Mountain Scholar :: Browsing by Author "Marconi, Mario C., committee member"

Browsing by Author "Marconi, Mario C., committee member"

Now showing 1 - 20 of 23

Open Access
Advances in single-pixel imaging toward biological applications
(Colorado State University. Libraries, 2014) Winters, David G., author; Bartels, Randy, advisor; Marconi, Mario C., committee member; Prasad, Ashok, committee member; Bernstein, Elliot R., committee member
In this work, we discuss two new methods for single-pixel imaging. First, we leverage advances in laser metrology and frequency synthesis to measure small shifts in the center frequency of an optical pulse. Pulses acquire such shifts when probing a transient optical susceptibility, as in impulsive stimulated Raman scattering, which we use to demonstrate the technique. We analyze the limits of this technique with regard to fundamental noise, and predict detection sensitivity in these limiting cases. We then present work on imaging in two dimensions, both x-y and x-z, using single element detectors. We accomplish this by multiplexing spatial frequency projections in time, allowing rapid two dimensional imaging without an imaging detector. As we eliminate the imaging detector, the sensitivity to scattering is dramatically decreased, allowing the method to be used deep in scattering tissue. Results are shown for several geometries and experimental configurations, demonstrating imaging capabilities across a variety of sample types, including fluorescent and biological samples.
Open Access
An experimental investigation of heaterless hollow cathode ignition
(Colorado State University. Libraries, 2020) Ham, Ryan K., author; Williams, John D., advisor; Yalin, Azer P., committee member; Marconi, Mario C., committee member; Tomasel, Fernando G., committee member
A hollow cathode is a specially designed plasma source that is capable of driving a large electron emission current throughout the course of a remarkably long lifetime. Given these characteristics, hollow cathodes are commonly used as electron sources in state-of-the-art plasma thrusters. Modern advancements in small-satellite technology have led to an increased demand for low-power electric propulsion systems. Given the high thrust-to-power ratio and flight-proven heritage of Hall-effect thrusters, efforts are currently being made to downsize these thrusters to a considerably small scale. By forgoing the use of a heater, heaterless hollow cathodes provide several advantages that are best realized in miniaturized Hall-effect thrusters. Unfortunately, the lack of a cathode heater gives rise to nontrivial complications in the process of igniting a plasma discharge, along with reason to believe that life-limiting cathode erosion could occur during ignition. These concerns have resulted in a lack of confidence that heaterless hollow cathode technology can endure the rigors of spaceflight qualification. In this research, heaterless hollow cathode ignition behavior was characterized. In doing so, it was found that repeatable and reliable instant start ignition behavior can be achieved when using a high propellant mass flow rate. To provide this flow condition without placing a large burden on a propellant feed system, a novel gas flow mechanism was developed and characterized. To investigate whether instant start ignition causes cathode erosion, a series of tests were performed in which heaterless hollow cathodes were subjected to a large number of ignition cycles. Microscopy revealed no indication of cathodic arc activity, and no other evidence of life-limiting erosion were observed. The instant start ignition process appears to be a viable approach to heaterless hollow cathode ignition, and we believe it provides a means for heaterless hollow cathode technology to be integrated into spaceflight propulsion systems.
