Browsing by Subject "Laser"

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A bright source of cold atoms
(2019-05-03) Lu, Yu, Ph. D.; Raizen, Mark G.; Niu, Qian; Hegelich, Bjorn; Caceres, Elena; Bajaj, Chandrajit
This dissertation describes a general approach of generating a high flux of cold atoms that can be confined in a magneto-optical trap. As an alternative to the state-of-the-art laser cooling method, this approach widens our ability to cool and control atoms without relying on a specific atomic transition and availability of laser. In this dissertation I will discuss the design, construction of the experiment and characterization of a pulsed cold atom source in detail. This work is based on a new paradigm: entrainment of atoms in the carrier gas of a supersonic beam, followed by the magnetic deceleration and trapping. Our methodology is based on the supersonic beam created by the expansion of a dense carrier gas from a pulsed release of gas through a small aperture. Cold noble gas emerging from this pulsed high pressure supersonic nozzle acts as a carrier, into which atoms of interest are then entrained. In our studies with the lithium atoms, up to 10¹¹ atoms per pulse could be entrained into a supersonic beam of helium at a translation temperature of below 100 mK. The supersonic valve is typically operated at a rate of below 1 Hz in the experiment. A much larger flux can be achieved at a higher repetition rate. These atoms are moving at a speed of 500 ms⁻¹ and need to be decelerated almost to a complete stop, in order to be trapped and cooled to quantum degeneracy. A 2.5-meter long moving-trap magnetic decelerator with 480 coil pairs was built and characterized. Atoms moving at a speed around 500 ms⁻¹ were trapped and decelerated to various final velocities ranging from 400 ms⁻¹ to 50 ms⁻¹, at a resulting temperature of 30 mK and a flux of 10⁸ ~ 10⁹ atoms per pulse. This whole process takes place within only 10 ms, at a repetition rate of 100 Hz 10¹⁰ ~ 10¹¹ atoms can be delivered per second. This approach is very general compared to the laser cooling, since most atoms in the periodic table have a magnetic moment in their ground state or can be optically pumped to a long-lived para-magnetic metastable state. In this dissertation, I show the working principle of each component of our experiment and characterize the atom beam at multiple stages. I describe in detail the construction and optimization of our magnetic decelerator, and demonstrate the control and monitoring system with the experiment results. Also implemented in this work is a flexible laser system that is composed of a reference diode laser and two tapered amplifiers to control and probe the internal states of atoms, as well as provide the trapping force. I also explore the optical molasses and chirped cooling techniques which help load the atoms into a magneto-optical trap. The successful demonstration of this method of creating a cold atom source leads us to believe that the magnetically decelerated supersonic atoms will play an important role in the area of cold atom physics
Beam diagnostics for the Texas Petawatt Laser Wakefield Acceleration Project
(2009-12) Bedacht, Stefan; Downer, Michael Coffin; Fink, Manfred
An overview of the beam diagnostics for the laser wakefield acceleration project at the Texas Petawatt Laser facility is presented. In this experiment, short and intense laser pulses of 165 fs and up to 190 J will be used to accelerate electrons up to the GeV energy range using laser wakefield acceleration. The density variation of the plasma generated in a helium gas cell will be measured with different optical detection systems such as frequency domain holography. Spectra of the transmitted laser beam and optical transition radiation will yield information about the energy transfer to the plasma and the energy of the electrons, respectively. In addition, a calorimeter will measure accelerated electron energies. Prior to the final experiment, preliminary frequency shift measurements and simulations on optical transition radiation were performed.
Blood shear stress and flow velocity : challenges for laser coagulation and hemostasis of large vessels
(2019-06-20) Katta, Nitesh; Milner, Thomas E.
