Sharp Spectral Variations of the Ultrafast Transient Light Extinction by Bimetallic Nanoparticles in the Near‐UV

Abstract Noble metal nanoparticles exhibit localized plasmon resonance modes that span the visible and near‐infrared spectral ranges and have many applications. Modifying the size, shape, and composition of the nanoparticles changes the number of modes and their properties. The characteristics of these modes are transiently affected when illuminating the nano‐objects with ultrashort laser pulses. Here, core–shell gold–silver nanocuboids are synthesized and their spectral signature in the stationary and ultrafast transient regimes are measured. Their dipolar transverse mode vanishes with increasing Ag‐shell thickness, while higher‐order modes grow in the near‐ultraviolet range where no plasmon resonance can be generated with single noble metal nanoparticles. These higher‐energy modes are associated with sharp spectral variations of the ultrafast transient light extinction by the bimetallic nanocuboids. By carrying out a theoretical investigation, the different contributions to this response are broken down and a physical interpretation of its spectral profile is provided. The transient optical signal is then shown to reveal resonance modes hidden in the stationary regime spectra.