In this work, the operating characteristics of quantum dot (QD) lasers with asymmetric barrier layers (ABLs) are studied. Several different cases are examined, in particular: 1) Effect of excited states on static and dynamic operating characteristics Within QDs, in addition to the lasing ground state, carriers can be captured into excited states, where they then decay into the ground state. This excited-state-mediated capture impacts the operating characteristics, limiting the maximum output power and modulation bandwidth. Three separate cases are considered: only indirect capture with electron-hole symmetry, both direct and indirect capture with electron-hole symmetry, and both direct and indirect capture of electrons but only indirect capture of holes. The impact of different parameters on the operating characteristics is studied, with values for maximizing the output power and modulation bandwidth being found. In addition, it is found that parasitic recombination in the active region in the space between QDs causes the output power to saturate at high injection currents for the cases of indirect capture for both electrons and holes and indirect capture for holes but direct and indirect capture for electrons, although the presence of the ABLs causes it to reach saturation at much lower currents. 2) QD laser with only a single ABL To be effective, the materials for ABLs must be carefully chosen to ensure that the band edges properly align to allow one carrier to enter the active region while preventing the other from overshooting it. Due to this requirement, it may arise that a suitable material only exists for one ABL but not the other. The performance of a QD laser with only a single ABL is considered and compared to a conventional QD laser. Specifically, the output power and characteristic temperature are calculated. While the single ABL laser only offers a negligible increase in output power compared to the conventional laser, it offers a considerable increase in characteristic temperature. 3) Analytical derivation of alpha factor in QD lasers with and without ABLs The alpha factor of a semiconductor laser describes the spectral linewidth broadening that occurs in semiconductor lasers due to changes in the refractive index due to the carrier density. While it has been studied experimentally, there has been little work done on deriving the alpha factor of QD lasers analytically. An expression for the alpha factor is found in this work using the real and imaginary parts of the complex susceptibility. For QD lasers with no inhomogeneous broadening, as well as ones with equilibrium filling of QDs with narrow line of QD size distribution, the alpha factor is independent of carrier density, and is therefore the same for any QD lasers, with or without ABLs. For QD lasers with equilibrium filling without a narrow line of QD size distribution, the alpha factor depends on carrier density, allowing for a potential difference between conventional and ABL QD lasers, however the difference between the two will be lessened. / Doctor of Philosophy / Semiconductor lasers are the most widely used laser, due in part to their ability to be controlled using electricity. Semiconductor lasers are used in a wide variety of consumer electronics, such as optical drives, as well as being used in fiber optic communications, where data is transmitted using the laser's light. Fiber optic communications transmit data by controlling the laser's output, where a high output (brighter light) represents a digital one, and a low output (dimmer light) represents a digital zero. Because semiconductor lasers can be directly controlled by changing the amount of current they receive, their output can easily be changed, allowing fast transfer of data. Despite their benefits, semiconductor lasers suffer from a drawback known as parasitic recombination. Parasitic recombination is a process that makes a significant portion of the current injected to generate useful light go to waste, which negatively impacts the laser's performance. One solution to parasitic recombination is the addition of asymmetric barrier layers (ABLs). By adding ABLs, parasitic recombination can be completely removed. In this work, several different cases of semiconductor quantum dot (QD) laser with ABLs are examined. Starting from a set of equations, the operating characteristics of the lasers in the different cases are found. First, the case of excited states is examined. The presence of excited states in semiconductor lasers impacts the rate that current can be converted to light, lowering their performance. By solving the starting rate equations, which describe the way different values change over time, the performance of the laser can be calculated. Specifically, the impact of several tunable parameters on the output power and modulation bandwidth are examined. The modulation bandwidth is how fast the laser output can be changed, which is equivalent to how fast data can be transmitted. Optimum values for the DC injection current, QD surface density (number of QDs per area), and laser cavity length are found.

A model was devised and used as the basis of a computer simulation to predict the shape of and physical phenomena in the keyhole during deep penetration laser welding. The shape of the weld cavity was determined as a part of the solution, and a convection-dominated vaporization model was utilized. Deep penetration welding is characterized by the formation of the keyhole. Beyond a certain threshold laser power, the laser beam rapidly evaporates material creating a strong back pressure, which pushes the molten material sideways forming a cavity. Hence, the laser power is effectively transferred to the bottom of the cavity and penetrates into the material until an energy balance is achieved around the keyhole. Around the keyhole three different regions (solid, liquid, and vapor) are analyzed, each region with its most suitable method. The heat transfer within the solid region is solved by Boundary Element Method. A thin layer approximation is made to simplify the analysis in the liquid region. A scaling analysis shows that fluid dynamics in the liquid region does not contribute significantly to the heat transfer in the liquid region. In the vapor region, a one-dimensional gas dynamic model is adopted from the literature. The solutions in the three regions are matched to satisfy conservation of mass at the liquid-vapor interface and of energy at the solid-liquid interface. Specifically, the matching technique of energy at the solid-liquid interface is called the matching scheme, and with it the shape of the solid-liquid interface is calculated. Then the shape of the liquid-vapor interface can readily be obtained from the shape of the solid-liquid interface and the thin liquid layer approximation. The matching scheme and the use of modules combine to make a model which is capable of predicting the shape of the solid-liquid interface; depth of penetration; surface temperature of the keyhole; pressure acting on the keyhole; energy distribution, such as the energy of vaporization, fusion, and conduction; and the thickness of the liquid layer. As a model material, pure iron was analyzed in this study. The calculated penetration depths are compared to empirical data, in order to verify the current study, and good agreement was observed.

