The lifetimes of the acoustic vibrations of metal nanostructures depend sensitively on the properties of the environment, such as the acoustic impedance and viscosity. In order to accurately study these effects, they have to be separated from the damping processes that are inherent to the nanostructure. Here we show that this can be done experimentally by investigating individual gold nanowires suspended over a trench in air and liquid environments. The experiments were done by ultrafast pump-probe microscopy, recording transient absorption traces at the same point on the nanowire in both environments. These first experiments were performed with water, and the measured vibrational quality factors due to the presence of water were compared to continuum mechanics calculations for a cylinder in a homogeneous environment. Good agreement was found between the experimental quality factors and the calculated values. The continuum mechanics analysis shows that damping is dominated by the acoustic impedance of the solvent rather than by its viscosity for the nanowires in the present experiments. This experimental technique opens up the possibility of studying the effect of viscosity on the high frequency vibrational motions of nanostructures for a variety of liquids.

Surface plasmon polariton (SPP) propagation in chemically synthesized gold nanobars was investigated using scanning transient absorption microscopy. The SPP propagation lengths were correlated to the size and shape of the nanobars, which were determined by atomic force microscopy. The average propagation length was found to be 12 ± 4 μm, and the measured values were independent of the excitation polarization. Finite element calculations were performed for gold nanobars in the presence of a glass substrate to model the experimental results. Comparison between the experimental and theoretical results shows that both bound and leaky SPP modes can be excited. The leaky mode is detected in the larger nanobars (widths greater than 500 nm), and the bound mode is detected in smaller nanobars. Highly directional and efficient plasmon coupling between a gold nanobar and a nearby gold nanoplate was also observed in the experiments.

Over the past decade, there have been a number of transient absorption studies of the acoustic vibrational modes of metal and semiconductor nanoparticles. This Perspective provides an overview of this work. The way that the frequencies of the observed modes depend on the size and shape of the particles is described, along with their damping. Future research directions are also discussed, especially how these measurements provide information about the way nano-objects interact with their environment.

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.

Single crystal, micrometer-sized nanoplates were formed by reducing Au(III) in the presence of surfactants using a modified polyol protocol. The shapes of the plates range from triangular to hexagonal. The nanoplates have {111} surfaces with an average edge length of 5 ± 2 μm and an average width of 107 ± 30 nm. Scanning electron microscopy (SEM) images reveal that the plates grow through a re-entrant groove created by twinning. The optical properties of the plates were studied by scattered light and transient absorption experiments. The scattered light measurements show that propagating surface plasmon polariton (SPP) modes of the Au nanoplates can be excited when a laser beam is focused at the edge of the nanoplate. We also demonstrate that the direction of propagation of the SPP modes can be controlled through the polarization of the laser beam. The transient absorption traces for single suspended nanoplates show oscillations, which are assigned to thickness vibrations of the plates. The quality factors for the oscillations are smaller than those recently measured for suspended gold nanowires, indicating possible contributions to the vibrational damping from surface bound molecules or from the crystal structure.