Open Access
Characterization of laser-produced plasmas as light sources for extreme ultraviolet lithography and beyond
(Colorado State University. Libraries, 2019) Yin, Liang, author; Rocca, Jorge J., advisor; Menoni, Carmen S., committee member; Marconi, Mario C., committee member; Yalin, Azer, committee member
Lithography is a critical process in the fabrication of integrated circuits. The continuous increase in computing power for more than half a century has depended in the ability to print smaller and smaller features, which has required the use of light sources operating at increasingly shorter wavelengths. There is keen interest in the development of high-power light sources for extreme ultraviolet (EUV) lithography at λ=13.5 nm and future beyond extreme ultraviolet (BEUV) lithography near λ=6.7 nm. The work conducted in this dissertation has characterized aspects of laser-produced plasmas (LPPs) that serve as light sources for EUV / BEUV lithography. The laser pulse shape dependence of the conversion efficiency of λ=1.03 μm laser into in-band 13.5 nm EUV emission in a Sn LPP was studied as a function of laser pulse shape and durations. Laser pulses of arbitrary temporal shape with variable energy and pulse widths were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. The pulse synthesizer is based on wave front splitting and pulse stacking for the generation of arbitrary shape laser pulses of Joule-level energy. Pulses ranging from hundreds of ps to several ns were generated with a single laser. The measurements showed the CE favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A significant increase in CE was observed when Q-switched pulses were substituted by square pulses of similar duration. Conditions were observed at which the EUV pulse duration significantly outlasts the laser pulse in the direction normal to the target surface, in contrast at grazing angles the measured EUV pulse duration is shorter and similar to the laser pulse duration. The physics leading to this angular anisotropy is discussed, along with the spectroscopic characterization of EUV emission and at-wavelength images that characterize the source size. Another aspect of this dissertation includes a comprehensive study of the emission from Gd and Tb LPPs in the λ=6.5 - 6.7 nm region. BEUV emission spectra were measured as a function of laser pulse duration (120 ps - 4 ns), emission angle, and spatial location within the plasma. At-wavelength images of the BEUV emitting plasma region were obtained as a function of irradiation parameters. The peak of the emission spectrum was observed to broaden and to shift to longer wavelengths as the laser pulses are shortened from ns to hundreds of ps. Transient self-consistent hydrodynamic/atomic physics simulations show that the picosecond irradiation creates significantly hotter plasmas in which the dominant emission originates from more highly ionized species. Gd LPP emission driven by nanosecond laser pulses best matched the reflectivity band of our La/B4C mirrors. Spatially resolved spectra of the Gd LPP were acquired for different laser parameters and were compared to simulations. The CE into in-band BEUV emission was determined by integrating angularly resolved measurements obtained using an array of calibrated energy monitors. A maximum CE of 0.47% / 0.45% for the Gd / Tb LPPs was obtained within a 0.6% bandwidth. The results are of potential interest BEUV lithography.
Open Access
Characterization of scandium oxide thin films for use in interference coatings for high-power lasers operating in the near-infrared
(Colorado State University. Libraries, 2010) Krous, Erik M., author; Menoni, Carmen S., advisor; Marconi, Mario C., committee member; Williams, John D., committee member
The work presented in this thesis aims to investigate scandium oxide (scandia), deposited using dual ion beam sputtering, as a high-index material for interference coatings to be implemented in high-power lasers. Ion beam sputtered scandia coatings have the potential to allow for the power scaling of high-power lasers operating in the near-infrared. Ion beam sputtering is the technique currently used by many commercial companies to produce low-loss, high-damage-threshold coatings required by lasers operating with high fluences. The development of scandia, and other thin film materials, requires the reduction of defects in the material through modification of growth processes and post deposition treatment. Material defects give rise to absorption of laser light and laser induced damage initiation sites. The growth parameter investigated in this work is the oxygen partial pressure in the deposition chamber during the reactive sputtering process of a metal Sc target to form Sc2O3. The film properties are sensitive to the oxygen partial pressure. At 2 μTorr oxygen partial pressure, the films are metallic and highly absorbing with an absorption, at λ = 1.064 μm, of > 104 ppm. The absorption decreases to 10 ppm at 5 μTorr oxygen partial pressure and at 38 μTorr, the absorption reaches a value of 35 ppm. This, along with the increase in absorption near the optical band edge, suggests an increase in shallow-type defect concentrations for increasing oxygen partial pressures. The observed defects contain unpaired electrons, as assessed by electron paramagnetic measurements, that have a paramagnetic absorption signal with principle g-values [gxx, gyy, gzz] = [2.018, 2.019, 2.058]. Generally, the concentration of the paramagnetic species increased with increasing oxygen partial pressure. These spin defects are possibly O2̅ interstitials in the deposited films. These defects contribute to an approximately 40% increase in the film stress observed in x-ray diffraction measurements and measurements of stress-induced fused silica substrate curvature.