No description available.

O Laser de Alta potência, também denominado, Laser Cirúrgico, tem sido usado em procedimentos médicos, para coagulação e vaporização dos tecidos. O maior problema, hoje encontrado, pela medicina no uso deste Laser, especialmente o laser de CO2, é a destruição térmica, causada pelo acúmulo de energia, convertida em calor, nos tecidos adjacentes, por ele irradiado. Uma vez absorvida esta energia, o tecido acumulará mudanças, que ocorrerão, durante o processo de irradiação e resfriamento. Estas variações poderão ir desde: uma mudança física, química e alterando completamente, os mecanismos biológicos, antes efetuados, pelo tecido irradiado. Este estudo tem como objetivo, analisar tais mudanças, verificando as diferenças morfológicas e geométricas, bem como utilizar experimentação para reconhecer, as zonas limitantes, ao processo de ablação. Para isto, quatro partes iguais de uma mandíbula bovina, foram usadas. Sendo que, cada uma destas, recebeu uma potência, diferente de laser. A cada amostra, foi aplicada a potência escolhida, em cinco diferentes tempos, mantendo uma distância aproximada de 250mm, entre uma aplicação e outra, com uma distância focal de 6mm. Em primeiro, observou-se característica macroscópica da região de interação, utilizando-se de um modelo zonal, para análise. Através de uma análise, mais aprofundada, com o uso de microscopia eletrônica, pode-se notar as variações imprevistas, ocorridas pelo processo de acúmulo e condução térmicos, advindos da ablação. A região de interação foi delineada em zonas de variações, denominadas as mesmas, como: Z I, Z2, e Z3. Sendo que a Z1, é o diâmetro formado pelo ponto de penetração da luz no tecido, ora irradiado.(Cratera). Z2 é o diâmetro envolvendo, a primeira camada de alteração do tecido irradiado, circundante a cratera. Z3, determina todo o diâmetro das alterações, por nós verificadas nas microfotografias. A partir deste ponto, usamos as amostras irradiadas, para calcular a quantidade de material removido, pela ablação. Através de um corte histológico, parafinado, analisamos todas profundidades, dos orifícios de entrada, formadas pela interação, por nós estudadas. Para cada lâmina, foi feita uma medida, da profundidade, formada pela entrada do raio, e pela material removido pela ablação. Tendo-se as medidas dos orifícios das crateras, as profundidades das mesmas, calculamos, aproximadamente, o material removido pela ablação. Considerando para isto, que os orifícios de penetrações, sendo, todos eles, de formatos de circunferências e as profundidades, em formatos Gaussianos. Obtendo, portanto, um formato conforme, para cada cratera. . Obtivemos, a partir do calculo matemático, o volume de massa de material retirado por ablação / The high power lasers are being widely used in surgical procedures, where there are vaporization and coagulation of the soft tissues. For hard tissues, such as, bones and teeth, pulsed lasers are stranded out because they may cause less thermal damage, as an example, the CO2 laser. Termal effects on tissue are recognized as being an accumulation of changes that occur during the entire period of irradiation and ensuing cooling (Mc KENZIE 1990). Located heating is affected by termal conductivity and convection by blood flow. Upon heating tissue experiences an alteration in a variety of physical, chemical and biological mechanism at the scale of chemical, organelle cell and tissue. Changes in tissue geometry and local microcirculation, could be affected. The purpose of these studies was to verify the physical and chemical variation and also reactions during the process of light exposure in the bone parts, especially the laser beam ablation patters. The generation of craters and other abnormal superficial termal damage, and craters that\'s show irregular wall contours. The relation and interaction between medicai CO2 laser beam and animal, human, and other biological tissues. For this experiment, a cattle\' s jaw, of four years old, were chosen for these experimental. Procedure to be described in this paper Those jaws were cut in four different pieces, in an effort, those parts were kept in the same shape and size, so no different variation of depth or length, would make it improper for the studies about to be made. Numbers were put in each different piece, showing in each sample, the time and the potency that were applied in that part. All samples were under favorable conditions (with long pulses and without water spray). Since classical pathology techniques for characterizing thermal damage to tissue are based on the microscopic examination of prepared tissue specimens for evidence of coagulation necrosis (nuclear psychoses, disintegration of organelles, hyalinization of collagen, loss of birefringence in muscle tissue, carbonization, etc.) Once the experimental appliance were done, a different colored circle were clear and optical visible around each bole, In a suggestion of that changes have happened in the material, used indeed. For almost, all the averages of the holes, in the electronic microscope, could be seeing the presence of minimal layer of superficial carbonization in the remaining bone. Therefore, the bone that received irradiation caused by CO2 laser, in a agreement with the proposal methodology, showed a variation of the tissue, that changed as the exposure changed the TIME/POWER. For each different pulse, the samples were analyzed. The tissues response to extreme temperature change in this zone usually consists of immediate coagulation necrosis, coagulation of vessels and macroscopic destruction of the tissue. An intermediary volume Between the necrosis core and the reversibly affected region is referred to as the Marginal Zone\". Therefore in this task, we called each analyzed zone as Z1 for the crater\'s diameter and Z2 for following areas, and finally, Z3 for the total diameter of the affected tissue. . All the measured zones, contains the shift from perfusion increases to coagulation, and the transition from structurally intact cells to dead cells. The most interest, issue that this study, may bring up, is for the clinical for the future\'s procedures because it contains the boundaries between the reversible and the irreversible tissue changes. Very few studies describe the presence and the address the consequences of the ablative aberrations, which can frequently and randomly happen during laser surgery. The knowledge of the different boundaries, of the affected areas, could predict severe impacts on the quality of the final surgical outcome, specially, when precision surgery techniques are required. The results of this discussed experiment shows how important it is to constantly and carefully observe, both the irradiated tissue\'s structure and the beam\'s broadening at the surface during the ablation.