Photo-coagulation of blood vessels offers stark advantages to current radiofrequency mediated devices, given the high specificity of blood absorption wavelengths (like 532nm, 1.064 μm). Existing literature has documented coagulation in treatment of pediatric vascular diseases like port-wine stains for microvascular hemostasis. Although laser treatments have been successful in smaller diameter blood vessels, treatment of larger sized vessels are less effective in achieving hemostasis. This report investigates the hypothesis that a primary limitation in coagulation of larger sized vessels comes from shear stress gradients with higher flow velocities along with temperature dependent viscosity changes. Laser coagulation at the 1.07 μm wavelength is tested in the chicken chorio-allantoic membrane (CAM) model highlighting limitations reported in the archival literature. COMSOL based finite element models are presented which includes hypothetical limitations in coagulation during laser irradiation. Laser fluence rate and dosimetry are adjusted based on experimental OCT imaging and angiography observations as well as COMSOL models to test the hypothesis that blood shear stress and flow velocity are important challenges for laser coagulation and hemostasis of large vessels
Characteristics of foamed asphalt binders for warm mix asphalt applications
(2014-08) Arega, Zelalem Alebel; Bhasin, Amit; Li, Wei (Of University of Texas at Austin); Prozzi, Jorge A; Zhang, Zhanmin; Juenger, Maria G
An increase in environmental awareness and energy concerns had recently prompted efforts to make pavement construction cheaper and more environmentally friendly. Warm mix asphalt (WMA) is an asphalt mixture production technology that promises to reduce production costs and greenhouse gas emissions. Foamed asphalt binder is increasingly being used to produce WMA. This dissertation addresses several issues related to the use of foamed asphalt binder for WMA applications. The first objective of the research presented in this dissertation is to develop a method and metrics to precisely quantify the characteristics of asphalt binder foams. Laboratory measurements were obtained using the newly developed method to evaluate the extent and stability of foams produced using different asphalt binders at different water contents and laboratory foaming devices. Results demonstrate that the method developed is promising in terms of its ability to provide a detailed history of the behavior of foamed asphalt binder as the foam collapses. In addition, results indicate that the method is sensitive to distinguish between foaming characteristics of different asphalt binders as well as different water contents and foaming devices. The second objective of this study was to relate intrinsic properties of the asphalt binder to its foaming characteristics. A physical model was developed for expansion of asphalt binder foam based on foam physics and fluid mechanics of micro-droplets. The model relates foamant water and asphalt binder mixing efficiency with the surface tension of the asphalt binder. The model can be used to predict which binder can be effectively foamed and used, and whether any chemical modification to the binder is necessary to achieve the same. Results indicate that only a small percentage of water is effective in foaming the asphalt binder. The last objective of this research was to evaluate the influence of foaming on asphalt binder residues and mixture workability and coatability. The influence of foaming process on the rheological properties of asphalt binder residue was investigated. In addition, the significance of foamed asphalt binder characteristics on mixture workability and coatability was evaluated. Results from this last part of the study can be used to optimize binder foaming such that the resulting mixture is coated and compacted without compromising performance.
Chirped pulse raman amplifier
(2009-12) Grigsby, Franklin Bhogaraju; Downer, Michael C., 1954-; Frommhold, Lothar W.; Becker, Michael F.; Keto, John W.; Sitz, Greg O.
All modern terawatt- and petawatt-class laser systems are based on the principle of chirped-pulse amplification (CPA). In this work, a compact subsystem that shifts a micro-joule portion of the chirped pulse energy to a new wavelength outside its original bandwidth, then amplifies it to millijoule energy without adding pump lasers, and without compromising the output of the fundamental CPA system in any significant way, has been developed and integrated into a standard terawatt-class CPA system. In this chirped pulse Raman amplifier sub-system, a 30 mJ portion of a chirped 800 nm fundamental pulse within the CPA system was split into two unequal portions, each of which impinged on a Raman-active barium nitrate, or Ba(NO3)2, crystal of length 5 cm. The weaker portion created a weak (15 J) first Stokes pulse (873 nm) by Stimulated Raman Scattering (SRS) in the first crystal, which then seeded a non-collinear four-wave-mixing process driven by the stronger portion of the split-off CPA pulse in the second crystal. The latter process amplified the first Stokes seed pulse to several millijoules with excellent beam quality. A study of Raman gain as a function of time delay between pump and Stokes pulse in the second crystal revealed a sharply peaked narrow interval ( 3 ps FWHM) of high gain and a wider interval ( 50 ps) of low gain. The amplified, chirped first Stokes pulse was successfully compressed to 100 fs duration using a grating pair of different line density than in the main CPA system, based on a comprehensive dispersion analysis of the optical path of the first Stokes pulse. The possibility of generating higher-order Stokes and anti-Stokes sidebands of the CPA pulse is also demonstrated. Further amplification of the sideband pulse by conventional methods, using an additional pump laser, appears straightforward. The chirped pulse Raman amplifier provides temporally synchronized fundamental and Raman sideband pulses for performing two-color, high-intensity laser experiments, some of which are briefly discussed. It can be integrated into any standard CPA system, and provides significant new versatility for high-intensity laser sources.
