The response of single crystal, cubic silver particles to ultrafast laser-induced heating has been examined experimentally and theoretically. The transient absorption traces display clear modulations due to coherently excited vibrational modes. Nanocube samples with edge lengths smaller than 50 nm show a single modulation, whereas samples larger than 50 nm show two vibrational modes. The results are compared to finite element calculations, where the cubes are modeled as having cubic crystal symmetry with the principal axes parallel to the sides of the particle. The action of the laser pulse is treated in two ways, first, as creating a uniform initial strain. In this case the predominant mode excited is the breathing mode. The period of this mode is in reasonable agreement with the vibrational periods measured for the smaller cubes and with the higher frequency modulation observed for the larger cubes. A nonuniform initial strain is also considered, which could arise from nonuniform heating for particles larger than the optical skin depth of the metal. In this case the predominant mode excited is a nontotally symmetric mode. The calculated periods from this analysis are in reasonable agreement with the lower frequency modulations observed for the larger samples. The results from this study show that, to within the accuracy of these measurements, the elastic constants of cubic silver nanoparticles are the same as bulk silver.

Time-resolved spectroscopy has been used to investigate the vibrational properties of hollow cubic nanoparticles: Au-Ag nanoboxes and nanocages. In these experiments laser-induced heating was used to coherently excite the breathing vibrational modes of the particle. The vibrational periods scale with the edge length of the particle, and the nanocages and nanoboxes showing equivalent responses despite a large difference in their morphology. The measured vibrational periods are compared to finite element calculations, where the particles are modeled as a hollow cube, with the principle crystal axes parallel to the sides of the cube. Very good agreement is obtained between the calculations and the experimental data, with the experimental frequencies being slightly lower than the calculated values (by ∼ 7 %). These results demonstrate the importance of accurately modeling the particles in order to interpret experimental data.Advances in synthetic techniques have made it possible to produce high quality samples of metal and semiconductor nanoparticles in a variety of different shapes, such as rods, 1,2,3 triangles, 4,5 cubes and boxes, 6 and branched structures. 7,8 These particles have unique optical properties, and are finding use in a variety of applications, from molecular sensing 9 to biological labeling 10 and photothermal therapy. 11,12 For metal particles the optical response is dominated by the surface plasmon resonance (SPR), which is a coherent oscillation of the conduction electrons across the particle surface. 13 Recent research has focused on understanding how the position and width of the SPR depends on the size and shape of the particles. 14 In this regard, single particle spectroscopy has been particularly useful. 15,16 These experiments have shown, for example, how radiation damping, 15,17 electron-surface scattering, 17,18 and the "lightning-rod effect" 19 control the width of the plasmon resonance.Time-resolved experiments also provide information about the properties of nanoparticles. For metals, transient absorption experiments conducted on a sub-picosecond timescale give information about electron-electron 20 and electron-phonon coupling. 21 At longer timescales (tens to hundreds of picoseconds) transient absorption measurements give data about how energy relaxes from the particle to the environment. 22,23 This information is important to photothermal therapy, where laser-induced heating of nanoparticles is used to kill selected cells. 11,12 For high quality samples, modulations due to coherently excited vibrational modes To obtain quantitative information from the vibrational spectroscopy experiments, an expression is needed that relates the measured frequencies to the particle dimensions and elastic constants. For spheres (both homogeneous and core-shell) and rods, analytic expressions are available for the important modes observed in the transient absorption experiments. 24,25,26, 27 However, for other shapes the vibrational modes must be calculated numerically. 28 In our recent stud...

Irradiating metal particles by an ultrafast laser pulse produces rapid heating of the lattice. This can lead to coherent excitation of the vibrational modes of the particle that correlate with the expansion co-ordinates. By comparing the measured periods to continuum mechanics calculations, these experiments can provide information about the elastic constants of the particle if the size and shape are known. In this paper recent results are presented for particles with cubic symmetry, specifically, nanocubes, nanoboxes (hollow cubes) and nanocages (nanoboxes with holes on the corners and/or facets of the box). The way the vibrational modes are assigned, and the information content of the experiments will be discussed, as well as the energy relaxation dynamics of the particles. Energy relaxation is important for the proposed use of the nanocages in phothermal therapy, where heat dissipation following laser excitation is used to selectively kill cells.