No description available.

Low-cost and compact blue/green lasers are desired for applications such as micromachining, laser display, underwater communications, high-density optical data storage, and medical diagnostics recently. Therefore various type of blue and green lasers have been studied, especially for intracavity frequency doubling of the diode-pumped solid-state laser, which can generate high blue/green powers with long lifetime. Unlike the green laser, which is originated from a four-level laser, the blue laser belongs to a quasi-three-level system, so it is more difficult to develop the blue laser source. The purpose of this experiment is to develop a high power and high efficiency blue laser with simple structure so that it can easily be commercialized. Furthermore, it is a very cost-effective approach to develop such solid-state lasers, if the laser output characteristics can be estimated precisely using software simulation. We used an optics software GLAD to model our four-level and quasi-three-level Nd:YAG laser systems. In experiment, we developed a blue laser system with a cavity length of 4 mm and with 71.7 mW of blue output power. Besides, we have succeeded in using GLAD to model a Nd:YAG plane-parallel laser cavity, which has an optical conversion efficiency of 68%. Finally, we designed an effective lens duct with a coupling efficiency up to 80%, even if there is no AR coating on lens duct¡¦s input or output facets.

The application of electronic-resonance enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) for the detection of nitric oxide (NO) and acetylene (C2H2) is experimentally demonstrated and the effects of various parameters on the ERE CARS signal investigated. In addition, the detection of dipicolinic acid (DPA) using “normal” CARS is demonstrated. For NO detection, the frequency difference between a visible Raman pump beam and Stokes beam is tuned to a vibrational Q-branch Raman resonance of the No molecule to create a Raman polarization in the medium. The second pump beam is tuned into resonance with the rotational transitions in the (1,0) band of the A2Σ+-X2Π electronic transition at 236 nm, and the CARS signal is thus resonant with transitions in the (0,0) band. A NO gas cell was used for the experiment to detect NO at various pressure levels. A significant resonant enhancement of the NO CARS signal was observed and good agreement between calculated and experimental data was obtained. For C2H2 detection, ERE CARS experiments were performed in a roomtemperature gas cell using mixtures of 5000 ppm C2H2 in N2. Visible pump and Stokes beams were used, with the frequency difference between the pump and Stokes tuned to the 1974 cm-1 Ϡ2 Raman transition of C2H2. An ultraviolet probe beam with the wavelengths ranging from 232 nm to 242 nm is scattered from the induced Raman polarization to generate the ERE CARS signal. The effects of probe wavelength and pressure on signal generation are discussed. CARS was used to detect the 998 cm-1 vibrational Raman transition from a sample of polycrystalline DPA. The transition is the breathing ring vibration in the pyridine ring structure in the DPA molecule. The DPA 998 cm-1 transition is detected with excellent signal-to-noise ratio and the full-width-at-half-maximum is very narrow, approximately 4 cm-1.

