Mechanical properties of complex solids and exotic thermal transport dynamics investigated with optical and extreme-ultraviolet transient grating techniques

Abstract

In this thesis we investigate the acoustic/mechanical properties and thermal transport phenomena in a range of solids using optical and extreme-ultraviolet transient grating (TG) techniques. A single-crystal (110)-oriented tungsten sample was subject to mild helium-ion bombardment and studied using acoustic TG spectroscopy. We observed a substantial and counterintuititive increase in the elastic anisotropy as a result of ion bombardment, consistent with previous ab initio calculations. The acoustic dispersion of a microgranular crystal composed of a hexagonal monolayer of polystyrene microspheres was measured over its entire Brillouin zone along the [gamma] - K direction. We observe three contact-based branches and ve spheroidal branches in addition to the surface acoustic wave of the glass substrate. We determine that both the contact and spheroidal modes are dispersive, collective vibrational modes of the system, and characterize a range of other dynamics in this system.

Reflection-geometry TG thermal transport measurements were performed on a bulk Si₉₃.₄Ge₆.₆ alloy for grating periods between 1 - 13.5 [mu]m. We obtain quantitative agreement with ab initio calculations using the variational solution of the phonon Boltzmann transport equation (BTE) under the relaxation time approximation (RTA). Nanoporous holey silicon membranes with feature sizes greater than 100 nm are studied with thermal transport TG measurements. The measured values of thermal diffusivity are in quantitative agreement with the results of ab initio RTA-BTE calculations assuming diffuse scattering from boundaries--i.e., the "Casimir formulation" of thermal transport through nanostructures. Hydrodynamic second sound over microscale distances was observed at temperatures above 100 K in graphite in a reflection-geometry TG measurement. We obtain semiquantitative agreement with ab initio linearized BTE calculations with the full scattering matrix.

TG measurements are extended to the extreme ultraviolet (EUV) spectral region using the EUV free-electron laser (FEL) sources of the FERMI light source at the Elettra Synchrotron Facility in Trieste, Italy. We report EUV-pump optical-probe TG measurements on a range of samples. We observe coherent phonon generation in BK-7 glass, diamond, and Bi₄Ge₃O₁₂ as well as highly non-diffusive and fully ballistic thermal transport in silicon and diamond, respectively, at a grating period of 277 nm. We then report EUV-pump EUV-probe TG measurements of amorphous Si₃N₄ and silicon, observing coherent acoustic signal and thermal transport signal at grating periods as small as 28 nm.