Effect of electrical conductivity of pure and doped lanthanum chromite on the onset of selective laser flash sintering
(2021-05-10) Matto González, Lezli Giselle; Kovar, Desiderio
Selective laser flash sintering (SLFS) utilizes a large electric field applied during laser scanning to selectively partially sinter ceramics. This technique has previously been demonstrated in aluminum nitride and yttria-stabilized zirconia and is promising because it opens the possibility of binder-free additive manufacturing of ceramics. The purpose of this research is to study the effect of the electrical conductivity of ceramics on the initiation of SLFS. The materials chosen for this work were lanthanum chromite (LC), an intrinsic electronic conductor, and lanthanum strontium chromite (LSC), a ceramic that has even higher electronic conductivity due to doping. The results obtained show that for SLFS to initiate in these materials there is a critical applied electric field that is two order of magnitude smaller than for YSZ and AlN, and that a minimum laser power of only 3 W for LC and 4 W for LSC is required for SLFS to initiate. This is again significantly smaller than what has previously been observed for YSZ and AlN. The unexpected lower laser power needed to initiate SLFS in LC compared to LSC could be due to microstructural characteristics of the powders that influence their packing density and provide more conductive paths in the pressed pellets. Studies of successive scans show that there is no significant history effect from previously parallel scanned lines on the current response of the successive lines which confirms that the effects of SLFS are localized to the near-scan regions. Varying scan directions also does not significantly affect the current measurement during SLFS in these materials. Results from partial scans that end on the negative electrode suggest that the charge carriers may follow the hot region under the scanning laser to the negative electrode, where the current measurement is obtained. Higher measured current for full scan lines compared to partial scan lines support the hypothesis that a combination of electrons produced at the positive electrode, temperature-activated intrinsic charge carriers, and extrinsic charge carriers present in LSC due to doping are the responsible for the relatively large current measured during SLFS of LSC even under modest laser powers and electric fields.
The effect of laser contrast and target thickness on laser-plasma interactions at the Texas Petawatt
(2014-12) Meadows, Alexander Ross; Hegelich, Bjorn Manuel
A two-year experimental campaign is described during which diamond-like carbon and plastic targets with thicknesses from 20 nanometers to 15 micrometers were irradiated by the Texas Petawatt Laser. Target composition and thickness were varied to modify the specifics of the laser-matter interaction. Plasma mirrors were selectively implemented to affect the contrast of the laser system and provide additional control of the physical processes under investigation. A number of particle diagnostics were implemented to measure the distribution of laser accelerated ions and electrons. In addition, optical diagnostics were fielded to measure the intensity profile of the laser and measure the density of the target pre-plasma. The results of these experiments suggest that the Texas Petawatt laser pulse has pre-pulse and pedestal features with intensities at least 10⁻⁸ of the main pulse. Micronscale targets were able to survive these features and maintain a relatively sharp density gradient until the arrival of the main laser pulse, allowing for ion acceleration. Electron spectra measured in this configuration show an average temperature of 10 MeV, with no v angular dependence out to at least 60 degrees. By contrast, interferometric plasma density measurements and a lack of any observable ion acceleration suggest that nanoscale targets were destroyed well before the main pulse. In this case, the peak of the laser pulse interacted with a cloud of plasma between 10⁻³ and 10⁻² of critical density. The contrast improvement offered by the implementation of plasma mirrors was seen to increase the maximum energy of laser accelerated protons from targets thicker than 1 micrometer. In addition, the plasma mirrors allowed nanoscale targets to survive pre-pulse and pedestal features and support the production of ion beams. Proton spectra show that ions were accelerated to greater maximum energies from nanoscale targets than from more traditional micron-scale targets. This effect can be attributed to a reduction in the target pre-plasma scale length upon the introduction of plasma mirrors. These results indicate that the manipulation of target properties and laser contrast can significantly affect the interaction between an ultrahigh intensity laser and a target.
