Browsing by Subject "Lasers"
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Item A laser-induced surface flow visualization technique using liquid crystal thermography(Texas Tech University, 2002-12) Hunt, Emily McFatherObservation of flow field characteristics such as flow separation and reattachment are important in many industries. Current methods for flow visualization can be difficult to implement, expensive, and highly intrusive. The objective of this project is to develop an inexpensive, user-friendly, non-intrusive measurement technique useful to engineers interested in surface flow visualization. This is accomplished using liquid crystals in conjunction with a laser heat source to generate a thermal tuft. The shape and size of the thermal tuft is used to characterize the flow field. Wind tunnel experiments are conducted to validate this concept and examine flow behavior over a flat plate in a low Reynolds number environment. The plate is coated with liquid crystals of one-degree and fivedegree bandwidths. A 150-mW infrared, diode laser provides a constant heat source and generates a high temperature thermal spot on the model. The results obtained during the wind tunnel experimentation show that an irradiated spot on a liquid crystal coated surface will produce a tuft. The shape and size of the thermal tuft is indicative of the direction and magnitude of the flow conditions. As the wind speed increases from 2 to 10 m/s, it was shown that the length of the thermal tuft increases linearly. The tail of the tuft was also found to follow the direction of flow. Turbulent and laminar flow conditions can be distinguished; however, the angle of attack could not be realized with this technique. Developing a technique for generating a matrix of heated spots on the model indicates that the results of using this method can be viewed over a large area. Overall, it was shown that this is an easy, inexpensive, and non-intrusive technique for visualizing flow on the surface of an object.Item A study of lasers optically pumped by HF and DF lasers(Texas Tech University, 1978-05) Ross, Randall IrvanNot availableItem A study of noise effects in phase reconstruction from phase differences(Texas Tech University, 1996-12) Fox, James LNot availableItem Analysis of laser power amplifiers.(Texas Tech University, 1975-08) Shaw, Dihwa ShirleyThe design of laser systems based on the HF chemical reaction presents special problems resulting from the shape of the active region. In this study, a Rigrod-type approach is used to analyze a master oscillator power amplifier. By suitably arranging two flat mirrors beside the active media, a multipass amplifier is devised. The advantage of the multipass amplifier is that the active medium can be driven to saturation more easily and also the gain is enhanced. This analysis is interesting because a multipass amplifier has never been analyzed successfully. A general design procedure is offered and a number of cases are discussed. In particular, a five pass MORA is designed to fit a spatial gain distribution of interest,Item Automated microfluidic platforms to facilitate nerve degeneration studies with C. elegans(2013-12) Ghorashian, Navid; Ben-Yakar, Adela; Pierce-Shimomura, Jonathan T; Zhang, Xiaojing; Sokolov, Konstantin V; Dunn, Andrew KWith its well-characterized genome, simple anatomy, and vast array of uses in molecular biology, the roundworm, Caenorhabditis elegans (C. elegans) is a well-established model organism in neurobiology. Concurrently, neurodegenerative diseases are some the most devastating and least understood ailments in modern medicine, making high-throughput approaches to understand their fundamental mechanisms imperative to developing new therapies. The worm's physical length-scales and simple genetics make it an ideal in vivo tool for high-throughput screening platforms. Concurrently, microfluidic technology has been used to make devices that manipulate these animals in a multitude of fashions to study various biological phenomena. With these considerations in mind, we have developed microfluidic platforms to facilitate optical interrogation of neurodegenerative and neuroregenerative phenomena in C. elegans for large-scale screens. First we developed a multiwell format device with 16 on-chip reservoirs to house and quickly deliver distinct worm populations to any liquid-format imaging platform. The system achieved unprecedented delivery speeds, avoided any population cross-contamination, and maintained animal viability. We then expanded this platform into a 64-well device that acted as a modular plug and play system for simple manipulation by conventional high-throughput liquid handling systems. The chip could be manipulated in the same fashion as a multiwell plate and interfaced with a novel pneumatic gasket system to achieve delivery speeds that were two-fold faster than those attained on the 16-well device. In addition, we worked to develop potential optical interrogation platform that could be fed populations of worms by the aforementioned delivery systems. This microfluidic chip consisted of an array of parallel traps to house individual worms over long durations for time-lapse studies of nerve regeneration after cuts to single axons mediated by a femtosecond pulse laser. Specifically, the