Open Access
Constrained dynamics of rolling balls and moving atoms
(Colorado State University. Libraries, 2011) Kim, Byungsoo, author; Putkaradze, Vakhtang, advisor; Tavener, Simon, committee member; Shipman, Patrick, committee member; Marconi, Mario C., committee member
This dissertation is devoted to the study of the dynamics, conservation laws and symmetries of rolling spheres, with special attention to applications to atomic and molecular systems. Previously known conservation laws of the rolling motion are associated with the nonholonomic version of Noether's theorem. Moreover, the conservation laws are related to the reduction by Lie symmetries of the dynamic equations of motion. Symmetries in the Noether's theorem and in the reduction by Lie symmetries are compared in their applications. In addition, we analyze the collective motion of the system of rolling particles for its statistical quantities under the constraint condition of rolling without slipping motion. The numerical simulations revealed some of qualitative characteristics in the statistical mechanics of the rolling-constrained system. As a separate topic, the study of the molecular dynamics is discussed in relation to the results of recent experimental achievements with the non-contact atomic force microscopy. We propose a novel scenario explaining the process of single-atom manipulation in terms of the classical resonance effect.
Open Access
Damping mechanisms in magnetic recording materials & microwave-assisted magnetization reversal
(Colorado State University. Libraries, 2014) Lu, Lei, author; Wu, Mingzhong, advisor; Gelfand, Martin P., committee member; Kabos, Pavel, committee member; Marconi, Mario C., committee member; Patton, Carl E., committee member
Understanding the damping of magnetization precession in magnetic recording materials is of both fundamental and practical significance. From the practical perspective, the relaxation processes not only set a natural limit to the time of magnetization switching which determines recording data rates, but also play critical roles in advanced magnetic recording techniques such as microwave-assisted magnetic recording and two-dimensional magnetic recording. Experimental and theoretical studies of magnon-electron scattering and two-magnon scattering (TMS) contributions to magnetization relaxations in magnetic recording head and media materials were conducted for the first time in this dissertation. The accuracy of ferromagnetic resonance (FMR) measurements was increased by the use of vector network analyzer (VNA) FMR techniques. Working equations of the grain-to-grain TMS and grain boundary TMS processes were developed based on the TMS models of Krivosik and Mo, and were applied to understand the relaxation mechanisms in various recording-related thin film materials. The dependences of the FMR behavior and relaxation rates on the external field orientation, the microwave frequency, and the temperature were investigated experimentally in the following three domains: the exchange-coupled composite media, the free layers of tunnel magneto-resistance readers, and FeCo alloy films for future writers. The theoretical models were used to analyze the experimental data and to understand the relaxation mechanisms. Microwave-assisted magnetization reversal (MAMR) is considered as a promising mechanism for further increasing the recording area density and pushing it beyond the super-paramagnetic limit. The MAMR operation was demonstrated with a 700-Gbit/in2 perpendicular media sample in this thesis study. For microwaves with frequencies close to the FMR frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.
Open Access
Development of a high energy TI:Sapphire laser for the excitation of extreme ultraviolet lasers
(Colorado State University. Libraries, 2011) Martz, Dale Herman, author; Rocca, Jorge J., advisor; Lee, Siu Au, committee member; Menoni, Carmen S., committee member; Marconi, Mario C., committee member
This dissertation describes the design, construction and characterization of a high energy chirped-pulse amplification Titanium-Sapphire laser system for the excitation of Extreme Ultraviolet (EUV) lasers. Compact EUV lasers have made possible nano-scale imaging, dense plasma diagnostics and photo-chemistry and photo-physics studies. They also have the potential to make possible a variety of new studies of surfaces and materials and enable the development of unique metrology and processing tools for industry. The components developed to realize high energy operation of the Titanium-Sapphire laser include the development of a Nd:Glass zig-zag slab pump laser and novel 800 nm multi-layer dielectric diffraction gratings for picosecond compression. The Titanium-Sapphire laser was used to pump several table-top EUV lasers. Increased average power operation of a 13.9 nm nickel-like silver laser generating 20 uW was demonstrated. This is the highest average power obtained from a compact EUV laser to date. Injection seeding of the 13.9 nm EUV amplifier produced laser beams with greatly improved beam characteristics which includes a large reduction in beam divergence and a near Gaussian far-field profile. The laser was also used to pump a gain-saturated table-top laser at 10.9 nm in nickel-like tellurium.
