Making waves: Radiation damping in metallic nanostructures.

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Ultrafast measurements of the dynamics of single nanostructures: a review.

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.

On the measurement of relaxation times of acoustic vibrations in metal nanowires.

The mechanical resonances of metal nanostructures are strongly affected by their environment. In this paper the way the breathing modes of single metal nanowires are damped by liquids with different viscosities was studied by ultrafast pump-probe microscopy experiments. Both nanowires supported on a glass substrate and nanowires suspended over trenches were investigated. The measured quality factors for liquid damping for the suspended nanowires are in good agreement with continuum mechanics calculations for an inviscid fluid that assume continuity in stress and displacement at the nanowire-liquid interface. This shows that liquid damping is controlled by radiation of sound waves into the medium. For the nanowires on the glass surface the quality factors for liquid damping are approximately 60% higher than those for the suspended nanowires. This is attributed to a shadowing effect. The nanowires in our measurements have pentagonal cross-sections. This produces two different breathing modes and also means that one of the faces for the supported nanowires is blocked by the substrate, which reduces the amount of damping from the liquid. Comparing the supported and suspended nanowires also allows us to estimate the effect of the substrate on the acoustic mode damping. We find that the substrate has a weak effect, which is attributed to poor mechanical contact between the nanowires and the substrate.

Strong Exciton-Plasmon Coupling in Silver Nanowire Nanocavities.

The interaction between plasmonic and excitonic systems and the formation of hybridized states is an area of intense interest due to the potential to create exotic light-matter states. We report herein coupling between the leaky surface plasmon polariton (SPP) modes of single Ag nanowires and excitons of a cyanine dye (TDBC) in an open nanocavity. Silver nanowires were spin-cast onto glass coverslips, and the wavevector of the leaky SPP mode was measured by back focal plane (BFP) microscopy. Performing these measurements at different wavelengths allows the generation of dispersion curves, which show avoided crossings after deposition of a concentrated TDBC-PVA film. The Rabi splitting frequencies (Ω) determined from the dispersion curves vary between nanowires, with a maximum value of Ω = 390 ± 80 meV. The experiments also show an increase in attenuation of the SPP mode in the avoided crossing region. The ability to measure attenuation for the hybrid exciton-SPP states is a powerful aspect of these single nanowire experiments because this quantity is not readily available from ensemble experiments.

Surface Plasmon Polariton Interference in Gold Nanoplates.

Transient absorption microscopy (TAM) measurements have been used to study the optical properties of surface plasmon polariton (SPP) modes in gold nanoplates on a glass substrate. For thin gold nanoplates, the TAM images show an oscillation in the signal across the plate due to interference between the "bound" and "leaky" SPP modes. The wavelength of the interference pattern is given by λ = 2π/Δk, where Δk is the difference between the wavevectors for the bound and leaky modes and is sensitive to the dielectric constant of the material above the gold nanoplate. Back focal plane imaging was also used to measure the wavevector of the leaky mode, which, in combination with the Δk information from the TAM images, enabled the bound mode wavevector to be determined. These experiments represent the first far-field optical measurement of the wavevector for the bound mode in metal nanostructures.

Brillouin Oscillations from Single Au Nanoplate Opto-Acoustic Transducers.

Brillouin oscillations, which are GHz frequency waves that arise from the interaction of light with acoustic waves, are experiencing increasing applications in biology and materials science. They provide information about the speed of sound and refractive index of the material they propagate in, and have recently been used in imaging applications. In the current study, Brillouin oscillations are observed through ultrafast transient reflectivity measurements using chemically synthesized Au nanoplates as opto-acoustic transducers. The Au nanoplates are semitransparent, which allows the Brillouin oscillations to be observed from material on both sides of the plate. The measured frequencies are consistent with the values expected from the speeds of sound in the different materials, however, the attenuation constants are much larger than those reported in previous studies. The increased damping is attributed to diffraction of the acoustic wave as it propagates away from the excitation region. This effect is more significant for experiments with high numerical aperture objectives. These results are important for understanding the relationship between frequency and spatial resolution in Brillouin oscillation microscopy.

Auger and Carrier Trapping Dynamics in Core/Shell Quantum Dots Having Sharp and Alloyed Interfaces.

The role of interface sharpness in controlling the excited state dynamics in CdSe/ZnSe core/shell particles is examined here. Particles composed of CdSe/ZnSe with 2.4-4.0 nm diameter cores and approximately 4 monolayer shells are synthesized at relatively low temperature, ensuring a sharp core-shell interface. Subsequent annealing results in cadmium and zinc interdiffusion, softening the interface. TEM imaging and absorption spectra reveal that annealing results in no change in the particle sizes. Annealing results in a 5-10 nm blue shift in the absorption spectrum, which is compared to calculated spectral shifts to characterize the extent of metal interdiffusion. The one- and two-photon dynamics are measured using time-resolved absorption spectroscopy. We find that biexcitons undergo biexponential decays, with fast and slow decay times differing by about an order of magnitude. The relative magnitudes of the fast and slow components depend on the sharpness of the core-shell interface, with larger fast component amplitudes associated with a sharp core-shell interface. The slow component is assigned to Auger recombination of band edge carriers and the fast decay component to Auger recombination of holes that are trapped in defects produced by lattice strain. Annealing of these particles softens the core-shell interface and thereby reduces the amount of lattice strain and diminishes the magnitude of the fast decay component. The time constant of the slow biexciton Auger recombination component changes only slightly upon softening of the core-shell interface.