Type of Document Dissertation Author Halabica, Andrej URN etd-11042009-100728 Title Coherent acoustic phonons in metal/dielectric superlattices Degree PhD Department Physics Advisory Committee
Advisor Name Title Richard Haglund Committee Chair Norman Tolk Committee Member Robert Magruder Committee Member Sharon Weiss Committee Member Sokrates Pantelides Committee Member Keywords
- laser annealing
- coherent acoustic phonons
- surface modes
Date of Defense 2009-10-27 Availability unrestricted AbstractPulsed 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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