platform was designed for regeneration studies in the C. elegans PQR neuron.Item Breakdown processes in laser triggered switching(Texas Tech University, 1983-05) Dougal, Roger AdelbertAn investigation of laser triggered switching using a 1.06 µ NdrYAG laser to trigger a uniform field gap filled with 800 Torr of N2 has been performed in the following triggering geometries: 1) Transverse not striking electrodes, 2) Coaxial not striking electrodes, 3) Coaxial striking target electrode, and 4) Coaxial striking recess in target electrode. Streak photography, time resolved spectroscopy, and current measurements reveal great differences in the breakdown processes for the various geometries. The major differences are attributable to laser field interaction with the incipient arc channel. All breakdown processes except types 2 and 3 look quite similar to an overvolted breakdown. Diagnostics of type 2 triggering have shown a laser assisted streamer propagating from the laser fireball to the opposite electrode. Streak photography shows the streamer precursor of the breakdown channel initially proceeds across the gap at a few times 10 cm/sec, but slows to about 2X10 cm/sec as it advances in the focal cone to regions of lower laser intensity. The laser interaction with the streamer produces a uniform, high conductivity channel which emits intense continuum light once ohmic heating raises the channel temperature. When delay is greater than the laser pulse length two distinct regions can be detected in the arc channel; one, laser assisted, showing the abrupt, uniform continuum luminosity, and the other not laser assisted, appearing much like a weakly overvolted breakdown process, exhibiting a glow discharge stage followed by thermalization proceeding from the ends of the glow region towards the middle at 10 cm/sec.Item COb2s laser refraction and heating effects in a magnetized plasma column(Texas Tech University, 1974-05) Molen, George MarshallRecent developments in the area of high intensity lasers have caused an increased interest among researchers in applying various new techniques to heating gaseous plasmas to thermonuclear ignition temperatures. Lasers should be ideally suited for such an application because of their ability to deposit large bursts of energy in extremely short time durations. The basic requirements are that the plasma be heated to temperatures of several hundred million degrees to overcome the Coulomb barrier through energetic collisions and be confined long enough to get a net energy gain from the fusion reactions. Two different approaches have been proposed to attain these conditions using lasers. One approach, inertial confinement, would use nanosecond or less duration laser pulses in the tens of kilojoules to irradiate a small pellet fabricated (for example) of solid deuterium-tritium. The material would be vaporized be the focused laser and further heated by absorption of laser light in the blowoff material. Compression resulting from an implosion due to thermal pressure and the recoil from the plasma's expansion would heat the pellet. The plasma would be allowed to expand freely, thus giving rise to the term inertial confinement as only the inertia of the particles would determine the time scale.Item Colloidal nanoparticles : a new class of laser gain media(2009-12) Morgan, Robert Douglas; Ditmire, Todd R.; Keto, John; Downer, Michael; Sitz, Greg; Desidero Kovar, DesideroDevelopment of high average power lasers has historically been limited by the properties of available gain media. As a result it is either too costly or impractical to employ lasers in many applications for which they would otherwise be well suited. We have synthesized a new type of colloidal laser gain material that should possess many of the advantages of solid state media without their primary disadvantage: poor thermal performance. The colloid consisted of an emulsion of 20% Nd+3 doped phosphate glass nanoparticles suspended in nonanoic acid. The spectroscopic properties of the material were found to be consistent with those of bulk Nd+3 doped materials and suitable for laser development.Item Development of broad spectrum technologies for high energy chirped pulse amplification(2007) Hays, Gregory Ross, 1972-; Ditmire, Todd R.We have developed several broad-spectrum technologies for high-intensity chirped pulse amplification. We present the design and performance of two 20 TW laser systems. THOR is a Ti:sapphire, 10 Hz, ultra-fast laser that produces femtosecond pulses with a peak intensity of 18.4 TW. The laser operates near the bandwidth limit of the medium maintaining sufficient spectrum to produce 38 fs pulses. This equates to a near transform limited time-bandwidth product of 0.490. The second laser system was developed to study broad-spectrum pulse amplification in mixed Neodymium-doped laser glasses. Our efforts were to produce a multi-Joule laser with sufficient bandwidth to compress near 100 fs using mixed-glasses in the final amplifier. We present the GHOST laser with modeling and experimental analysis of the precise gain ratios between the mixed glasses. GHOST examines the bandwidth limit of the mixed-glass architecture in order to produce the broadest amplified spectrum with the shortest compressed pulsewidth. The laser has a total gain of 4x109 