Open Access
Development of a petawatt class Ti:Sapphire laser for the excitation of extreme radiation sources
(Colorado State University. Libraries, 2020) Rockwood, Alex Pratt, author; Rocca, Jorge J., advisor; Lee, Siu Au, committee member; Roberts, Jacob L., committee member; Marconi, Mario C., committee member
This dissertation describes the design, construction and characterization of a high peak power, high repetition rate, Titanium-Sapphire laser system. This chirped-pulse amplification (CPA) laser delivers femtosecond pulses of up to 0.85 PW peak power. By utilizing pump laser amplifiers with a slab configuration high repetition rate are achieved, 3.3Hz, the highest at which Petawatt-class lasers have been operated to date. This 800nm laser also has a high power, ultra-high contrast 400 nm beamline. By frequency doubling the 800 nm with a KDP crystal at ≥ 40% conversion we are able to achieve a contrast of > 1 × 10-12. The ability to focus this second harmonic beam to ~1.2 μm Full Width at Half Maximum (FWHM) spot size made it possible to achieve intensities up to ~ 6.5 ×1021 W/cm2. With these high intensities and high contrast this laser is a powerful tool in many applications especially in the study of laser/matter interactions at relativistic plasmas. This Ti:Sapphire laser was used for the excitation of plasma based soft x-ray (SXR) lasers. prior to this work compact, repetitively fired, gain-saturated x-ray lasers had been limited to wavelengths above λ = 8.85 nm. We were able to demonstrate SXR lasers operating in the gain-saturated regime down to wavelengths as low as λ = 6.85 nm in Ni-like Gd. Gain was also observed at λ = 6.4 nm, and λ = 5.8 nm in Ni-like Dy. As an application of plasma-based SXR lasers, single shot Fourier holograms covering a large area of view were demonstrated using an 18.9nm laser with high spatial coherence based on dual plasma amplifier. Compact SXR lasers have made possible applications in nano-scale imaging, dense plasma diagnostics and a variety of new studies of materials and surfaces. Other applications that were enabled by this Petwatt-class laser discussed elsewhere include the study of the interaction of relativistic laser pulses with aligned nanostructures, producing record conversion efficiency of optical laser light into picosecond x-ray pulses with photons of > 1 KeV energy and flashes of deuterium-deuterium fusion neutrons.
