Resilience of Fano Resonances in Gold–Silver Nanostructures under Laser-Induced Morphological Transformations

In this study, we explore how the distribution of silver influences the optical and structural properties of pentatwinned gold nanorods, focusing on their transformation under ultrafast laser irradiation. Ag–Au–Ag heterostructures were synthesized, exhibiting a longitudinal plasmon resonance at 800 nm. These structures were irradiated with ultrashort laser pulses, resulting in Ag diffusion from the tips to the sides of the nanorods at low laser fluences. The diffusion process led to the formation of core–shell Au@Ag structures, reducing aspect ratio variability and narrowing the localized surface plasmon resonance (LSPR) band. At higher fluences, the nanorods underwent reshaping into spherical AuAg alloy nanocrystals, causing the disappearance of the longitudinal LSPR and the emergence of a new plasmon band at shorter wavelengths. A Fano resonance (FR), originating from the interference between the localized transverse plasmon mode and the continuum of Au interband transitions, persisted at low fluences but vanished as the structures alloyed. Simulation results confirmed that the FR is sensitive to core–shell configurations and is useful as a marker for detecting structural changes. The findings highlight the potential of laser irradiation for tailoring plasmonic nanomaterials for sensing and photonics.