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.

Acoustic Vibration Modes of Gold–Silver Core–Shell Nanoparticles

“…As already analyzed in Ref. [32], they originate either from dipolar modes characterized by in-phase induced charges at the inner and outer NP interfaces (longitudinal L 0 et transverse T 0 dipolar modes), or multipolar transverse modes stemming from mirror charges at the inner and outer interfaces (transverse T i modes, i > 0). The larger i, the higher the energy of the mode.…”

“…The TA maps present plasmonic bleaching features which can be associated with the respective resonance modes identified in the stationary regime [32]. In particular, the TA map of the bare AuNR sample excited at 380 nm (see Supplementary Materials Section S3) shows two typical plasmonic bleaching signals which can be attributed to the transient modification of the transverse surface plasmon resonance (TrSPR) and the longitudinal one (LgSPR): The LgSPR is excited by an electric field polarization along the AuNR longitudinal axis, whereas the TrSPR corresponds to an electric field polarization perpendicular to the longitudinal axis.…”

“…First, AuNRs were synthesized by seed-mediated growth, which involves a prereduction step with salicylic acid [29]. Then, a silver shell was epitaxially grown onto the gold nanorods and its thickness was controlled by adjusting the amount of silver precursor and ascorbic acid as a reducing agent in the reaction mixture [30][31][32]. The silver-to-gold ratio, denoted by "eq", was varied (i.e., eq = 0, 0.8, 2, 4, 8), leading to important changes in the optical properties of the nanoparticles.…”

“…The sample characterization was already published elsewhere [32]. Size and shape distribution are assessed by transmission electron microscopy (TEM) performed at IMAGIF (I2BC CNRS, Gif s/Yvette, France) using a JEOL JEM-1400 microscope operating at 120 kV with a filament current of about 55 µA.…”

“…Five samples are synthesized, from eq = 0 (bare AuNRs) to eq = 8 (AuNRs coated with a thick Ag shell). As shown by TEM [32], Ag grows preferentially along the transverse dimension of the NPs; therefore, the overall aspect ratio (AR) decreases, and the initial rod-like NPs become cuboidal. In addition, the edges and corners become sharper with increasing Ag shell thickness.…”