High-speed transient effects in quarternary lasers

Single-grain Silicon Technology for Large Area X-ray Imaging

The Time Dependent Schrödinger Equation In A Strong Laser Field

Low-threshold GaAs/AlGaAs lasers have, for the first time, been grown selectively on 10 μm stripe openings patterned in oxide on Si substrates. Lateral current confinement provided by side facets reduces edge leakage, and results in threshold currents as low as 75 mA for a 10 μm by 210 μm device, a nearly two-fold improvement over comparable etched ridge waveguide lasers. Spectrum measurements show single longitudinal mode emission near 850 nm. This adaptation of selective heteroepitaxial growth for lateral current confinement of AlGaAs/GaAs lasers on Si substrates, adopted from similar work on GaAs substrates, offers potential for significant threshold current reductions of lasers integrable with Si.

Over the last two decades III-nitride optoelectronic devices have experienced an impressive evolution in terms of performance. However, their potential is far from being fully exploited. Although they offer bandgaps from the deep UV to the infrared spectral range, lasing is currently restricted to the 336?536 nm wavelength region. Several challenges still need to be overcome. For example, the thermal stability of the InGaN active region becomes an issue in long-wavelength (? > 500 nm) laser diodes: high indium content (In > 25%) quantum wells (QWs) have been shown to markedly degrade when kept above 900 °C. For this reason, the growth temperature of the subsequent p-type waveguide and cladding layers must be sufficiently low to avoid QW degradation, which is accompanied by a reduction in the internal quantum efficiency. Thanks to the high vacuum environment, molecular beam epitaxy (MBE) allows growing these layers at a much lower temperature compared to metalorganic vapor phase epitaxy (MOVPE), and could be used to reduce the thermal budget on the active region, further extending the wavelength range. Thus the aim of this thesis is the realization of long wavelength laser diodes by combining high efficiency active regions grown by MOVPE with p-doped layers grown by ammonia-MBE. Hence, the p-type doping by ammonia-MBE is first scrupulously studied and optimized on 500 nm thick (Al)GaN layers. In order to comprehensively address the physical processes governing the doping efficiency, the role of the growth temperature, the V-III ratio, and the growth rate is investigated. In addition, the Mg cell configuration resulting in the best doping profile control and reproducibility is studied as well. Electrochemical capacitance-voltage profiling is extensively used to study the compensation mechanisms. The optimization leads to the achievement of layers exhibiting low dopant compensation (below 5%) up to high doping levels and resistivity values as low as 0.4 Ohm­ cm. These layers are then implemented on top of active regions grown by MOVPE and lasing is obtained for the first time in hybrid structures exhibiting state-of-the-art electrical characteristics. The threshold voltage is as low as 4.3 V in an index guided structure with 800x2 ?m² ridge dimension designed for violet emission (? = 400 nm). These excellent current-voltage characteristics are achieved owing to the doping profile control in the cladding and contact layers which is confirmed by the low contact resistance value of 5E-4­ cm². Lasing is also demonstrated beyond 500 nm, keeping state-of-the-art electrical characteristics, despite the reduced doping in the cladding layer. These devices also exhibit values for internal optical losses as low as 6 cm?1. The growth of tunnel junctions is also attempted. Low specific resistance values are demonstrated in ammonia-MBE grown GaN tunnel homojunctions. The absence of hydrogen passivation and the reduced memory effects of MBE are considered to be crucial for the realization of these devices together with the stability of the doping at reduced growth temperatures. n-p-n structures featuring a tunnel junction in series with a p-n junction are then used to spread the current across light-emitting diodes. A device architecture featuring a buried tunnel junction to be used for current injection inmicro-LEDs or vertical cavity surface emitting lasers is also demonstrated and excellent current confinement properties are obtained.

