and enabling rapid recovery of liquid-phase breakdown.