Coherent acoustic phonons in metal/dielectric superlattices
Pulsed infrared laser annealing experiments on metal implanted dielectric matrices were performed, using a free electron laser as a source of infrared radiation at 8 and 9 *m wavelengths. This was the first study where such wavelengths were used for laser annealing of composite materials. Our results show that the annealing successfully modified nanocomposites consisting of Au and Ag nanoparticles embedded in a dielectric matrix. Fast nucleation and growth of Au nanoparticles in both SiO2 and Al2O3 matrices were observed, while nanoparticle dissolution due to this rapid thermal annealing process was observed in Ag-implanted SiO2. These experiments demonstrate the unique effects of fast thermal heating of the matrix on the size and size distribution of embedded metal nanoparticles, using photons with energies far below the bulk bandgap of the matrix. A set of composite materials in the form of Au/Al2O3 superlattices was also prepared using electron beam evaporation. Time-resolved femtosecond laser spectroscopy was successfully applied to study the vibrational properties of these multilayers. A comprehensive experimental and computational study of the effects of varying Au layer thickness on the excitability and detectability of the first and second surface acoustic phonon mode was undertaken. The frequency of these modes was compared to the theoretical calculation and the slight differences were attributed to the effect of the nanoparticle structure of the Au films, which was confirmed using TEM and optical spectroscopy. In addition to the surface modes, propagating phonon modes were observed in the Au(5 nm)/Al2O3(45 nm) superlattice. These belong to the lowest minibranch of the zone-folded longitudinal phonon dispersion curve and travel in the form of a propagating pulse, which is reflected at the substrate and surface interfaces. By detecting two echoes of the pulse, it was possible to experimentally determine the effective sound velocity in the superlattice.
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