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Plasma Heating in Highly Excited GaN/AlGaN Multiple Quantum Wells

Description: Plasma Heating in Highly Excited GaN/AIGaN Multiple Quantum @@lvEu Wells w f + 1998 %p, K. C. Zeng, R. Mair, J. Y. Liz and H. X. Jiang a) ` fabrication and understanding of MQW lasers [2-5]. For the design of these lasers, one on RT optical studies. Our results revealed that in the GaN/AIGaN MQWS, plasma heating strongly effects the carrier distribution between the confined and unconfined band-to-band and fke excitonic transitions [7]. In the MQW sample under low the unconfined states as determined from the band structure. sample under high Lxc, we varied the excitation intensity by one order of magnitude from 0.110 to IO. The carrier density is estimated to be about N=1012/cm2 (at UC= 0.1 Io) to 1013/cm2 (at 1=== l.). We plotted the PL spectra for four representative excitation fimction of injected carrier density N (open squares). The ratio starts at a value of about 18% for N=1012/cm2 (& = O. lb), and reaches a value over 64 `XO for N=1013/cm2 (& = regions is a loss to optical gain. The carrier density is ve~ high in our experiment and an electron-hole plasma (EHP) state is expected. Because the carrier transfer process plasma temperature. The laser pump energy is about 4.3 eV, which is far above the energy band gap of the sample studied here. This may result in a hot carrier population carrier densities and plasma temperatures. Using a phenomenological expression based The calculated ratio of carriers in the unconfked to the confined states (Ima~ kf) as a finction of carrier density at different temperatures are plotted in Fig. 3 (solid lines). The figure shows that the experiment results can only be explained by plasma heating of the injected carriers at high & ( TP > TJ. The transparency carrier densities for GaN/AIXGal.XN MQW structures ...
Date: October 9, 1998
Creator: Botchkarev, A.; Chow, W.W.; Jiang, H.X.; Lin, J.Y.; Mair, R.; Morkoc, H. et al.
Partner: UNT Libraries Government Documents Department

Joule losses in linear THETA-pinches: laser vs magnetic heating

Description: From fifth symposium on engineering problems of fusion research: Princeton. New Jersey. USA (6 Nov 1973). The dc joule losses per unit length in a THETA -pinch compression coil are calculated and compared to the thermonuclear energy production for two different methods of plasma heating. In the first method, conventional staged THETA -pinch heating is assumed. In the second method, laser heating by long wavelength irradiation from thc ends is assumed. Reactor parameters are calculated. and it is shown that for circulating power fractions to be 20% or less, the plasma radius must be at least a few cm in size. (auth)
Date: January 1, 1973
Creator: Ellis, W.R.
Partner: UNT Libraries Government Documents Department

Real Time Dynamics of Laser Activated Interface Processes at the Molecular Scale

Description: Nanotechnology is one of the most interesting and challenging frontiers of science and technology. We are motivated by the belief that progress will come from improved understanding and control of structure, dynamics and reactivity at interfaces. First, we provide a summary of our projects and key findings. The following pages provide a more detailed account.
Date: December 30, 2007
Creator: Borguet, Eric
Partner: UNT Libraries Government Documents Department

Characterizing the effects of scale and heating rate on micro-scale explosive ignition criteria.

Description: Laser diode ignition experiments were conducted in an effort to characterize the effects of scale and heating rate on micro-scale explosive ignition criteria. Over forty experiments were conducted with various laser power densities and laser spot sizes. In addition, relatively simple analytical and numerical calculations were performed to assist with interpretation of the experimental data and characterization of the explosive ignition criteria.
Date: January 1, 2005
Creator: Hafenrichter, Everett Shingo & Pahl, Robert J.
Partner: UNT Libraries Government Documents Department

Directed light fabrication of iron-based materials

Description: Directed light fabrication (DLF) is a process that fuses gas delivered metal powders within a focal zone of a laser beam to produce fully dense, 3-dimensional metal components. From a computer generated solid model, deposition ``tool paths`` are constructed that command the laser movement to fabricate near net shape parts a layer at a time. Among potential candidate systems to study, iron-based alloys are particularly attractive for rapid prototyping. To evaluate the processing parameters in the DLF process, studies have been performed on the microstructural development in 1-dimensional and 2-dimensional Fe-based components. For example, continuous microstructural features are evident, implying a continuous liquid/solid interface during processing. In addition, solidification cooling rates have been determined based upon secondary dendrite arm spacings in Fe-25wt. % Ni and 316 stainless steel. Cooling rates vary from 10{sup 1}{minus}10{sup 5} K s{sup {minus}1}, and the solidification behavior has been simulated using macroscopic heat transfer analyses.
Date: January 1, 1995
Creator: Thoma, D.J.; Charbon, C.; Lewis, G.K. & Nemec, R.B.
Partner: UNT Libraries Government Documents Department