An electrostatic approach for producing nanoparticulate membranes using laser ablation of microparticle aerosols
(2011-08) Davis, Claire Elisabeth; Kovar, Desiderio; Becker, Michael F.
The Laser Ablation of Microparticle Aerosols (LAMA) process produces nanoparticles by ablating microparticles that are entrained in an aerosol. Two of the main advantages of this process are that the particles produced are charged (preventing agglomeration) and bare (without a capping layer). Two different techniques are possible to collect the nanoparticles. In this work, the charged state of the particles formed was utilized to collect them electrostatically. This approach has the additional advantage that particles can be selected according to their size. The focus here was a particular application for gas separation. The nanoparticles produced were directly collected in a polymeric liquid, which was then irradiated with ultraviolet light to form a rubbery film. These membranes were tested for olefin/paraffin gas separation, a challenge that finds many applications, notably in the petroleum industry.
Experimental studies of laser driven proton acceleration from ultrashort and highly intense laser pulse interaction with overdense plasma
(2014-12) Kuk, Donghoon; Ditmire, Todd R.
The generation of high current multi-MeV protons and ions by irradiation of short pulse high intense laser on an ultra-thin target has been observed and subjected great interest in recent. When ultra-thin overdense target is irradiated by focused ultraintense laser pulse, hot electrons are generated by various mechanisms and they generate energetic ion beams. In TNSA, a quasi-electrostatic field is produced on the target rear surface when the the laser pulse interacts with overdense target, driving hot electrons go torward the target rear surface. However, this mechanism results in a range of field gradients leading to a broad proton energy distribution typically. To overcome the issue, an alternative accelration mechanism has been presented to achieve the quasi-monoenergetic proton acceleration and the mechanism is called Radiation Pressure Acceleration. In the RPA, the radiation pressure push electrons into the target smoothly and setting up an electrostatic field by the laser pressure. In this thesis, we study two alternative experimental methods for the quasi-monoenergetic proton acceleration and find experimental feasibility of the presented methods from other research groups.
Fire retardant polyamide 11 nanocomposites/elastomer blends for selective laser sintering
(2016-05) Ortiz, Rogelio; Bourell, David Lee; Koo, Joseph H.
Additive manufacturing (AM) had previously been used solely for prototyping and visualization purposes, but in recent years, this technique has shifted to the idea of producing end-use parts. This has already been successfully done in some areas via selective laser sintering (SLS). Unfortunately, current polymeric materials for processing via SLS do not meet the requirements of the majority of commercial applications. Hence, this thesis presents efforts to develop a multifunctional polyamide 11 (PA11) polymer with enhanced thermal, mechanical, and flammability properties for SLS through the use of nanotechnology.