In this thesis, nanometer scale semiconductor lasers and micrometer scale polymer based dye lasers are our focus in bringing the miniaturized lasers to applications in data transmission; ultra-small chemical / biological sensors; and ultra-compact spectroscopic sources. Combining the advantage of electrically driven semiconductor lasers and the advantage of a broad emission spectrum of dye molecules would utilize the highly dense multi-functional lab-on-a-chip by integrating microfluidic PCR, microfluidic fluorescent detection system, and compact visible and NIR detectors which are commercially available. On the other hand, in the meantime of pushing the size limit of the laser cavities, new phenomena with the nanoscale lasers enable further exploration and understanding in fundamental physics. In the first part of this thesis, two sub-micron scale semiconductor lasers are presented. The smallest lasers utilizing the disk structures—with diameters of approximately 600 nm—were realized in the InGaP/InGaAlP quantum well material system at room temperature, featuring ultra-small mode volumes of approximately 0.03 cubic?microns, and exhibiting single-mode operation at low threshold powers. And the first visible photonic crystal ultra-small mode volume lasers, with cavity volumes of approximately 0.01 cubic?microns, are realized in the same material system. They are ideally suited for use as spectroscopic sources and both of them can be lithographically tuned from 650 – 690 nm. In the second part of this thesis, two sub-millimeter-scale polymer-based dye lasers—a poly(dimethylsiloxane) (PDMS)-based mechanically tunable DFB dye laser and a poly(methylmethacrylate) (PMMA)-based second-order circular grating distributed feedback dye laser—are presented. Both of them are compatible with microfluidic technology, which gives freedom in integrating the lasers with the microfluidic chips. Compared to the soft lithography used in the PDMS-based dye laser, the nanoimprint lithography used in the PMMA-based dye laser would be more useful for fabricating ultra-small dye lasers and enabling mass production in the near future. At the end of the thesis, a nano-linewidth metal grating mask pattern transferred transient grating (MPT-TG) technique is described as a potential technique using the ultra-small lasers for molecular-dynamics study in solutions.

Rosacea is a common skin disease clinically characterized by skin flushing, persistent erythema, papules/pustules and telangiectasia. Laser therapy is an ideal option for the treatment of rosacea-associated telangiectasia and rhinophyma. The most commonly used lasers include 585-/595-nm pulsed dye lasers, long-pulsed 1 064-nm neodymium:yttrium-aluminum-garnet (Nd:YAG) laser, carbon dioxide (CO2) laser, erbium:yttrium-aluminum-garnet (Er:YAG) laser, fractional laser, and so on. Intense pulsed light and pulsed dye laser have shown favorable therapeutic effects on rosacea-associated erythema and telangiectasia with high safety and short downtime. Long-pulsed Nd:YAG laser can only be used to target wide blood vessels and to treat resistant telangiectasia. CO2 laser, Er:YAG laser and fractional laser can be used to improve rhinophyma. In clinic, combined treatment using multiple lasers may result in better therapeutic outcomes. Key words: Rhinophyma; Lasers; Lasers, dye; Therapeutic uses; Intense pulsed light

Laser treatment has become a popular method for resolving peri-implantitis, but the full range of its effects on implant surfaces is unknown. The purpose of the present investigation was to analyze the influence of different clinically applicable erbium:yttrium-aluminum-garnet (Er:YAG), carbon dioxide (CO2), and diode laser parameters on titanium surfaces that were either polished or sandblasted, large-grit, acid-etched (SLA). Six polished and six SLA titanium disks were irradiated at nine different power settings (n = 54 polished, 54 SLA) with Er:YAG, CO2, or diode lasers. The CO2 and diode lasers were used in continuous wave mode, and the Er:YAG laser was used in a pulsed manner. The surface of each disk was analyzed by scanning electron microscopy and confocal white light microscopy. Each disk was irradiated on six circular areas of 5 mm in diameter with the same specific laser setting for 10 seconds. Within the chosen parameters, the CO2 and diode laser did not cause any visible surface alterations on either the polished or SLA disks. In contrast, both polished and SLA disks showed surface alterations when irradiated with the pulsed Er:YAG laser. The SLA surfaces showed alteration after 10 seconds of irradiation with Er:YAG laser at 300 mJ/10 Hz. The surfaces of the polished disks did not show alteration with the Er:YAG laser until they were irradiated at the higher energy of 500 mJ/10 Hz for 10 seconds. The results of confocal white light microscopy were in agreement with scanning electron micrographs. In contrast to continuous-wave diode and CO2 laser irradiation, pulsed Er:YAG laser irradiation caused distinct alterations with power settings beyond 300 mJ/10 Hz on the SLA surface and 500 mJ/10 Hz on the polished surface. Thus, it is only safe to use the Er:YAG laser for implant surface irradiation with settings no higher than 300 or 500 mJ/10 Hz.