Equation of state measurements at extreme pressures using laser-driven shocks

Description: The regime of high density and extreme pressure in hydrogen is very difficult to approach theoretically since it is a strongly correlated, partially degenerate composite of molecules, atoms, and electrons. For this reason, a number of theoretical models of the EOS of hydrogen have been proposed. This makes reliable experimental data essential as a guide to theory. We have accessed this regime by shocking liquid D2 to pressures at and above the metallic transition where we measured the thermodynamic properties of the shocked state.
Date: December 3, 1998
Creator: Cauble, R C; Celliers, P M; Collins, G W; DaSilva, L B & Gold, D M
Partner: UNT Libraries Government Documents Department

Laser-generated shock waves and applications to advanced materials

Description: The use of lasers for the generation and application of high-pressure shock waves offers unique advantages and challenges. In contrast to impact systems, the range of pressures and strain rates is substantially greater using laser drive. The ability to change the temporal shape of the drive pulse allows a variety of strain-rate conditions to be obtained. In addition, high time-resolution in situ diagnostic methods are relatively simple to implement. Lasers can be at a disadvantage compared to impact methods in terms of shock generation, simplicity of the states achieved, the difficulty of characterizing bulk properties, and sample size. I will review the physics of laser-driven shock physics, diagnostic methods, and applications, with an emphasis on material physics. I will also present some views on important new directions for this area of research.
Date: January 19, 1996
Creator: Holmes, N.C.
Partner: UNT Libraries Government Documents Department

Femtosecond laser materials processing

Description: The use femtosecond pulses for materials processing results in very precise cutting and drilling with high efficiency. Energy deposited in the electrons is not coupled into the bulk during the pulse, resulting in negligible shock or thermal loading to adjacent areas.
Date: February 1, 1997
Creator: Stuart, B.C.
Partner: UNT Libraries Government Documents Department

Development of motion control method for laser soldering process

Description: Development of a method to generate the motion control data for sealing an electronic housing using laser soldering is described. The motion required to move the housing under the laser is a nonstandard application and was performed with a four-axis system using the timed data streaming mode capabilities of a Compumotor AT6400 indexer. A Microsoft Excel 5.0 spreadsheet (named Israuto.xls) was created to calculate the movement of the part under the laser, and macros were written into the spreadsheet to allow the user to easily create this data. A data verification method was developed for simulating the motion data. The geometry of the assembly was generated using Parametric Technology Corporation Pro/E version 15. This geometry was then converted using Pro/DADS version 3.1 from Computer Aided Design Software Inc. (CADSI), and the simulation was carried out using DADS version 8.0 from CADSI.
Date: May 1, 1997
Creator: Yerganian, S.S.
Partner: UNT Libraries Government Documents Department

Ultrafast studies of solution dynamics

Description: This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). Fast chemical dynamics generally must be initiated photochemically. This limits the applicability of modern laser methods for following the structural changes that occur during chemical and biological reactions to those systems that have an electronic chromophore that has a significant yield of photoproduct when excited. This project has developed a new and entirely general approach to ultrafast initiation of reactions in solution: laser-induced temperature jump (T-jump). The results open entire new fields of study of ultrafast molecular dynamics in solution. The authors have demonstrated the T-jump technique on time scales of 50 ps and longer, and have applied it to study of the fast events in protein folding. They find that a general lifetime of alpha-helix formation is ca 100 ns, and that tertiary folds (in apomyoglobin) form in ca 100 {mu}s.
Date: October 1, 1997
Creator: Woodruff, W.H.; Dyer, R.B. & Callender, R.H.
Partner: UNT Libraries Government Documents Department

Experiments and computational modeling of pulverized-coal ignition. Semiannual report, Apr 1, 1998--Sep 30, 1998

Description: Under typical conditions of pulverized-coal combustion, which is characterized by fine particles heated at very high rates, there is currently a lack of certainty regarding the ignition mechanism of bituminous and lower rank coals. It is unclear whether ignition occurs first at the particle-oxygen interface (heterogeneous ignition) or if it occurs in the gas phase due to ignition of the devolatilization products (homogeneous ignition). Furthermore, there have been no previous studies aimed at determining the dependence of the ignition mechanism on variations in experimental conditions, such as particle size, oxygen concentration, and heating rate. Finally, there is a need to improve current mathematical models of ignition to realistically and accurately depict the particle-to-particle variations that exist within a coal sample. Such a model is needed to extract useful reaction parameters from ignition studies, and to interpret ignition data in a more meaningful way. The authors propose to examine fundamental aspects of coal ignition through (1) experiments to determine the ignition mechanism of various coals by direct observation, and (2) modeling of the ignition process to derive rate constants and to provide a more insightful interpretation of data from ignition experiments. They propose to use a novel laser-based ignition experiment to achieve their objectives. The heating source will be a pulsed, carbon dioxide laser in which both the pulse energy and pulse duration are independently variable, allowing for a wide range of heating rates and particle temperatures--both of which are decoupled from each other and from the particle size. This level of control over the experimental conditions is truly novel in ignition and combustion experiments. Laser-ignition experiments also offer the distinct advantage of easy optical access to the particles because of the absence of a furnace or radiating walls, and thus permit direct observation and particle temperature measurement. The ignition mechanism ...
Date: October 31, 1998
Creator: Chen, John C. & Owusu-Ofori, Samuel
Partner: UNT Libraries Government Documents Department

