UC Berkeley

Two particular experimental approaches have been experiencing increased interest, that of dynamic spectroscopy and that of 2D measurements. The first allows us to obtain a better understanding of the electronic and chemical processes taking place under shock loading while the second provides a detailed view into the microscopic and macroscopic heterogeneities which directly influence material behavior and have been difficult to identify in the bulk material. In order to better understand these behaviors and their origins, two approaches have been utilized. A novel dynamic broadband optical reflectivity diagnostic was developed to probe changes in material properties on the scale of electronic structure and an existing, but only recently developed high resolution velocimetry system was used to observe the aforementioned changes on larger, microstructural scales.

In order to expand understanding of the chemical and mechanical responses of condensed matter to dynamic shock compression two projects were undertaken. The first was the development of a broadband optical reflectivity diagnostic with both time and wavelength resolution. The Shock Wave Optical Reflectivity Diagnostic (SWORD) has enabled us to study the dynamic optical reflectivity in shocked samples over the visible and near-infrared, across a time span of nanoseconds and with a resolution of 0.5 ns and 10 nm. Laser velocimetry was used in tandem with the SWORD to determine kinematic properties of the shocked samples, such as pressure and density. This novel diagnostic has been applied to the semiconductor-to-metal transition in single-crystal germanium and used to observe both a general reflectivity increase on metallization and a wavelength dependent response as a function of pressure.

The second project involved the application of the recently developed two dimensional Velocity Interferometry System for Any Reflector (2D VISAR, alternatively Janus High Resolution Velocimeter, JHRV) to study anisotropic shock wave propagation and dynamic heterogeneous deformation and fracture in diamond. In combination with the aforementioned laser velocimetry (also VISAR), we have obtained velocity histories and two-dimensional velocity maps and images of the shocked target at various time points after breakout. Significant anisotropy in both the elastic and inelastic waves was observed in single-crystal diamond samples. Characteristic length scales for the fracture of polycrystalline diamond samples of varying grain size were also determined.