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Ultrafast excitation of nanoparticles can excite the acoustic vibrational modes of the structure that correlate with the expansion coordinates. These modes are frequently seen in transient absorption experiments on metal nanoparticle samples and occasionally for semiconductors. The aim of this review is to give an overview of the physical chemistry of nanostructure acoustic vibrations. The issues discussed include the excitation mechanism, how to calculate the mode frequencies using continuum mechanics, and the factors that control vibrational damping. Recent results that demonstrate that the high frequencies inherent to the acoustic modes of nanomaterials trigger a viscoelastic response in surrounding liquids are also discussed, as well as vibrational coupling between nanostructures and mode hybridization within the nanostructures. Mode hybridization provides a way of manipulating the lifetimes of the acoustic modes, which is potentially useful for applications such as mass sensing. 

The properties of the leaky surface plasmon polariton (SPP) modes in gold nanostripes were investigated using scattered light microscopy. Both bare gold nanostripes and stripes coated with a thin polymer film containing a near-infrared absorbing dye were examined. Real-space microscopy images were employed to determine the SPP propagation length, while Fourier space images provided measurements of the wavevector. Frequency versus wavevector dispersion curves were generated by performing experiments at different excitation wavelengths, and the slopes of these curves yielded the SPP group velocities. For the bare nanostripes the group velocity was determined to be vg = (0.92 ± 0.05)c0 and for the dye-coated nanostripes it was vg = (0.85 ± 0.06)c0, where c0 is the speed of light. The SPP lifetimes were estimated by combining the group velocity and propagation length measurements. The results show that the lifetime of the gold SPPs is significantly reduced when the nanostripes are coated with the dye. At the peak of the dye absorption curve the change in the SPP dephasing rate induced by the dye–polymer film was found to be 0.07 fs–1. Finite element simulations show that the increased dephasing is due to a combination of energy transfer from the SPP modes to the dye, as well as increased radiation damping due to changes in the dielectric environment of the nanostructures. These findings provide insights into the energy transfer processes in plasmonic systems, which can be leveraged to optimize the design of plasmonic devices for applications in sensing, imaging and nanophotonic circuits. 

The fundamental and n = 3 overtones of Au nanoplate thickness vibrations have been studied by transient absorption microscopy. The frequencies of the n = 3 overtone are less than 3× the frequency of the fundamental. This anharmonicity is explained through a continuum mechanics model that includes organic layers on top of the nanoplate and between the nanoplate and the glass substrate. In this model, anharmonicity arises from coupling between the vibrations of the nanoplate and the organic layers, which creates avoided crossings that reduce the overtone frequencies compared to the fundamental. Comparison of the experimental and calculated quality factors shows that coupling occurs to the top organic layer. Good agreement between the measured and calculated quality factors is obtained by introducing internal damping for the nanoplate. These results show that engineering layers of soft material around metal nanostructures can be used to control the vibrational lifetimes. 

The vibrational modes of semiconductor and metal nanostructures occur in the MHz to GHz frequency range, depending on dimensions. These modes are at the heart of nano-optomechanical devices, and understanding how they dissipate energy is important for applications of the devices. In this paper ultrafast transient absorption microscopy has been used to examine the breathing modes of a single gold nanoplate, where up to four overtones were observed. Analysis of the frequencies and amplitudes of the modes using a simple continuum mechanics model shows that the system behaves as a free plate, even though it is deposited onto a surface with no special preparation. The overtones decay faster than the fundamental mode, which is not predicted by continuum mechanics calculations of mode damping due to radiation of sound waves. Possible reasons for this effect include frequency dependent thermoelastic effects in the nanoplate, and/or flow of acoustic energy out of the excitation region. 

Membrane viscosity is an important property of cell biology, which determines cellular function, development and disease progression. Various experimental and computational methods have been developed to investigate the mechanics of cells. However, there have been no experimental measurements of the membrane viscosity at high-frequencies in live cells. High frequency measurements are important because they can probe viscoelastic effects. Here, we investigate the membrane viscosity at gigahertz-frequencies through the damping of the acoustic vibrations of gold nanoplates. The experiments are modeled using a continuum mechanics theory which reveals that the membranes display viscoelasticity, with an estimated relaxation time of ca. ps. We further demonstrate that membrane viscoelasticity can be used to differentiate a cancerous cell line (the human glioblastoma cells LN-18) from a normal cell line (the mouse brain microvascular endothelial cells bEnd.3). The viscosity of cancerous cells LN-18 is lower than that of healthy cells bEnd.3 by a factor of three. The results indicate promising applications of characterizing membrane viscoelasticity at gigahertz-frequency in cell diagnosis. 

Photothermal microscopy is a powerful method for investigating biological systems and solid state materials. Using a modulated pump to excite the sample, a continuous probe beam monitors the change in the refractive index of the sample due to the modulated heating. These experiments are typically performed at high frequencies to reduce the 1/f noise, achieving a higher signal to noise ratio. In this paper, we explore how the resolution and sensitivity of the photothermal experiments change when the modulation frequency is brought down below 100kHz. In the instance that the pump and probe are cofocused at the sample, the resolution is determined by the size of the pump beam. On the other hand, when a widefield pump is used, significant broadening occurs for frequencies under 20kHz. This broadening is attributed to thermal diffusion. However, the amount of broadening is less than that expected from the thermal diffusion length, which is about 1.7μm at 10kHz for nanoparticles in glycerol. We also explore the situation where the point spread functions of the pump and probe beams are smaller than the particle size as well as how the penetration depth depends on the properties of the pump and probe beams.