Strain-compensated InGaAsSb-AlGaAsSb quantum-well (QW) lasers emitting near 2.5 /spl mu/m have been grown by solid-source molecular beam epitaxy. The relatively high arsenic composition causing a tensile strain in the Al/sub 0.25/GaAs/sub 0.08/Sb barriers lowers the valence band edge and the hole energy level, leading to an increased hole confinement and improved laser performance. A 60% external differential efficiency in pulsed mode was achieved for 1000-/spl mu/m-long lasers emitting at 2.43 /spl mu/m. A characteristic temperature T/sub 0/ as high as 163 K and a lasing-wavelength temperature dependence of 1.02 nm//spl deg/C were obtained at room temperature. For 2000 /spl times/ 200 /spl mu/m/sup 2/ broad-area three-QW lasers without lateral current confinement, a low pulsed threshold of 275 A/cm/sup 2/ was measured.

We report electrically pumped λ = 4 μm vertical-cavity surface-emitting lasers (VCSELs) that significantly extend the wavelength range covered with electrically pumped VCSELs into the mid infrared. A single stage active region with eight type-II quantum wells provides sufficient gain, while lateral current confinement and waveguiding are accomplished with the proven buried tunnel junction technology. These devices operate in continuous wave (CW) up to −7 °C and in pulsed operation up to 45 °C. Their CW threshold pump power levels are below 10 mW at temperatures well accessible by thermo-electric cooling, and their maximum single-mode output power is around 0.18 mW. Single-mode operation with side-mode suppression ratio of more than 20 dB and a continuous electro-thermal tuning range as large as 19.2 nm are achieved.

Objective To preliminarily optimize the energy and density of laser radiation for the early treatment of hyperplastic scars in a rabbit ear model, and to explore possible therapeutic mechanisms. Methods Sixty-one hyperplastic scars were successfully established on the ears of 10 healthy New Zealand white rabbits with large ears, and randomly divided into 2 groups: 1-week group (30 scars) and 3-week group (31 scars) . These 2 groups were separately divided into 5 subgroups: group A treated with laser at a density of 100 PPA and an energy of 10 mJ, group B with laser at a density of 100 PPA and an energy of 50 mJ, group C with laser at a density of 169 PPA and an energy of 10 mJ, group D with laser at a density of 169 PPA and an energy of 50 mJ, and group E receiving no treatment. There were 6 scars in each group, except the group E in the 3-week group. Two healthy New Zealand white rabbits with large ears were not subjected to modeling, and served as group F (blank control group) . Immunohistochemical study was performed to determine the expression of matrix metalloproteinase (MMP) -13 in the skin tissues from the rabbit ears 1 week after the treatment. Three weeks after the treatment, the skin tissues from the rabbit ears were subjected to hematoxylin-eosin (HE) staining and Masson staining. Then, the structure of scars was observed, and scar elevation index was calculated. Statistical analysis was carried out by Kruskal-Wallis H test for the comparison of scar elevation index, and by one-way analysis of variance (ANOVA) for the comparison of the average absorbance value of MMP-13. Results As HE staining revealed, the groups A, B, C and D all showed thicker dermis and increased number of collagen fibers compared with the group F (normal skin tissues) , but showed thinner dermis, decreased number and more ordered arrangement of collagen fibers compared with the group E (untreated scar tissues) . No obvious difference was observed in the thickness of the dermis among the groups A, B, C and D. The scar elevation index significantly differed among the 6 groups (H= 22.757, P 0.05) . Conclusions Non-ablative fractional laser is effective for the treatment of early-stage hyperplastic scars. At the same laser density, 50-mJ laser was superior to 10-mJ laser for the treatment of hyperplastic scars, likely because high-energy laser can stimulate the recombi-nation of extracellular matrices and up-regulated MMP-13 expression to a greater extent. Key words: Cicatrix, hypertrophic; Lasers; Matrix metalloproteinase 13; Fractional laser; Energy

