No abstract available

[en] We describe measurements we have made of the emittance of a high-current, moderate-energy ion beam after transport through a permanent-magnet electrostatic plasma lens. The results indicate the absence of emittance growth due to the lens, when the lens is adjusted for optimal beam focusing. The measured emittance for a 16 keV Cu²⁺ ion beam formed by a vacuum arc ion source was about 0.4 π · mm · mrad at a beam current of 50 mA rising more-or-less linearly to 1.5 π · mm · mrad at 250 mA, and was conserved in beam transport through the lens. These results have significance for the application of high-current ion sources and the electrostatic plasma lens to particle accelerator injection

[en] Here, the equilibrium shape of 6H-SiC nanostructures and their surfaces were studied by analyzing nano-void (~10 nm) shapes, which were introduced in monocrystalline 6H-SiC by high-temperature neutron irradiation, using transmission electron microscopy. The nano-voids were determined to be irregular icosahedrons truncated with six (1¯100), twelve (1¯103), one smaller top-basal, and one larger bottom-basal planes, which suggests that (1¯100) and (1¯103) are the next stable surface class after the basal planes. The relatively frequent absence of the (1¯100) surface in the nano-voids indicated that the (1¯103¯) surface type is energetically rather stable. These non-basal surfaces were found not to be atomically flat due to the creation of nanofacets with half unit-cell height in the c-axis. The (1¯100) and (1¯103) surfaces were classified as two and four face types according to their possible nanofacets and surface termination, respectively. We also discuss the surface energy difference between the (1¯103¯) and (1¯103) face types in relation to the energy balance within the equilibrium, but irregular, polyhedron, in which the (1¯103) surface had double the surface energy of the (1¯103¯) surface (~3900 erg/cm"2).

[en] We introduce experimental findings of actively controlled anisotropic near-field coupling in a metamaterial composed of two concentric in-plane split ring resonators and a closed-ring resonator functioning at terahertz (THz) frequencies. Transparency windows are determined due to the coupling effect between two dipole-like modes for both polarizations of the THz field, but at different frequencies. By properly incorporating silicon islands into the metamaterial unit cells, the transparency windows can be optically controlled with a modulation depth reaching 68% and 62% for horizontal and vertical polarizations, respectively. The results are of value for designing active THz anisotropic devices.

[en] We study the effect of carbon segregants on the spin and orbital moments of L1₀ FePt granular media using x-ray magnetic circular dichroism (XMCD) spectroscopy and report an effective decoupling of the structural film properties from the magnetic parameters of the grains. The carbon concentration reduces the grain size from (200 ± 160) nm² down to (50 ± 20) nm² for 40 mol. %C and improves sphericity and the order of grains, while preserving the crystalline order, spin and orbital moments, and perpendicular magnetocrystalline anisotropy. Here, we identify the primary cause of enhanced saturation and coercive fields as the reduced demagnetization fields of individual grains. The ability to shrink grains without impairing their magnetic properties is a critical requirement for the commercialization of Heat-Assisted Magnetic Recording.

[en] In controlling the thermal properties of the surrounding environment, we provide insight into the underlying mechanisms driving the widely used laser direct write method for additive manufacturing. In this study, we find that the onset of silver nitrate reduction for the formation of direct write structures directly corresponds to the calculated steady-state temperature rises associated with both continuous wave and high-repetition rate, ultrafast pulsed laser systems. Furthermore, varying the geometry of the heat affected zone, which is controllable based on in-plane thermal diffusion in the substrate, and laser power, allows for control of the written geometries without any prior substrate preparation. In conclusion, these findings allow for the advance of rapid manufacturing of micro- and nanoscale structures with minimal material constraints through consideration of the laser-controllable thermal transport in ionic liquid/substrate media.

[en] In this study, CCD-based thermoreflectance imaging and finite element modeling are used to study the two-dimensional (2D) temperature profile of a junction-down broad-area diode laser facet subject to back-irradiance. By determining the temperature rise in the active region (ΔΤAR) at different diode laser optical powers, back-irradiance reflectance levels, and back-irradiance spot locations, we find that ΔΤAR increases by nearly a factor of three when the back-irradiance spot is centered in the absorbing substrate approximately 5 μm away from the active region, a distance roughly equal to half of the back-irradiance spot FWHM (9 μm). This corroborates prior work studying the relationship between the back-irradiance spot location and catastrophic optical damage, suggesting a strong thermal basis for reduced laser lifetime in the presence of back-irradiance for diode lasers fabricated on absorbing substrates.

[en] Here, the ability to create optical materials with arbitrary index distributions would prove transformative for optics design and applications. However, current fabrication techniques for graded index (GRIN) materials rely on diffusion profiles and therefore are unable to realize arbitrary distribution GRIN design. Here, we demonstrate the laser direct writing of graded index structures in protein-based hydrogels using multiphoton lithography. We show index changes spanning a range of 10"–"2, which is comparable with laser densified glass and polymer systems. Further, we demonstrate the conversion of these written density variation structures into SiO_2, opening up the possibility of transforming GRIN hydrogels to a wide range of material systems.

[en] In this paper, the tetragonality (c/a) of a PbZr_0_._2Ti_0_._8O_3 (PZT) thin film on La_0_._7Sr_0_._3MnO_3/0.72Pb(Mg_1_/_3Nb_2_/_3)O_3-0.28PbTiO_3 (PMN-PT) substrates was controlled by applying an electric field on the PMN-PT substrate. The piezoelectric response of the PZT thin film under various biaxial strains was observed using time-resolved micro X-ray diffraction. The longitudinal piezoelectric coefficient (d_3_3) was reduced from 29.5 to 14.9 pm/V when the c/a ratio of the PZT film slightly changed from 1.051 to 1.056. Finally, our results demonstrate that the tetragonality of the PZT thin film plays a critical role in determining d_3_3, and in situ strain engineering using electromechanical substrate is useful in excluding the extrinsic effect resulting from the variation in the film thickness or the interface between substrate.

[en] Dielectric and piezoelectric properties for Zn_1_-_xMg_xO (ZMO) thin films are reported as a function of MgO composition up to and including the phase separation region. Zn_1_-_xMg_xO (0.25 ≤ x ≤ 0.5) thin films with c-axis textures were deposited by pulsed laser deposition on platinized sapphire substrates. The films were phase pure wurtzite for MgO concentrations up to 40%; above that limit, a second phase with rocksalt structure evolves with strong (100) texture. With increasing MgO concentration, the out-of-plane (d_3_3_,_f) and in-plane (e_3_1_,_f) piezoelectric coefficients increase by 360% and 290%, respectively. The increase in piezoelectric coefficients is accompanied by a 35% increase in relative permittivity. Loss tangent values fall monotonically with increasing MgO concentration, reaching a minimum of 0.001 for x ≥ 0.30, at which point the band gap is reported to be 4 eV. As a result, the enhanced piezoelectric response, the large band gap, and the low dielectric loss make Zn_1_-_xMg_xO an interesting candidate for thin film piezoelectric devices, and demonstrate that compositional phase transformations provide opportunities for property engineering.