Transient Absorption Studies of Single Metal Nanoparticles

The vibrational response from single silver nanocubes and nanowires to ultrafast laser induced heating is studied using transient absorption spectroscopy. Modulations due to coherent excitation of the breathing modes of the particles are clearly seen in the traces for both systems. Fitting the data to a damped cosine function yields the periods and homogeneous lifetimes of the vibrational motion. Different particles show different periods and lifetimes. The distribution of periods is believed to come from the size distribution in the samples. In both samples the lifetime distribution is significantly broader than the period distribution. A weak correlation between the periods and the lifetimes was found for the nanocubes, whereas the nanowires showed a moderate correlation between the period and lifetime. These results imply that the local environment has a strong effect on the lifetimes.The nanowires images and polarization analysis showed that light is preferentially absorbed at the ends of the nanowire due to coupling between the propagating surface plasmon polariton (SPP) modes and the near-IR pump laser. The propagating SPP mode travels down the wire at light-like speeds and as it moves it dephases, which causes lattice heating and coherently excites the breathing vibrational mode of the nanowires. Experiments with spatially separated pump and probe beams show that the transient absorption signal decays exponentially with distance. This shows that the SPP dephasing creates a gradient in both the electronic and lattice temperatures. These results also imply that the vibrational mode excited is not a normal mode of the wire. The timescale for the damping of the vibrational motion for the nanowires is similar to that for the nanocubes Ì¢‰âÂ' which shows that the shape of the particle (1D compared to 3D) does not strongly affect the way acoustic energy flows into the environment.