with a net gain of 260 from glass. The measured optimum gain ratio of 3.3 (G[subscript phos]/G[subscript sil]) produced 14.4 nm (FWHM) of bandwidth with a 103 fs pulsewidth. This constitutes a time-bandwidth product of 0.398. Additionally we have investigated two novel laser glasses in an effort to generate high energy (>1 kJ), broad spectrum pulses from a chirped-pulse amplification Nd:glass laser. Both glasses have significantly broader spectra (>38 nm FWHM) than currently available Nd:phosphate and Nd:silicate glasses. We present calculations for small signal pulse amplification to simulate spectral gain narrowing. The technique of spectral shaping using mixed-glass architecture with an optical parametric chirped-pulse amplification front-end is evaluated. Our modeling shows amplified pulses with energies exceeding 10 kJ with sufficient bandwidth to achieve 120 fs pulse widths are achievable with the use of the new laser glasses. With further development of current technologies, a laser system could be scaled to generate one exawatt in peak power. Finally we report controlled enhancement of optical third harmonic generation from hydrodynamically expanding clusters of noble gas atoms several hundred femtoseconds following ionization and heating by ultrashort pump pulses.Item Dual femtosecond ti: sapphire laser for ultrafast optical sampling two-color pump/probe studies(Texas Tech University, 1994-05) Luo, Ningyi DanielA pair of self-synchronous Ti: Sapphire lasers have been setup for two-color pump/probe detection in the sub-picosecond time regime. The two 75 femtosecond self-mode-locked Ti:Sapphire lasers, pumped by a cw Argon ion laser, are operated asynchronously at slightly different repetition rates to provide continuously varying dynamic delay times. The two Ti:Sapphire lasers are tunable at 700-890 nm. The shorter wavelength pulses from one laser are used as a pump source, while the longer wavelength pulses are used as a probe. The sum-frequency pulses generated by the cross-correlation of the two laser pulses are used to define the "time-zero" position and trigger the pump/probe process. The experiment is triggered at the difference frequency, and the signal can be averaged many times allowing a weak signal to build up. Dual-time scale is involved with the interpretation of the signal, which allows the experiment to be carried on the real time scale and the signal to be recorded on a much reduced equivalent time scale. Excited state lifetime measurement of laser HITCI has proven that this technology is practically feasible. Several advantages have been seen: (1) independent wavelength tunability of the pump and probe lasers; (2) variable femto- to nano-second pump/probe time delay; (3) fast (^is-ms) data collection time so that the effects of laser phase noise, temperature drift and sample variability are greatly reduced; (4) compact optical layout, without the need for optical delay lines and modulators, and thus, simple optical alignment. This study sheds light on the development of a novel compact high speed optical instmment, which will be useful in time-resolved spectroscopy, optical communication, optical computing, and medical diagnosis. With the addition of a BBO second harmonic generator to one of the two Ti:Sapphire lasers to generate femtosecond tunable UV/blue laser pulses, this dual-laser system can be used to study the critical hydrated electron problem. That was the original motivation of designing and developing this state-of-the-art instmment.Item Focusing light within turbid media with virtual aperture culling of the eigenmodes of a resonator(2012-12) Tom, William James; Dunn, Andrew Kenneth, 1970-; Downer, Michael; Emelianov, Stanislav; Milner, Thomas E; Rylander, Henry GVirtual aperture culling of the eigenmodes of a resonator (VACER) is a technique to focus light within turbid media at arbitrary locations. A seed pulse of light is directed through a phase-conjugate mirror (PCM) into a turbid medium. Though much of the light may be lost, any light which reaches the second PCM is phase conjugated and thus returned to the first PCM where the light will be phase conjugated again. Amplification by the PCMs can prevent decay of the light cycling between the PCMs. Introducing a mechanism which filters light based on position enables attenuation of the modes not traveling through the center of the virtual aperture resulting in a focusing of light at the center of the virtual aperture. The seed pulse and the positioning of the PCMs on opposite sides of the virtual aperture ensure that modes cannot bypass the virtual aperture. Magnetic fields and ultrasound waves are potential means for implementation of a virtual aperture. Generally, only weak filtration mechanisms like magnetic fields and ultrasound waves are innocuous to turbid media. Fortunately, weak effects can strongly cull modes in VACER because the filtration mechanism affects the modes during each pass between PCMs and the modes compete. A combination of theory and computational modeling prove that sound physical principles underlie VACER. Moreover, computational modeling reveals how mode overlap, the seed pulse, and other variables impact VACER performance. Good experimental performance is predicted.Item Interaction of clusters with ultra short X-ray free