Generation, measurement and application of x-rays from laser-plasma electron accelerators
(2021-12-03) Hannasch, Andrea Jeanne; Downer, Michael Coffin; Ditmire, Todd; Paban, Sonia; Ketcham, Richard; Cowan, Tom
This dissertation presents a comprehensive study of the generation mechanisms, diagnostic techniques and possible applications of few keV to 100 MeV x-rays generated by laser wakefield electron accelerators. Chapters 1-3 review the principles of x-ray science and laser wakefield acceleration, and 3 mechanisms by which laser wakefield accelerators produce x-rays: 1) betatron oscillations of the electrons while still accelerating; 2) inverse Compton scatter (ICS) x-rays involving electron oscillations induced when electrons collide with a counter-propagating laser pulse after exiting the accelerator; 3) bremsstrahlung from the impact of accelerated electrons with a solid target. Chapters 4-6 then present original, recently published work, starting in chapter 4 with experiments that characterized secondary x-rays from a laser wakefield accelerator at Helmholtz-Zentrum Dresden Rossendorf. In this work, a laser wakefield accelerator was driven by the 150 TW DRACO laser system and produced electrons tunable in energy from 250 to 350 MeV. I co-designed and built a compact calorimeter consisting of a stack of x-ray absorbers alternating with imaging plates. This single device enabled me to unfold spectra of all three major types of x-rays, both individually and in mixtures: 1) few-keV betatron x-rays, 2) ICS x-rays that were spectrally peaked at ~1 MeV photon energy, and 3) broadband bremsstrahlung with an average energy of ~30 MeV and a high energy tail extending beyond 100 MeV photon energy. Chapter 5 presents results obtained at The University of Texas in which I extended the work in chapter 4 and used a redesigned compact calorimeter to characterize secondary x-rays generated from a GeV-class accelerator. In this work, the accelerator was driven by the 1 PW Texas Petawatt Laser (TPW) which accelerated electrons to energies ranging from 500 MeV to 2 GeV. The compact calorimeter was redesigned for improved sensitivity to photons from 1 MeV to >100 MeV and enabled me to unfold ICS x-rays that were peaked at ~10 MeV photon energy, and broadband bremsstrahlung with average energies ~80 MeV. Chapter 6 then presents additional results obtained on the DRACO laser system in which I characterized the capabilities of a LPA and plasma mirror to generate ICS x-rays in both a linear and nonlinear regime. I used a CsI(Tl) scintillator to characterize the strength and divergence of ICS x-rays generated by retro-reflecting the accelerator’s spent drive laser pulse back onto the accelerated electrons using a plasma mirror. These measurements showed that the laser-electron interaction ranged from sub-relativistic to relativistic, depending on the plasma mirror distance from the accelerator exit. Finally, chapter 7 presents unpublished results from the TPW and presents unfolded spectra from a bremsstrahlung target scan in which a series of targets ranging from 25 μm-thick Kapton to 7.6 mm-thick Pb were used to produce Bremsstrahlung with average energies ranging from 60 MeV to >100 MeV. Chapter 7 also presents preliminary results from the application of bremsstrahlung x-rays to nuclear activation of copper. This dissertation concludes with a summary of the presented results and a discussion of future directions for laser plasma produced x-ray science.
High resolution retinal imaging to evaluate laser and light safety in the retina for near and long term health effects
(2012-12) Pocock, Ginger Madeleine; Snodderly, D. Max; Rylander, H. Grady (Henry Grady), 1948-; Markey, Mia K.; Milner, Tom E.; Oliver, Jeffrey W.
The purpose of this research was to investigate detect and monitor laser-tissue interactions at threshold and potentially sub-threshold levels of injury. High resolution imaging modalities can provide a deeper understanding of candidate biomarkers disease and injury at the molecular, cellular, and tissue-levels which can be used to identify and diagnose early stages disease and damage. In addition, multi-scale and multi-modal imaging have also been used to identify inherent biomarkers of retinal disease and injury. Monitoring tissue changes can be mapped back to biological changes at the cellular and sub-cellular level. Diseases often alter tissue on the ultra-structural level yet retinal clinical diagnosis often monitor changes in tissue at the organ level. If injury and disease is detected and diagnosed during an “early” stage of development, treatments and drug interventions may prevent further spread of the pathology. Non-invasive imaging is expected to be a valuable tool for in vivo medical research as well as for the diagnosis and management of disease. In addition to developing new imaging tools and techniques to image the retina, the identification of inherent biomarkers of disease and health using diagnostic methods are almost equally as important. Using the inherent optical properties of retinal tissue, we can non- invasively quantify differences in the absorption and reflection of light to gauge the risk for visual disability or worse yet irreversible vision loss as a result of retinal disease and chronic light exposure. The research presented with in this dissertation is three separate studies aimed at identifying light injury and potential biomarkers indicating the risk of light mediated development of disease.
Hydrodynamic instabilities of radiative blast waves
(2013-12) Kim, In Tai; Ditmire, Todd R.
