Transverse shifts and time delays of spatiotemporal vortex pulses reflected and refracted at a planar interface

Transverse (Hall-effect) and Goos–Hänchen shifts of light beams reflected/refracted at planar interfaces are important wave phenomena, which can be significantly modified and enhanced by the presence of intrinsic orbital angular momentum (OAM) in the beam. Recently, optical spatiotemporal vortex pulses (STVPs) carrying a purely transverse intrinsic OAM were predicted theoretically and generated experimentally. Here we consider the reflection and refraction of such pulses at a planar isotropic interface. We find theoretically and confirm numerically novel types of OAM-dependent transverse and longitudinal pulse shifts. Remarkably, the longitudinal shifts can be regarded as time delays, which appear, in contrast to the well-known Wigner time delay, without temporal dispersion of the reflection/refraction coefficients. Such time delays allow one to realize OAM-controlled slow (subluminal) and fast (superluminal) pulse propagation without medium dispersion. These results can have important implications in various problems involving scattering of localized vortex states carrying transverse OAM.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-based Single-crystal Superalloy Part III: Auspicious Control of Dendrite Tip Undercooling

Location-dependent dendrite tip undercooling is numerically elucidated to predict crystallography-assisted resistance to centerline grain boundary formation and morphology transition of stray grain formation ahead of dendrite tip in the ternary Nickel-Chromium-Aluminum molten pool during course of nonequilibrium solidification for explanation arduous solidification behavior control of microstructure melioration. Heat input is not so salient as welding configuration for auspicious solidification behavior and beneficial microstructure development. Advantageous symmetry of welding configuration efficiently lessens dendrite tip undercooling for prevalent dendrite morphology stability of planar interface with alleviation of columnar/equiaxed transition (CET) phenomenon. The bimodal distribution of undercooling ahead of dendrite tip is symmetrically dominant for (001)/[100] growth crystallography with capability of increasing morphology of interface kinetics for epitaxial growth and guarantees single-crystal potential. Alternatively, the distribution of undercooling ahead of dendrite tip is asymmetrically prevalent for (001)/[110] growth crystallography with inefficiency of nonhomologous solidification behavior for discontinuous intersection of solidification interface. Undercooling ahead of dendrite tip inside [010] growth region is not so wide as inside [100] growth region, where thermometallurgically initiates unstable solidification interface and inferior solidification behavior, with unfavorable crystallography in the case of asymmetrical (001)/[110] welding configuration. The smaller heat input is applied, the narrower undercooling ahead of dendrite tip is acquired to significantly mitigate microstructure anomalies with favorable solidification conditions, meliorate metallurgical properties and potentially improve weldability with viability of epitaxial columnar morphology and vice versa. Optimum heat input, especially low laser power and high welding speed together, is a viable and robust way to limit plethora of undercooling and easily decrease solidification behavior anomalies. When low laser power or rapid welding speed is chosen, low heat input not only lessens [100] dendrite growth region, where is spontaneously vulnerable to columnar/equiaxed transition, as ramification of prominent dendrite tip undercooling, but also metallurgically ameliorates [001] dendrite growth region, where morphologically aids epitaxial growth and activates stable planar interface, with achievable diminution of dendrite tip undercooling. Symmetrical (001)/[100] welding configuration, in which undercooling ahead of dendrite tip is preferably narrower than asymmetrical (001)/[110] welding configuration, is one of the most important ingredient for auspicious control of dendrite tip undercooling, once other welding conditions are similar. The main reason, why welding conditions (both low heat input and (001)/[100] welding configuration) is quite superior to welding conditions (both high heat input and (001)/[110] welding configuration), is attributable to favorable crystallography-dependent thermometallurgical factors to suppress inhomogeneous microstructure as long as solidification conditions within marginal stability range. Satisfying crack-free microstructure development is strongly interdependent on kinetics-related solidification behavior through scrupulous control of dendrite tip undercooling to balance between microstructure amelioration and weld depth requirement. The mechanism of columnar/equiaxed transition elimination, by which kinetic driving forces of abnormal microstructure development within high-undercooling region on either left or right side of weld pool is diminished through challenging method of crystallography-dependent dendrite tip undercooling control, is therefore proposed. Finally, there is reasonable consensus between numerical analysis results and experiment results. The numerical analysis provides credible insight into where is liable to microstructure anomalies and why dendrite tip undercooling suppresses stray grain formation for successful laser surface modification of Ni-based single-crystal superalloy.

Quantitative ultrasound assessment of the influence of roughness and healing time on osseointegration phenomena

The evolution of bone tissue quantity and quality in contact with the surface of orthopedic and dental implants is a strong determinant of the surgical outcome but remains difficult to be assessed quantitatively. The aim of this study was to investigate the performance of a quantitative ultrasound (QUS) method to measure bone-implant interface (BII) properties. A dedicated animal model considering coin-shaped titanium implants with two levels of surface roughness (smooth, Sa = 0.49 µm and rough, Sa = 3.5 µm) allowed to work with a reproducible geometry and a planar interface. The implants were inserted in rabbit femurs and tibiae for 7 or 13 weeks. The ultrasonic response of the BII was measured ex vivo, leading to the determination of the 2-D spatial variations of bone in contact with the implant surface. Histological analysis was carried out to determine the bone-implant contact (BIC) ratio. The amplitude of the echo was significantly higher after 7 weeks of healing time compared to 13 weeks, for both smooth (p < 0.01) and rough (p < 0.05) implants. A negative correlation (R = − 0.63) was obtained between the ultrasonic response and the BIC. This QUS technique is more sensitive to changes of BII morphology compared to histological analyses.