electron laser pulses(2012-08) Kandadai, Nirmala Krishna; Ditmire, Todd R.; Hallock, G. A.Biomolecular imaging has become one of the most exciting potential applications of the Linear Coherent Light Source (LCLS), which is a source of intense femtosecond X-rays. It has been predicted that a highly intense pulse with pulse lengths on the order of a few femtoseconds should be sufficient to capture the image of a biomolecule before it is destroyed. However, the rate at which a large biomolecule explodes during exposure is a large unknown, and will likely be one of the major factors in determining if such imaging will succeed. Clusters were chosen as a size dependant model system, ideal to study the evolution of complex systems in X-ray fields. From earlier intense near-infrared (IR) experiments, it is known that depending on size and Z constitution, clusters explode by Coulomb or hydrodynamic forces. These two limits have very different cluster explosion times and signatures. Coulomb explosion is too fast to allow imaging, whereas a hydrodynamically expanding cluster is a much slower process. The ionization process leading to cluster explosion is strongly wavelength dependent as one passes from IR through XUV to the X-ray regime because the kinetic energy of the released electrons determines the charge imbalance within the cluster, and therefore, determines the explosion dynamics. Unlike in previous experiments performed with near IR or XUV pulses, irradiation by photons at the LCLS will lead to the ejection of energetic photo- and Auger- electrons which could easily escape from the cluster, leaving behind positive ions. The buildup of this charge during exposure can lead to a Coulomb explosion of the sample. On the other hand, if the charge accumulates, the photoelectrons will be held inside the cluster, where they could contribute to the cluster temperature and form a nanoplasma and expand hydrodynamically. The main goal of the thesis was to study the explosion dynamics of clusters generated due to their interaction with intense X-rays and look at its dependencies on the X-ray energy, photon fluence, absorption cross sections, sample constituency and sample size. This thesis also compares the results from X-rays with the corresponding results obtained using ultrashort XUV and Infrared lasers.Item Laboratory visualization of laser-driven plasma accelerators in the bubble regime(2010-08) Dong, Peng; Downer, Michael Coffin; Becker, Michael; Ditmire, Todd; Lang, Karol; Shvets, GennadyAccurate single-shot visualization of laser wakefield structures can improve our fundamental understanding of plasma-based accelerators. Previously, frequency domain holography (FDH) was used to visualize weakly nonlinear sinusoidal wakes in plasmas of density n[subscript e] < 0.6 × 10¹⁹/cm³ that produced few or no relativistic electrons. Here, I address the more challenging task of visualizing highly nonlinear wakes in plasmas of density n[subscript e] ~ 1 to 3× 10¹⁹/cm³ that can produce high-quality relativistic electron beams. Nonlinear wakes were driven by 30 TW, 30 fs, 800 nm pump pulses. When bubbles formed, part of a 400 nm, co-propagating, overlapping probe pulse became trapped inside them, creating a light packet of plasma wavelength dimensions--that is, an optical "bullet"--that I reconstruct by FDH methods. As ne increased, the bullets first appeared at 0.8 × 10¹⁹/cm³, the first observation of bubble formation below the electron capture threshold. WAKE simulations confirmed bubble formation without electron capture and the trapping of optical bullets at this density. At n[subscript] >1× 10¹⁹/cm³, bullets appeared with high shot-to-shot stability together with quasi-monoenergetic relativistic electrons. I also directly observed the temporal walk-off of the optical bullet from the beam-loaded plasma bubble revealed by FDH phase shift data, providing unprecedented visualization of the electron injection and beam loading processes. There are five chapters in this thesis. Chapter 1 introduces general laser plasma- based accelerators (LPA). Chapter 2 discusses the FDH imaging technique, including the setup and reconstruction process. In 2006, Dr. N. H. Matlis used FDH to image a linear plasma wakefield. His work is also presented in Chapter 2 but with new analyses. Chapter 3, the main part of the thesis, discusses the visualization of LPAs in the bubble regime. Chapter 4 presents the concept of frequency domain tomography. Chapter 5 suggests future directions for research in FDH.Item Laser micro/nano scale processing of glass and silicon(2006) Theppakuttai Komaraswamy, Senthil Prakash; Chen, ShaochenItem Laser micro/nano scale processing of glass and silicon(2006-05) Theppakuttai Komaraswamy, Senthil Prakash, 1977-; Chen, ShaochenThe revolutionary progress in semiconductor, communication, and information industries based on electronic and photonic technologies demands for the development and enhancement of new laser processes to support micro and nanotechnologies. This dissertation is aimed at exploring the use of lasers for micro and nano scale processing of glass and silicon, the most commonly used materials in the IC industry. The objective of the dissertation is two fold: a) use lasers for locally micro bonding glass and silicon wafers, and b) use lasers for