Identification and Characterization of Point Defects in Nonlinear Optical Crystals: Barium Gallium Selenide and Lithium Triborate

Increased interest in developing new nonlinear optical materials has led to 3rd generation materials, such as BaGa4Se7 and LiB3O5. These two nonlinear optical crystals have the potential to enable development of tunable lasers operating across the mid-wave infrared and the ultraviolet regions, respectively. Coherent light sources are used in the development of enhanced technologies with commercial and defense-based applications such as: health and environmental monitoring devices, next-generation infrared countermeasures, and ballistic missile booster exhaust plume characterization. The performance of optical materials in high-powered military applications is largely limited by the presence of defects. These defects adversely affect performance through unwanted optical absorption, resulting in the buildup of excess heat, causing material deterioration or complete failure. High-sensitivity and high-resolution experimental techniques, such as electron paramagnetic resonance, enable identification and characterization of defects in materials. In partnership and cooperation with U.S. industrial crystal growth companies, this information is shared, improving material quality and performance

Oxygen Vacancies in LiB\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e5\u3c/sub\u3e Crystals and Their Role in Nonlinear Absorption

LiB3O5 (LBO) crystals are used to generate the second, third, and fourth harmonics of near-infrared solid-state lasers. At high power levels, the material’s performance is adversely affected by nonlinear absorption. We show that as-grown crystals contain oxygen and lithium vacancies. Transient absorption bands are formed when these intrinsic defects serve as traps for “free” electrons and holes created by x rays or by three- and four-photon absorption processes. Trapped electrons introduce a band near 300 nm and trapped holes produce bands in the 500-600 nm region. Electron paramagnetic resonance (EPR) is used to identify and characterize the electrons trapped at oxygen vacancies (the unpaired electron is localized on one neighboring boron). Self-trapped holes and lithium vacancies with the hole trapped on an adjacent oxygen are also observed with EPR. At room temperature, we predict that most of the unwanted defect-related ultraviolet absorption created by a short laser pulse will decay with a half-life of 29 µs

Laser ablation for the synthesis of carbon nanotubes

Single walled carbon nanotubes are produced in a novel apparatus by the laser-induced ablation of moving carbon target. The laser used is of high average power and ultra-fast pulsing. According to various preferred embodiments, the laser produces and output above about 50 watts/cm.sup.2 at a repetition rate above about 15 MHz and exhibits a pulse duration below about 10 picoseconds. The carbon, carbon/catalyst target and the laser beam are moved relative to one another and a focused flow of "side pumped", preheated inert gas is introduced near the point of ablation to minimize or eliminate interference by the ablated plume by removal of the plume and introduction of new target area for incidence with the laser beam. When the target is moved relative to the laser beam, rotational or translational movement may be imparted thereto, but rotation of the target is preferred

Laser ablation for the synthesis of carbon nanotubes

Single walled carbon nanotubes are produced in a novel apparatus by the laser-induced ablation of moving carbon target. The laser used is of high average power and ultra-fast pulsing. According to various preferred embodiments, the laser produces an output above about 50 watts/cm.sup.2 at a repetition rate above about 15 MHz and exhibits a pulse duration below about 10 picoseconds. The carbon, carbon/catalyst target and the laser beam are moved relative to one another and a focused flow of side pumped, preheated inert gas is introduced near the point of ablation to minimize or eliminate interference by the ablated plume by removal of the plume and introduction of new target area for incidence with the laser beam. When the target is moved relative to the laser beam, rotational or translational movement may be imparted thereto, but rotation of the target is preferred

SAFETY PERFORMANCE EVALUATION ON THE NEBRASKA TURNED-DOWN APPROACH TERMlNAL SECTION

One full-scale vehicle crash test was conducted on the Nebraska Turned-Down Approach Terminal Section. Test NETD-1 was conducted with a 1984 Dodge Colt weighing 1,887-lbs (test inertial). Impact conditions were 59.0 mph and 0 degrees with a 1.25-ft offset toward the roadway. The test was conducted and reported in accordance with the requirements specified in the Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances, National Cooperative Highway Research Program (NCHRP) Report No. 230. The safety performance of the Nebraska Turned-Down Approach Terminal Section was determined to be unacceptable according to the NCHRP 230 criteria

SAFETY PERFORMANCE EVALUATION OF THE STEEL-BACKED LOG RAIL

The Coordinated Federal Lands Highways Technology Improvement Program (CTIP) was developed with the purpose of serving the immediate needs of those who design and construct Federal Lands Highways, including Indian Reservation roads, National Park roads and parkways, and forest highways. A wide assortment of guardrails, bridge rails and transitions are being used on roads under the jurisdiction of the National Park Service and other Federal agencies. These guardrails, bridge rails and transitions are intended to blend in with the roadside in order to preserve the visual integrity of the parks and parkways. However, many of them have never been crash tested (1,2). A testing program was developed in order to ensure that the safety hardware used in these areas are safe for the traveling public. The Steel Backed Log Rail was included in the second Federal Highway Administration (FHWA) testing program - Guardrail Testing Program II

Acceleration is the Key to Drag Reduction in Turbulent Flow

A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes and Reynolds number. The drag was reduced by as much as 30\%. Past work has suggested that the drag reduction scales with the velocity amplitude of the motion, its period, or the Reynolds number. Here, we find that the key parameter is simply the acceleration, which reduces the complexity of the phenomenon by two orders of magnitude. This insight opens new potential avenues for reducing fuel consumption by large vehicles and for reducing energy costs in large piping systems.Comment: 8 pages, 5 figure

SAFETY PERFORMANCE EVALUATION ON THE NEBRASKA TURNED-DOWN APPROACH TERMlNAL SECTION

One full-scale vehicle crash test was conducted on the Nebraska Turned-Down Approach Terminal Section. Test NETD-1 was conducted with a 1984 Dodge Colt weighing 1,887-lbs (test inertial). Impact conditions were 59.0 mph and 0 degrees with a 1.25-ft offset toward the roadway. The test was conducted and reported in accordance with the requirements specified in the Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances, National Cooperative Highway Research Program (NCHRP) Report No. 230. The safety performance of the Nebraska Turned-Down Approach Terminal Section was determined to be unacceptable according to the NCHRP 230 criteria

FULL-SCALE VEHICLE CRASH TEST ON THE IOWA STEEL TEMPORARY BARRIER RAIL

One full-scale vehicle crash test was conducted on the Iowa Steel Temporary Barrier Rail. Test 15-1 was conducted with at 5,500 pound vehicle at 22.5 degrees and 60.6 mph. The overall test length of the barrier was 200 feet. The barrier was shop fabricated and transported to the test site in 20 foot length sections. The cross-section of the barrier consisted of two stacked steel HP 14x73 (A36) shapes with the edges of the flanges placed back to back and held together by welded steel straps spaced 5 feet on centers. The inside box section between the HP shapes was filled with concrete. The height of the barrier was 29 inches. The 20 foot length sections were bolted together at the test site. The location of the vehicle impact was 100 feet from the end of the barrier installation. This was also the location where two sections were bolted together. The test was evaluated according to the safety criteria in NCHRP 230 and also in the AASHTO guide specifications, performance level 2. The safety performance of the Iowa Steel Temporary Barrier Rail was determined to be satisfactory

Transition-metal ions in β-Ga\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e crystals: Identification of Ni acceptors

Excerpt: Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band