Open Access
Extreme ultraviolet laser ionization mass spectrometry: probing materials at the micro and nano scales
(Colorado State University. Libraries, 2023) Rush, Lydia Alexandra, author; Menoni, Carmen S., advisor; Duffin, Andrew M., advisor; Farmer, Delphine K., committee member; Marconi, Mario C., committee member; Rocca, Jorge J., committee member
The focus of this dissertation is the use of 50 to 10 nanometer wavelength extreme ultraviolet (EUV) laser light as a next generation probe for mass spectrometry analyses at the micro (>100 nanometers) and nano (≤100 nanometer) spatial scales. While the unique properties of EUV light have revolutionized the semiconductor industry through nanoscale lithography fabrication, the use of EUV lasers with analytical instruments, like mass spectrometers, for high spatial resolution chemical analyses is a relatively untapped area. This unexplored territory is owed partly to only recently bringing EUV lasers to an accessible "bench-top" scale. Herein I show how EUV laser ionization can be used with different types of mass spectrometers as a new route for interrogating nuclear and geologic materials with micro and nano scale lateral spatial resolution. I focus on the application of a compact capillary discharge EUV laser operating at a wavelength of 46.9 nanometers connected to a time-of-flight (TOF) mass spectrometer, called the EUV TOF. I also show for the first time how the 46.9 nm EUV laser ionization source can be connected to a commercial magnetic sector mass spectrometer, called the EUV magnetic sector. Specifically, I demonstrate that the EUV TOF instrument can measure the 235U/238U isotope ratio in 100 nm sized pixels in a heterogeneous uranium fuel pellet that was made by blending different feedstocks together. The results show that the EUV TOF maps similar micrometer sized areas of 235U/238U heterogeneity as nanoscale secondary ionization mass spectrometry (NanoSIMS), indicating that EUV laser ionization can be used to accurately probe complex nuclear materials within the scope of the study. I also show that the EUV TOF can be used to measure 206Pb/238U and 232Th/238U isotope ratios at the 8 µm scale in select geologic matrices of silicates, zircons, monazites, and iron manganese within error (±2σ) using a single non-matrix matched calibration standard. However, the precision on the ratio measurements was low for useful geologic applications, ranging between 1-10% at elemental concentrations exceeding hundreds of ppm because of the limitations of using a TOF for isotope ratio measurements. To this end, I show the current development of the new EUV magnetic sector instrument that uses the EUV laser ionization source with a commercial double-focusing sector-field multi-collector mass spectrometer with the aim of achieving more precise (<1%) and sensitive (≤ppm) isotope ratio measurements at high spatial scales (<10 µm down to the nanoscale). The EUV magnetic sector is being developed to probe more complex isotopic systems in nuclear and geologic materials that was not possible with the TOF mass spectrometer. The work here shows that the 46.9 nm wavelength EUV laser ionization source can be interfaced with Thermo Fisher's commercial sector-field multi-collector mass spectrometer called the Neptune by removing its inductively coupled plasma (ICP) region. The Neptune's ion optics, electric sector, and magnetic sector were modified for acceptance of the pulsed EUV-generated ions. These modifications resulted in ions from ≤2 µm diameter craters created by EUV laser ablation and ionization being successfully focused, separated by mass, and detected using the Neptune's electron multipliers. However, further system upgrades to the Neptune's detectors are needed for accurate isotope ratio measurements at high spatial scales because the 10 to 30 nanosecond wide EUV-generated ion pulses are on the order of the electron multipliers' dead time. With proper detectors, the EUV magnetic sector's accuracy, precision, sensitivity, efficiency, and spatial resolution can be measured in future experiments. The demonstration of the EUV magnetic sector instrument here represents the first time that an EUV laser ionization source has been used with a sector-field mass spectrometer, paving the way for future high spatial resolution isotope ratio analyses.
Open Access
Fast electronic driver for optical switches
(Colorado State University. Libraries, 2012) Woolston, Mark R., author; Rocca, Jorge J., advisor; Marconi, Mario C., committee member; Menoni, Carmen S., committee member; Yalin, Azer P., committee member
Electronically controlled optical switches are critical components in many optical systems, including pulsed lasers. Solid-state optical switches based upon the Pockels effect are widely utilized in research and industry, however Pockels cells require electronic drivers capable of switching several kilovolts quickly and cleanly. This thesis reviews Pockels cell designs and their typical applications in laser systems, discusses common drive circuit topologies found in literature, and describes the development of a fast, stable electronic driver for half-wave congured Pockels cell optical switches. In a crowded optical environment, it is frequently desirable to locate the Pockels cell at some distance from the driver electronics. The driver was developed to be capable of 1.4 ns optical transition times when connected to a 6 pF Pockels cell via 1.2 meters of 50 ohm coaxial cable. The driver is designed to operate in colliding-pulse mode at 6-8 kV, with 80 ampere typical switch currents. Total switch propagation delay is less than 100 ns, and thermal drift has been measured at less than 50ps/C°. These pulsers are currently used to drive Pockels cells in colliding pulse mode in pulse picking and slicing applications where optical rise times of < 2 ns and low drift are needed. Novel non-invasive diagnostic techniques for measuring and graphing pulse propagation in a repeatable manner along collapsing avalanche transistor chains are presented.
