Bridging the gap to mesoscale radiation materials science with transient grating spectroscopy

Direct mesoscale measurements of radiation-induced changes in the mechanical properties of bulk materials remain difficult to perform. Most widely used characterization techniques are either macro- or microscale in nature, focusing on overall properties or overly small areas for analysis. Linking the atomic structure of irradiated materials directly with their radiation-affected properties remains one of the largest unmet challenges in radiation materials science. By measuring the change in surface acoustic wave speed as a function of relative orientation on metallic single crystals, we demonstrate that transient grating (TG) spectroscopy experiments have the sensitivity necessary to detect radiation-induced material property changes. We also show that classical molecular dynamics (MD) simulations can be used to accurately simulate orientation-based changes in surface acoustic wave speed in TG experiments, by comparing with experimental measurements and theoretical predictions. The agreement between theory, simulation, and experiment gives confidence in classical MD as a predictive tool to simulate defect-based changes in elastic properties, which cannot yet be fully treated by theory. This ability is of critical importance for the informed use of TG spectroscopy to measure material property changes induced by radiation damage, which may vary by amounts formerly too small for reliable in situ detection. Finally, our MD simulation framework is used to study the effect of an imposed vacancy population on the acoustic response of several materials. The results of these studies indicate that TG experiments are well suited to the ex situ and in situ study of radiation-induced material property changes.National Science Foundation (U.S.) (Grant 1122374)National Science Foundation (U.S.) (Grant CHE-1111557)Transatomic Power (Award 023875-001)U.S. Nuclear Regulatory Commission (MIT Nuclear Education Faculty Development Program. Grant NRC-HQ- 84-15-G-0045

Point source in a phononic grating: stop bands give rise to phonon-focusing caustics

We use locally-excited gigahertz surface phonon wavepackets in microscopic line structures of different pitches to reveal profound anisotropy in the radiation pattern of a point source in a grating. Time-domain data obtained by an ultrafast optical imaging technique and by numerical simulations are Fourier transformed to obtain frequency-filtered real-space acoustic field patterns and k-space phononic band structure. The numerically-obtained k-space images are processed to reveal an intriguing double-horn structure in the lowest-order group-velocity surface, which explains the observed non-propagation sectors bounded by caustics, noted at frequencies above the bottom of the first stop band. We account for these phonon-focusing effects, analogous to collimation effects previously observed in two- and three-dimensional lattices, with a simple analytical model of the band structure based on a plane wave expansion. As the frequency is increased, a transition to dominant waveguiding effects along the lines is also documented

Negative refraction and focusing of elastic Lamb waves at an interface

We study negative refraction and focusing of elastic waves in a simple mechanical system comprised of a freestanding plate with a step change in thickness. A point-focused and intensity-modulated laser source is used to excite backward-propagating Lamb waves on one side of the step, and the displacement field is probed using an optical interferometer. Conversion between forward- and backward-propagating modes at the interface leads to negative refraction, and we demonstrate for the first time the operation of a flat lens, similar to that predicted by Veselago in negative-index media, for guided elastic waves in isotropic media. We propose that guided elastic waves provide a convenient and powerful experimental test bed for the study of negative-index physics.National Science Foundation (U.S.) (Grant No. CMMI 0448796)National Science Foundation (U.S.) (Grant No. IIP 0712496

Surface Brillouin scattering of opaque solids and thin supported films

Surface Brillouin scattering (SBS) has been used successfully for the study of acoustic excitations in opaque solids and thin supported films, at both ambient and high temperatures. A number of different systems have been investigated recently by SBS including crystalline silicon, amorphous silicon layers produced by ion bombardment and their high temperature recrystallisation, vanadium carbides, and a nickel-based superalloy. The most recent development includes the measurement of a supported gold film at high pressure. The extraction of the elastic constants is successfully accomplished by a combination of the angular dependence of surface wave velocities and the longitudinal wave threshold within the Lamb shoulder. The application of surface Green's function methods successfully reproduces the experimental SBS spectra. The discrepancies often observed between surface wave velocities and by ultrasonics measurements have been investigated and a detailed correction procedure for the SBS measurements has been developed