High power laser for peening of metals enabling production technology

Description: Cryogenic energy-dispersive x-ray detectors are being developed because of their superior energy resolution ((less than or equal to) 10 eV FWHM for keV x rays) compared to semiconductor EDS systems. So far, their range of application is limited due to their comparably small size and low count rate. We present data on the development of superconducting tunnel junction (STJ) detector arrays to address both of these issues. A single STJ detector has a resolution around 10 eV below 1 keV and can be operated at count rates of order 10,000 counts/s. We show that the simultaneous operation of several STJ detectors does not diminish their energy resolution significantly, while increasing the detector area and the maximum count rate by a factor given by the total number of independent channels.
Date: June 18, 1998
Creator: Daly, J.; Dane, C.B.; Hackel, L.A. & Harrison, J.
Partner: UNT Libraries Government Documents Department

Laser desorption from and reconstruction on Si(100) surfaces studied by scanning tunneling microscopy

Description: Laser irradiated Si(100) surfaces were studied with an ultrahigh-vacuum scanning tunneling microscopy (STM) system. Our observations indicate that only the dimerized outermost atomic layer is removed if the laser fluence is below the melting threshold with a photon energy larger than the band gap. The newly exposed layer, surprisingly, did not have a dimerized atomic structure, but rather, resembled that of a bulk-terminated structure. The uncovered layer remained atomically smooth (no vacancies) even after 90% of the outermost layer was removed. A possible explanation of these observations is that atom removal occurs by a preferential breakage of the atomic bonds in defect sites. When the laser fluence was increased to levels above the melting threshold, extensive surface roughening occurs.
Date: July 1, 1995
Creator: Xu, Jun; Overbury, S.H. & Wendelken, J.F.
Partner: UNT Libraries Government Documents Department

Laser encapsulation of metallic films in SiO{sub 2}

Description: Thin films of gold, copper and iron deposited on silica were driven into the substrate by a laser pulse. This transport takes place only when the irradiation is performed at a laser energy density of 0.7 J/cm{sup 2} or lower. Cross sectional transmission electron microscopy (TEM) of the irradiated specimens reveals two distinctive stages in the encapsulation process. In the first, the film melts and clusters into small particles and in the second one the particles are driven into the substrate by the laser pulse. The particle size of encapsulated metal varies from 5 to 50 nm. Selected area diffraction of the large particles and lattice fringe images of the smaller particles reveal pure metals, e.g., gold, copper or iron. Titanium films laser irradiated are not encapsulated in silica; instead, these films react with silica forming an amorphous compound. Apparently, one of the conditions required for encapsulation is that the metal should not react with the substrate material. On subsequent irradiation at a laser energy density of 1.5 J/cm{sup 2}, ablation of silica partially exposes the metallic particles. Strong bonding between a new film deposited after irradiation and the substrate is obtained because these particles anchor the freshly deposited film. Anchoring is clearly revealed by cross sectional TEM. The mechanisms of encapsulation are discussed using results from TEM and adhesion testing.
Date: December 31, 1995
Creator: Pedraza, A.J.; Cao, S.; Lowndes, D.H. & Allard, L.F.
Partner: UNT Libraries Government Documents Department

Dynamics of laser ablation for thin film growth by pulsed laser deposition

Description: Fundamental gas dynamic and laser-material interactions during pulsed laser deposition are explored through sensitive imaging and plasma spectroscopic diagnostics. Two recent phenomena, plume-splitting in background gases and the unusual dynamics of graphite ablation for amorphous diamond film growth, are presented.
Date: February 1, 1996
Creator: Geohegan, D.B. & Puretzky, A.A.
Partner: UNT Libraries Government Documents Department

Laser-solid interaction and dynamics of laser-ablated materials

Description: An annealing model is extended to treat the vaporization process, and a hydrodynamic model describes the ablated material. We find that dynamic source and ionization effects accelerate the expansion front of the ablated plume with thermal vaporization temperature. The vaporization process and plume propagation in high background gas pressure are studied.
Date: September 1995
Creator: Chen, K. R.; Neboeuf, J. N.; Wood, R. F.; Geohegan, D. B.; Donato, J. M.; Liu, C. L. et al.
Partner: UNT Libraries Government Documents Department