Selective Laser Sintering (SLS) and Selective Laser Melting (SLM) are parent Additive Manufacturing processes using a laser to solidify metallic, ceramic, polymer or composite powders. During the process, the object is built layer by layer. A laser source is responsible for the consolidation, by local heating. The light is deviated by a scanning head according to the instructions of an STL file. Then partial fusion of the particles takes place, followed by a solidification of the liquid created. Kinetics of these steps are very high and play an important role in the final microstructure (rearrangement of the particles, pore creation, residual stresses). In the case of polymers and composites the technique is now well understood and widely used, but for metals and ceramics it suffers from a lack of precision, surface roughness and poor mechanical properties. The goal of this Ph.D. work is to understand the effect of the thermal gradients on the consolidation process, using different laser parameters (power, pulse frequency, scan speed,…). In SLS, microscopic thermal gradients arise, due to the fact that the laser is pulsed and that only the exterior of the powder particles is molten. A thermal model to describe the interaction between a laser beam and a (spherical) grain is proposed. This model allows for the incorporation of the latent heat of fusion and for a realistic surrounding. The absorbed laser intensity is modeled by means of the Mie theory. Experiments where two particles of powder are isolated and illuminated by the laser are carried out in order to measure the interparticular necks and the volume of liquid formed for different repetition rates. The thermal model leads to good predictions of the particles final sintering state. Fluid flow models are investigated in order to determine the dynamic of the molten liquid. The main issue is to explain the capillary flow mechanisms leading from the molten material to the neck formation. Two models are derived to simulate the fluid flow between the particles. The first one simulates a capillary flow between two parallel plates. The second one deals with energetic considerations arising from Frenkel's principle. In both cases, the final output is the liquid life-time necessary to get the interparticular neck lengths experimentally observed. We point out that the two models predict the same liquid lifetime, although they are derived with different hypotheses. In SLM, continuous lasers are used and one has to deal with macroscopic thermal gradients, since the particles are completely molten. A very high laser power is used and the negative thermal effects (like thermal stresses or balling effects) are important. A solution to control them is to adjust the scanning strategy. Four scanning strategies are investigated for material with different thermal conductivities. The three-dimensional model used to describe the laser-matter interactions and the temperature evolution of the scanned powder bed allows for a finite latent heat (Stefan-problem) and for conductivity modifications due to the consolidation. We show that this finite element thermal model can be efficiently used to anticipate most of the problems (like cracks or balling) arising in practice. The benefit of avoiding thermal gradients is shown, in particular by EBSD analysis. Finally, applications of the SLS/SLM technique on different pieces built during this work are shown. A new way to build support structure is also proposed.

Objective To investigate the dose-effect relationship between Q-switch ruby laser and skin pigmentation in Guinea pigs. Methods Ten Guinea pigs were included in this study, and the back of each pig was divided into 5 areas, negative control area receiving no UVB irradiation or laser treatment, positive control area receiving UVB irradiation but no laser treatment, low-, middle- and high-dose areas receiving UVB irradiation followed by the treatment with laser at an energy density of 1.5, 3.0 and 6.0 J/cm2 for 4, 2, and 1 session, respectively, with an interval of 1 week. Melanin content index (MCI) was determined, and confocal laser scanning microscopy (CLSM) and histopathology were performed to observe melanin granules and melanocytes in the skin tissue from Guinea pigs before the initial laser treatment, 2 and 4 weeks after the final laser treatment. Results The best response was observed in the low-dose areas, and there was a complete clearance of hyperpigmentation in these areas, which maintained 8 weeks without recurrence, while different degrees of skin pigmentation recurred in the middle- and high-dose areas. Moreover, the low-dose areas showed a significantly lower melanin content index (MCI) compared with the middle- and high-dose areas (F = 4.2,P < 0.05), together with a marked reduction in the extent of pigment deposition, melanin granules, melanocyte number and density. Conclusions The efficacy of Q-switched ruby laser on skin pigmentation in Guinea pigs is correlated with the energy density and treatment frequency, and, low intensity and high frequency are often associated with a better treatment outcome. Key words: Lasers; Skin pigmentation; Dose-response relationship, radiation; Guinea pigs