We present the results from a series of experimental investigations into the hydrodynamic instabilities that occur in radiative blast waves. In particular, we examine the Vishniac instability in which the perturbation modes oscillate in time and, for certain mode numbers and polytropic index of the medium, can exhibit a growth in their amplitudes. Experiments were conducted on the GHOST laser laboratory in which a source of atomic clusters was irradiated by a 1J-2J, 115fs laser pulse to produce cylindrical blast waves. The thrust of this thesis falls into two categories. First, we analyze the effects radiative cooling has on the evolution of blast waves such as the lowering of the effective polytropic index and consequently the lowering of their deceleration parameter. Radiation from the blast wave surface results in a preheated ionization precursor in the upstream material and is indicated by a gradual decline in the electron density profile of the blast wave rather than a sharp jump. This mechanism, if strong enough, can also create a secondary shock wave to form ahead of the main blast wave. The second set of experiments investigates the temporal evolution of longitudinal perturbations induced on the blast waves by use of a transverse interferometric beam that modifies the cluster medium prior to the onset of the main pump beam. These perturbations are analyzed and compared to theory set forth in Vishniac's mechanism for oscillatory instabilities and their growth rate.
Isochoric heating of copper to Warm Dense Matter state using protons produced through laser solid-laser interactions
(2013-05) Feldman, Samuel Henry; Ditmire, Todd R.
This thesis examines the equation of state of copper at Warm Dense Matter states, between 1-100 eV and .1-10 times solid density. Protons accelerated off a thin metal foil irradiated with a high intensity laser beam flash heat solid density copper to between 5-10 eV before significant expansion occurs. The measured temperature and expansion are compared against simulations using various equations of states. The production and characterization of the laser system and proton beam used to heat the matter is also presented.
Laser and waterjet cutting for theatrical costumes
(2016-05) Collings, Justin Shaw; Glavan, James; Mickey, Susan E
As a costume technician, I am interested in ways we can incorporate new and different technologies into our craft. The use of laser cutting technology has become increasingly prevalent in the fashion industry to create one of a kind fabrics and embellishments on garments. I am interested in how we can use laser cutting to create costumes. For my thesis project, I will create three historically based costume pieces using a laser cutter and a waterjet cutting machine. The projects will be one: a jeweled metal crown inspired by the ducal crown of France, Italy, Spain, and Portugal as illustrated in Heralischer Atlas, by Hugo Gerhard Stroh; two, a 16th Century leather breastplate with laser cut and etched leather embellishments; and finally a re-creation on an early 20th century Edwardian lace collar and yoke.
Laser Forming of Ti-6Al-4V: Research Overview 58
(2000) Kobryn, Pamela A.; Semiatin, S. Lee
Laser forming is a solid-freeform-fabrication process which is being investigated for titanium-component manufacturing based on its cost-reduction potential. However, before it can be transitioned to production, the relationships between processing parameters, input materials, and deposit properties must be understood. In the present work, efforts were undertaken to characterize these relationships. These efforts included a comparison of different laser-forming processes, an investigation of the effect of processing parameters on deposit structure, determination of microstructure evolution via laser-processing experiments and thermal FEM modeling, and a study of texture/microtexture evolution. An overview of the results of this laserforming research are provided in this paper
Laser line scanning processing system for wrinkling in nation during coating
(2011-05) Doblar, Peter Anthony; Beaman, Joseph J.; Wood, Kristin
One of the major limiting factors in fuel cell production is the time and effort that are required in the ink coating process of the Nafion film that is at the heart of what makes a fuel cell work. The principle reason that this issued has not been tackled by the industry at large is the inherent difficulties that arise. First and foremost is the rapid and extensive expansion of the material upon contact with the liquid ink causing the Nafion film to wrinkle while being processed. In the drive to help mitigate this issue it must be understood by what conditions and severity that wrinkling occurs. The method chosen to detect this was to develop a laser profile scanner to record and output the severity of any wrinkles present in the film. This thesis showcases and explains the laser scanning system designed specifically for this problem and material.