Open Access
Gain-saturated repetetive soft X-ray lasers with wavelengths spanning 9-30 nm and lasing down to 7.4 nm
(Colorado State University. Libraries, 2011) Alessi, David Alan, author; Rocca, Jorge J., advisor; Lee, Siu Au, committee member; Menoni, Carmen S., committee member; Marconi, Mario C., committee member
This dissertation describes the development of table-top soft X-ray lasers with wavelengths ranging from 30 nm to 7.4 nm. The laser transisitons occur within collisionally excited states of nickel-like and neon-like ions which are created from laser ablation of solid targets. A Nd:glass slab laser system was developed to provide 20J (and then upgraded to 40J) of laser light at 527 nm for pumping a table-top chirped pulse amplification Ti:sapphire laser. With this increase in pump energy, the Ti:sapphire system is capable of producing 12J uncompressed laser pulses at a 1Hz repetition rate. Stretched and compressed pulses from this Ti:sapphire laser system operating near 800 nm are used to both ionize the material to a high degree and heat the free electrons in these plasmas to temperatures required for high gain. Simulations from a 1.5D hydrodynamic/atomic model indicate a peak gain of 90 cm-1 for the 8.8 nm laser transition in nickel-like lanthanum is reached with an electron temperature of ~850 eV and a density of 6×1020 cm-3. By using the grazing incidence pumping geometry, gain saturated operation was demonstrated in the 2p53p 1S0 →2p5 3s 1P1 transition of neon-like titanium (λ = 32.6 nm) and vanadium (λ = 30.4 nm), as well as in the 3d94d1S0→3d9 4p1P1 transition in nickel-like tellurium (λ = 10.9 nm) and lanthanum (λ = 8.8 nm). Strong lasing was also demonstrated in the same neon-like transition in chromium (λ = 28.6 nm), as well as the same nickel-like transition in cerium (λ = 8.5 nm), praseodymium (λ = 8.2 nm), neodymium (λ = 7.9 nm) and samarium (λ = 7.4 nm). This is the first demonstration of the generation of bright gain-saturated sub-9-nm wavelengths with a table-top laser operating at 1-Hz repetition rate. The short wavelength, microjoule pulse energy, picosecond pulse duration and repetitive operation of these lasers will enable new applications such as sequential imaging of ultrafast nano-scale dynamic phenomena to be realized on a table top.
