ALBERT

Description: Femtosecond laser ablation is used in applications which require low damage surface treatments, such as serial sectioning, spectroscopy, and micromachining. However, dislocations are generated by femtosecond laser-induced shockwaves and consequently have been studied in strontium titanate (STO) using transmission electron microscopy (TEM) and electron backscatter diffraction analysis. The laser ablated surfaces in STO exhibit dislocation structures that are indicative of those produced by uniaxial compressive loading. TEM analyses of dislocations present just below the ablated surface confirm the presence of ⟨ 110 ⟩ dislocations that are of approximately 35° mixed character. The penetration depth of the dislocations varied with grain orientation relative to the surface normal, with a maximum depth of 1.5  μ m. Based on the critical resolved shear stress of STO crystals, the approximate shockwave pressures experienced beneath the laser irradiated surface are reported.

Description: The three-dimensional microstructure of levitation melted TiNi 1.20 Sn has been characterized using the TriBeam system, a scanning electron microscope equipped with a femtosecond laser for rapid serial sectioning, to map the character of interfaces. By incorporating both chemical data (energy dispersive x-ray spectroscopy) and crystallographic data (electron backscatter diffraction), the grain structure and phase morphology were analyzed in a 155  μ m × 178  μ m × 210  μ m volume and were seen to be decoupled. The predominant phases present in the material, half-Heusler TiNiSn, and full-Heusler TiNi 2 Sn have a percolated structure. The distribution of coherent interfaces and high-angle interfaces has been measured quantitatively.

Description: Using both experiment and 2D3V particle-in-cell (PIC) simulations, we describe the use of specular reflectivity measurements to study relativistic ( Iλ 2  〉 10 18  W/cm 2 ⋅ μ m 2 ) laser-plasma interactions for both high and low-contrast 527  n m laser pulses on initially solid density aluminum targets. In the context of hot-electron generation, studies typically rely on diagnostics which, more-often-than-not, represent indirect processes driven by fast electrons transiting through solid density materials. Specular reflectivity measurements, however, can provide a direct measure of the interaction that is highly sensitive to how the EM fields and plasma profiles, critical input parameters for modeling of hot-electron generation, evolve near the interaction region. While the fields of interest occur near the relativistic critical electron density, experimental reflectivity measurements are obtained centimeters away from the interaction region, well after diffraction has fully manifested itself. Using a combination of PIC simulations with experimentally inspired conditions and an analytic, non-paraxial, pulse propagation algorithm, we calculate reflected pulse properties, both near and far from the interaction region, and compare with specular reflectivity measurements. The experiment results and PIC simulations demonstrate that specular reflectivity measurements are an extremely sensitive qualitative, and partially quantitative, indicator of initial laser/target conditions, ionization effects, and other details of intense laser-matter interactions. The techniques described can provide strong constraints on many systems of importance in ultra-intense laser interactions with matter.