nanopatterning glass and silicon substrates by circumventing the diffraction limit of light. In the first part of the thesis, glass and silicon wafers are bonded locally in microscale by a pulsed Nd:YAG laser. Glass is transparent to the wavelength used and hence the laser beam passes through the glass wafer and is absorbed by silicon. As a result, silicon is melted and upon resolidification bonding is realized between the two substrates. The transient melting and resolidification of the substrates is studied experimentally and compared to the simulation results of a finite element numerical model. The bonded areas are studied in detail using a scanning electron microscope and a chemical analysis is done to understand the bonding mechanism. In the second part of the thesis, nanopatterns are created on glass and silicon substrates by circumventing the diffraction limit of light. The nanofeatures are created by irradiating silica and gold nanospheres deposited on a substrate. In case of silica spheres, features approximately half the diameter of the sphere were obtained by utilizing the optical field enhancement around the spheres. In case of gold spheres, features as small as 40 nm were realized by the excitation of coherent resonant electron plasma oscillations. The effect of sphere size, laser wavelength, polarization, incident angle, and energy were studied experimentally. Finally, these experimental results are compared with the numerical results from a multidimensional, heat transfer model.Item Laser mode analysis(Texas Tech University, 1977-12) Kwatra, Ajit Pal SinghA new integral equation for optical resonators has been derived from the boundary value problem. This equation, called the Source Integral Equation (SIE), has been solved approximately and mode shapes compared with those from the widely used Fox-Li Integra! Equation (FLIE). A double resonance condition (constraints on both mirror separation and transverse dimension) has been obtained from a self-consistency condition. Approximations to the resonance condition have been devised for the limiting case of flat mirrors. The results obtained from a Functional Equation Method (FEM) are compared with traditional computer solutions. The locations of the peaks and valleys agree well and the overall mode shapes are similar. This verifies the utility of the FEM for spherical mirrors of circular aperture. The FLIE and SIE results from the FEM, are compared to confirm the validity of the FEM approach for the SIE. The SIE results are compared to the traditional computer results to find effects of the corrections of the SIE to the FLIE.Item Optical methods for microscopic particle size measurement(Texas Tech University, 1997-12) Kustov, Vadim MichailovichNot availableItem Optically controlled diffuse discharges for switching applications(Texas Tech University, 1986-08) Hutcheson, George ZohnDiffuse discharges containing electronegative gases, at present, are the only means capable of fast, repetitive, long-life operation as opening switches. Optical control of diffuse discharge switches has been proposed as both a means of sustaining and of enhancing the performance of such switches. Processes considered in this dissertation are photo ionization. photo detachment. and several approaches to optically enhanced attachment. Ultraviolet (UV) ionization has been used for several years now as a means of preionizing self-sustained diffuse discharges and, recently, has been used as a sustainment mechanism for diffuse discharge opening switches. Time-resolved measurements of the optical emission of a spark array, similar to those used for preionization or sustainment of diffuse discharges, are presented. Results of experiments in atmospheric pressure diffuse discharges containing admixtures of attachers, sustained and initiated by such a spark array, are also presented. Photo detachment is considered as a mechanism which could decrease switch losses and decrease switch closure time by counteracting dissociative attachment. Experimental results are presented demonstrating optically increased current densities, of as much as 900%, in externally sustained and externally initiated, atmospheric pressure, diffuse discharges containing 0^. This process proceeds through photodetachment of 0 by 590 nm light generated by a dye laser (0 +hu -• 0+e). Optically enhanced attachment through the generation of vibrationally excited attachers appears particularly attractive as a means to decrease turn-off or opening times for diffuse discharge switches. For some molecules attachment cross-sections can be increased by orders of magnitude by vibrational excitation. The influence of this effect on the resistivity of a discharge through IR pumping of NH.;^ and CpH^aF by a low power CO^ laser in continuous low pressure discharges is presented. UV enhanced attachment is also shown in gases containing molecules such as C^HpF^. CpHpClp, and CpH-Cl where UV photo dissociation produced vibrationally excited, strongly attaching, molecules (e.g.. C2H2F2+hu-C2H2+HF(v>1)).Item Several new infrared laser pumped lasers(Texas Tech University, 1978-05) Bushnell, Andrew HughNot availableItem Surface modification of ceramic and metallic alloy substrates by laser raster-scanning(2003-12) Ramos Grez, Jorge Andrés, 1969-; Bourell, David Lee