A experimental study on short pulse generation from praseodymium-doped fluoride mode-locked fibre lasers and gain-switched DFB semiconductor laser is presented[l]. The fiber lasers were modelocked either by using a phase modulator (PM) or cross phase modulation (XPM). In the first case, pulses of 33 ps and 75 mW (peak power) at 419 MHz were obtained. In the second case, we used a 1.55 µn gain-switched diode laser to modulate the phase of the 1.3 um laser field via intracavity XPM. This produced 50 ps, 13 mW pulses at 2.8 GHz. In this second laser we also observed stable dark pulses with 140 to 220 ps duration, making this laser attractive as a dark soliton generator. In the case of the gain-switched (GS) diode laser, we obtained (after spectral filtering) 5 ps, 20 mW at 1 GHz. Using this GS laser we propagated solitons in 50 km of standard fibre, thus demonstrating its potentials as a simple and inexpensive soliton quality laser source.

Accurate focus positioning of a laser beam is an important basis for improving laser engraving, cutting and welding accuracy, but the traditional methods are not fit for automatically finding high-power laser focuses. The plasma caused with laser ablating metal is able to emit a large amount of ultraviolet radiation. Therefore, in this paper a focus automatic positioning method for high-power infrared lasers is proposed and the relevant device is fabricated with a GaN Schottky photodiode as sensor and a 304 stainless-steel laser target as consumable material. As a contrast experiment, the depths of the laser ablation pits on the stainless-steel target are measured with a confocal microscope to find the deepest pits. The focal position difference is 24 m between the two positioning methods, when the experiments using a 1064-nm-wavelength fiber laser engraving machine with a pulse width of 100 ns, are petition frequency of 20 kHz and an average power of 10 W.

Laser fusion, likely the ultimate solution to the crisis of human energy, is highly valued by the international community and has always been the focus of international research. It turns out that the biggest scientific obstacle of laser fusion is the effective control of the high-energy-density nonlinear flows during implosions. The research of high-energy-density nonlinear flows covers many different fields, such as high-energy-density physics, plasma physics, fluid mechanics, computing science, strong impact physics, and high pressure atomic physics. Meanwhile, the capability of multi-material and multi-scale numerical simulations as well as large laser facility with high output power is also needed. As an emerging research field, it is full of all kinds of novel phenomena to be explored. In addition, hydrodynamic instabilities and the subsequent turbulent mixing in high-energy-density flows, are also important processes in astrophysical phenomena (e.g., galaxy collision and merging, stellar evolution, formation of protostars and supernova explosion) and involve with the core content of astrophysics. This paper reviews, firstly the status and progress, as well as the challenges and opportunities of high-energy-density nonlinear flows research. Secondly, it introduces hydrodynamic instabilities during implosions in central ignition laser fusion, among which, key factors related to the bottleneck of implosion performance of the National Ignition Facility (NIF) in the United States are condensed. Next, it summarizes the development of hydrodynamic instability experiments in laser fusion abroad. Finally, it lists some key achievements on the fundamental issues of hydrodynamic instabilities by the laser fusion implosion physics team in China over the last three years. This team has been engaged in the research and control of nonlinear flows in laser fusion implosions, as well as the research and design of target physics. A lot of improvements have been made in recent years on the theoretical analysis and numerical simulation of outstanding issues for hydrodynamic instabilities in laser fusion implosions, and the design and analysis of experiments on large lasers, which greatly promoted the development of this research direction in China.