Open Access
Impact of thermal management on vertical-cavity surface-emitting laser (VCSEL) power and speed
(Colorado State University. Libraries, 2011) Safaisini, Rashid, author; Lear, Kevin L., advisor; Marconi, Mario C., committee member; Reising, Steven C., committee member; Sites, James R., committee member
Increasing the modulation bandwidth and output power of vertical-cavity surface-emitting lasers (VCSELs) are of great importance in a variety of applications such as data communication systems. The high temperature generated in the active region of VCSELs is one of the main limiting factors in achieving high power and high speed operation. This work is focused on investigating the effects of thermal management on improving AC and DC properties of VCSELs and achieving higher thermal performance devices. Thermal heatsinking is obtained by surrounding the VCSEL mesas with high thermal conductivity materials such as copper and also using passive heatsinking by flip-chip bonding the laser dies on a GaAs heat spreader. The research includes fabricating and characterizing 980 nm bottom-emitting and 670 nm top-emitting oxide-confined VCSELs. This dissertation is divided into three main parts: high-power, high-speed 980 nm VCSEL arrays, low thermal resistance 670 nm VCSELs, and temperature dependent dynamics of 980 nm VCSELs. Experimental work performed on fabricating and characterizing 980 nm, bottom-emitting, oxide-confined VCSEL arrays and single elements is presented first. The result of DC and AC characterization confirms the effectiveness of Cu electroplating of mesas and flip-chip bonding in reducing VCSELs' thermal resistance to obtain lower operating temperatures. Uniformity of frequency response and operating wavelength across the arrays also motivates managing thermal issues and is an indication of uniform distribution of current and heat flux on the array. This research resulted in record VCSEL arrays with frequency response of approximately 8 GHz and operating CW power of 200 mW. These 28-element, 18µm aperture diameter arrays represent the highest power reported for a VCSEL or VCSEL array with greater than 1 GHz modulation bandwidth. The second part of this dissertation details the fabrication steps and DC characterization of visible, 670 nm, top-emitting, oxide-confined VCSELs. Since achieving high operating temperatures is one of the main challenges in realizing improved red VCSELs, the effect of mesa heatsinking on improving their DC behavior using copper electroplating of mesas is studied. Thermal modeling of the copper plated VCSELs also facilitates better understanding and analysis of the experimental results. A photomask and process flow were designed to fabricate VCSELs with a variety of mesa diameters and inner and outer plating sizes to investigate the major direction of heat flow in the VCSELs and decrease VCSEL thermal resistance and thus increase the output power. Although copper plating significantly reduces thermal resistance, it did not substantially increase maximum operating temperature of the red devices and also put the mesas under stress that might not be desired. This study led us to analyzing the effects of stress on the VCSEL mesas which is induced by the copper films. Finally, the temperature dependence of 980 nm VCSEL dynamics is investigated using noise spectra measurement. This analysis provides some useful insights in understanding how temperature alters VCSEL properties and how these properties can be improved. A VCSEL with 7 µm aperture diameter was fabricated from the same epitaxial material and followed the same processing steps as the VCSEL arrays. Relaxation oscillation frequencies and damping factors as functions of bias current and stage temperature were extracted. These results along with the VCSEL DC measurement were used to estimate the laser differential gain as a function of temperature. The differential gain was shown to be relatively temperature independent over a temperature range of 10 °C to 70 °C with an average value of approximately 12×10-16 cm2. This research led us to the conclusion that improving the output power at elevated temperatures should yield better frequency response in this case. The VCSEL output power reduction was observed to be the major cause of bandwidth reduction at elevated temperatures for the device under test. This work is the first report on the measurement of temperature dependence of VCSEL dynamics.
Open Access
Improved resolution and speed in nonlinear microscopy
(Colorado State University. Libraries, 2010) Masihzadeh, Omid, author; Bartels, Randy A., advisor; Roberts, Jacob Lyman, committee member; Menoni, Carmen S., committee member; Marconi, Mario C., committee member
Optical microscopy is an important tool for biomedical research. New techniques for microscopy enable new capabilities for studying biological systems. Moreover, in optical microscopy, the polarization state of the focal field strongly influences the images formed due to the impact of focal spot size, adjusting the relative strength and phase of both transverse and longitudinal field components, and manipulating inter- action with the sample under study. In particular, coherent nonlinear microscopies, such as third harmonic generation (THG), and second harmonic generation (SHG), offer rich possibilities for new control over the imaging process. In the first part of this dissertation, I demonstrate that control over the spatial polarization state of the focal field can be used to improve the spatial resolution in a laser-scanning THG microscopy. First, we show a detailed design of our nonlinear scanning microscope, then we introduce a non-iterative algorithm for measurement of spatially inhomogeneous polarization distributions in third-harmonic generation microscopy. We also, show control of spatial polarization state of the focal field through imaging of a spatial light modulator to the focus of a microscope objective. Then, we introduced a novel technique for enhancing resolution in THG microscopy, through spatial polarization shaping at the focal field. In the second part of this dissertation, we show an alternative method to laser- scanning nonlinear microscopy in biological tissue, namely, nonlinear holographic microscopy. First, we introduce the foundation of nonlinear holographic microscopy by reviewing linear off-axis holography. We start by introducing digital recording in off- axis holography, its limitations, and show how through holography we can obtain 3-D images from 2-D data. We then explore numerical reconstruction of the object field from the recorded holograms. Finally, we expand this technique to SHG nonlinear holographic microscopy to construct 3-dimensional images of biological tissues.
