Joseph M. McLellan

Time-resolved spectroscopy of silver nanocubes: Observation and assignment of coherently excited vibrational modes

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.

Spinel domes with integrated electromagnetic interference protection

Magnesium aluminate spinel is a durable electro-optical material with high visible through mid-IR transmission. Technology Assessment and Transfer (TA and T) reports on their efforts to integrate electromagnetic interference protection into spinel domes through the use of metallic grids. TA and T is developing an approach to embed noble metal EMI grids in spinel domes by encapsulating the grid with a germanate glass that has matching refractive index and coefficient of thermal expansion to spinel. Achievements to date and an outlook on this method as a viable manufacturing route are presented.

Vibrational spectroscopy and energy relaxation of nanocubes, nanoboxes, and nanocages

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.