This thesis is about the synthesis and characterization (electron microscopy and spectroscopy) of chromium oxide condensates prepared by a dynamic process of pulsed laser ablation (PLA) or pulsed laser ablation in liquid (PLAL) regarding the composition, size, shape, structure and internal stress of the condensates under the influence of laser parameters and dopant. Firstly, dense chromium oxide nanocondensates dissolved with ca. 4 atomic % Si, according to energy dispersive X-ray analysis, were fabricated by PLA on a clamped Cr/Si target in air purged with oxygen for a very rapid heating/cooling and hence pressure effect. Transmission electron microscopic observations indicated the predominant corundum-type Si4+:£\-Cr2O3 nanocondensates are hexagonal in shape with significant internal compressive stress, and the minor spinel-like Si4+:Cr3O4 nanocondensates are octahedral in shape with considerable tetragonal distortion. The predominant Si4+:£\-Cr2O3 condensates tended to coalesce over stepwise (0001) or lateral (1-102) surface to generate dislocations until parallel epitaxial relationship was exactly reached via a Brownian rotation process of the particles. X-ray diffraction indicated that the internal compressive stress was quite released for the coarsened/coalesced condensates. The laser ablation results in this part shed light on the condensation effect, as an alternative to a solidification process, on the formation of Cr-rich oxide particles on the surface of Cr4+ doped YAG fiber. In addition, the £\-Cr2O3 single crystal nanocondensates were fabricated by pulsed laser ablation in air purged with oxygen and characterized by analytical electron microscopy regarding shape dependent local internal stress of the anisotropic crystal. The nanocondensates formed predominantly as rhombohedra with well-developed {01-12} surfaces and occasionally hexagonal plate with thin {11-20} edges and blunt corners. Such nanocondensates showed Raman shift for the CrO6 polyhedra, indicating a local compressive stress up to 5 GPa on the average. Careful analysis of the lattice fringes revealed a local compressive stress (0.5% strain) at the thin edge of the hexagonal plates and a local tensile stress (0.3-1.0 % strain) near the relaxed {-1012}, {10-11} and (0001) surfaces of truncated rhombohedra. The combined effects of nanosize, capillarity force at sharp edge, and specific surface relaxation account for the retention of a local internal compressive stress built up in an anisotropic crystal during a very rapid heating-cooling process. Furthermore, amorphous chromium oxide nanocondensates were fabricated by energetic PLA on Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with a bimodal interspacing 0.259-0.266 and 0.355-0.371 nm which are close to that of specific lattice planes of the stable £\-type structure, i.e. (-1104) and (01-12) having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains (PBC), respectively. Such amorphous nanocondensates were observed in-situ to became more polymerized by forming (01-12)-like layers and then fully crystallized as £\-Cr2O3 for further (01-12)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient £\-Cr2O3 yet at higher frequencies due to an internal compressive stress up to ca. 4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase. Moreover, the chromium oxide condensates nearly spherical in shape ranging from 0.1 to 0.2 micron in diameter were fabricated by laser ablation on Cr target at a very high power density of 1.8¡Ñ1012 W/cm2 for a very rapid heating and cooling effect. Analytical electron microscopic observations of such spherical particulates revealed three types: (1) £\-Cr2O3 single crystal with (-1101), (-1012) and (1-210) facets, (2) spinel-like Cr3O4 polycrystals with spherulitic texture, i.e. a rather corrugated solidification front, (3) recrystallized Cr3O4 polycrystals derived from type 2 by radiant heating. The microstructure and phase difference among the particulates can be attributed to varied extent of supercooling under the influence of rather complicated Cr2+ solute trapping of the molten and solid phases in the Cr3O4-O pseudo-binary in vacuum. The chromium oxide condensates being spherical yet full of facets, with significant internal compressive stress up to ca. 3.4 GPa according to Raman shift, and with UV-absorbance close to violet light due to the presence of internal stress and/or Cr2+, may have potential optoelectronic and catalytic applications. Finally, analytical electron microscopic observations indicated that the chromium oxide nanocondensates fabricated by PLAL (in water) are predominantly dodecahedral Cr3O4 with varied extent of tetragonal (t-) distortion from the spinel (sp)-type following the Bain relationship [21-1]sp//[01-1]t; (011)sp//(100)t. The t-Cr3O4 nanocondensates have {101}-twining due to tetragonal distortion and/or a coalescence event. The additional x(200) and 2x(211) commensurate superstructures can be attributed to periodic presence of Cr2+, Cr3+ and/or H+ in the 4- and/or 6-coordinated lattice sites with an internal compressive stress up to ~ 5 GPa according to X-ray photoelectron and vibrational spectroscopic evidences. The presence of internal stress and Cr2+ ion caused red shift of the UV absorbance, thus shedding light on potential optoelectronic applications of the Cr3O4 nanocondensates.

Thesis (Ph. D.)--University of California, San Diego and San Diego State University, 2004. / Vita. Includes bibliographical references (leaves 192-200).

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.