Mapping the nanoplasma regime using harmonics
(2017-12) Korzekwa, Richard Clement; Downer, Michael Coffin; Breizman, Boris; Ditmire, Todd; Fink, Manfred; Varghese, Philip
Laser-irradiated microclusters exhibit a range of useful and scienti cally-relevant phenomena, including e cient absorption, harmonic generation, and anomalous dispersion. These properties make them useful for applications such as particle accelaration, nuclear fusion, and phase-matching for UV and X-ray sources. The generation of third harmonic light by intense illumination of pre-heated expanding clusters can be used as a way of mapping the nanoplasma regime, characterized by partial removal of the ionized cluster's electron core. We present measurements of this harmonic generation, showing a drop in expansion-related enhancement between probe intensity I[subscript pr] = 1.5 x 10¹⁶ W/cm² and I[subscript pr] = 1.0 x 10¹⁸ W/cm², where the harmonic generation vanishes. This is consistent with our model of cluster ionization, expansion, and polarizability, which we also use to estimate the cluster size distribution.
Mid-infrared type-I diode laser design using molecular beam epitaxy
(2020-08-14) Sifferman, Scott Daniel; Bank, Seth Robert; Belkin, Mikhail A; Wang, Yaguo; Wasserman, Daniel M; Yu, Edward T
The mid-infrared region of the electromagnetic spectrum, particularly in the wavelength range between 3 and 5 µm, is important for a number of applications in spectroscopy, gas sensing, infrared countermeasures, and communications. Despite these motivations, mid-infrared laser development has lagged behind that of visible and near-infrared technology. This is in part because semiconductor laser sources, while they exist across the mid-infrared, suffer from one or several drawbacks such as high power consumption, high threshold currents, low characteristic temperatures, limited wallplug efficiency, parasitic non-radiative recombination processes, or reduced carrier confinement. The latter impediment, specifically reduced carrier confinement of holes, is endemic to the active regions of GaSb-based type-I quantum-well diode lasers as the optical emission wavelength is extended past 3 µm. In this work, we present our efforts toward enhancing mid-infrared active regions to extend the emission wavelength of type-I emitters. Through the use of highly-strained, high indium-content quantum wells we demonstrate type-I diode laser operation from aluminum-free active regions up to 3.62 µm, and photoluminescence emission from type-I quantum wells out past 4 µm. Additional studies focused on the effect of using bismuth during the growth of these materials. While increased compressive strain in the quantum well alloy enables greater hole confinement at longer emission wavelengths, it also leads to material roughening and defect formation that restrict the number of and thickness of strained regions that can be grown before material quality irreparably degrades. We observed that by using bismuth as a surfactant during the growth of highly-strained GaIn(As)Sb alloys, material degradation was suppressed as these materials were grown well beyond classical critical thickness limits. We were also able to leverage the epitaxial growth conditions used for highly-strained, high indium-content quantum wells to incorporate dilute amounts of bismuth, up to 3%, into the quantum well materials. The addition of bismuth to the quantum well alloys modifies the valence band to provide additional hole confinement, leading to brighter emitters with up to 34% higher peak intensity. It also resulted in overall lower materials strain without reducing the emission wavelength or performance. This opens a promising approach to overcome strain-related limitations to laser performance and emission wavelength, allowing for device designs with increased numbers of quantum wells and potentially reducing the effects of gain saturation. An additional path toward improved mid-infrared devices is to switch the quantum well barrier material from GaSb to a lattice-matched AlGaAsSb alloy. This is the same strategy employed for many other mid-infrared type-I diode lasers, albeit for emission wavelengths less than 3.1 µm. By changing the barrier alloy, the quantum well valence band offset is increased, providing stronger hole confinement. Coupling these barriers with the highly-strained, high indium-content quantum wells results in a 3× improvement in peak photoluminescence and a >30% reduction in emission linewidth for quantum wells operating up to 4.2 µm. Using this coupled approach, we propose a laser diode device designed to operate at 4.1 µm.
Power System and Energy Storage Models for Laser Integration on Naval Platforms
(2015-06) Gattozzi, A. L.; Herbst, J. D; Hebner, R. E; Blau, J. A; Cohn, K. R; Colson, W. B; Sylvester, J. E; Woehrman, M. A.
High power solid state laser systems are being developed for advanced weapons and sensors for a variety of Department of Defense applications including naval surface combatants. The transient power and cooling requirements of these emerging technologies present significant challenges to the electric power distribution and thermal management systems, particularly for applications requiring back fit of the new systems onto existing platforms with limited electric power generation and cooling capacities.