Ultrafast Dynamics of Colloidal Copper Nanorods: Intraband versus Interband Excitation

Colloidal copper nanorods (NRs) display transverse and longitudinal localized surface plasmon resonances. The longitudinal localized surface plasmon modes are tunable through the near-infrared electromagnetic radiation energies with NR aspect ratios. Visible and near-infrared transient optical response of the copper NRs is investigated under excitation conditions spanning intraband and interband excitation (0.79-3.50 eV). In both the visible and near-infrared regions, the spectral response of the samples under intraband excitation (<2 eV) differs substantially from their response under interband excitation (>2 eV). However, the timescale of the electron-phonon coupling estimated from pump fluence-dependent measurements (τ _ep) is less sensitive to excitation conditions than reports for gold. τ _ep shortens slightly from ≈616 fs with intraband excitation (at visible probe energies) to ≈565 fs with interband excitation. The observed dynamics correspond to an average sample electron-phonon coupling parameter varying across all conditions from 4.4 × 10¹⁶ to 6.4 × 10¹⁶ J m^-3 K^-1, which is similar to bulk copper. Furthermore, coherent acoustic phonons are observed for the longitudinal localized surface plasmon resonance with a range of oscillatory periods reflecting sample size dispersion.