Open Access
Investigation of laser cooling and trapping of atomic silicon: towards the development of a deterministic single ion source
(Colorado State University. Libraries, 2023) Ronald, Samuel R., author; Lee, Siu Au, advisor; Fairbank, William M., Jr., advisor; Rocca, Jorge J., committee member; Marconi, Mario C., committee member
The laser cooling and magneto-optical trapping of silicon atoms were investigated experimentally. These are the first steps towards the development of a deterministic single ion source suitable for single ion implantation of a Kane quantum computer. We identified the 3s23p2 3P2 → 3s3p3 3Do3 transition at 221.74nm as a cycling transition suitable for laser cooling. We also identified the 3s23p2 1D2 → 3s3p3 3Do3 at 256.26nm as a repump transition coupling a lower metastable state with the upper cooling state. Two deep ultraviolet (DUV) laser systems were implemented to provide the cooling and repump laser light. Both systems utilized two stage second harmonic generation to quadruple the frequency of a fundamental laser to produce the DUV light. The cooling laser system utilized frequency quadrupling of a tunable cw Ti:Sapphire ring laser to produce up to 90mW at 221.74nm. The repump laser system utilized frequency quadrupling of an external cavity diode laser to produce up to 35mW at 256.26nm. A silicon atomic beam source operating at 1400°C was developed that produced a beam of free silicon atoms for laser studies. The atomic beam characteristics were analyzed, and the velocity distribution was manipulated via laser cooling. Careful spectroscopic studies were performed on the cooling and repump transitions. Frequency references for the DUV lasers were investigated in Te2 and I2 with Doppler free saturated absorption spectroscopy, using the first doubling stage output of the cooling and repump laser, respectively. Specific hyperfine components of the molecular transitions in Te2 and I2, suitable for frequency references, were identified and measured. Locking of the cooling laser on the Te2 reference was demonstrated. A magneto-optic trap (MOT) was implemented in the silicon atomic beam. A CCD optical system to image the fluorescence from atoms in the MOT was developed and achieved single atom ii detection capability. MOT trapping of silicon atoms was attempted. The low flux of atoms in the MOT velocity capture range precluded any observation of trapped atoms. A Zeeman slower, based on a novel design utilizing a variable pitch helical solenoid, was designed, simulated, and constructed to improve the flux of slow atoms. No magneto-optic trap was observed due to insufficient laser power for simultaneous Zeeman slowing and magneto-optic trapping. Investigations were performed for one dimensional laser cooling, via a Zeeman slower, along the atomic beam motion direction. Atomic beam velocity distribution profiles were observed to be modified when the Zeeman slower was on. The parameter space of Zeeman slower currents, laser power and detuning, was explored. A simulation of the atom motion over the 1m long flight path under the influence of the Zeeman slower was carried out and found to agree with the observed results.
Open Access
Modeling of laser-created plasmas and soft x-ray lasers
(Colorado State University. Libraries, 2010) Berrill, Mark Allen, author; Rocca, Jorge J., advisor; Marconi, Mario C., committee member; Menoni, Carmen S., committee member; Lee, Siu Au, committee member
This dissertation describes the development of computer models to simulate laser created plasmas used to generate soft x-ray lasers. These compact short wavelength lasers have substantial average powers and very high peak brightness, that make them of significant interest for many applications. A better understanding of the plasmas is necessary to advance the development of these lasers into more compact, efficient, and higher power sources of coherent soft x-ray light. The plasma phenomena involved are complex, and require a detailed computer model of the coupled magneto-hydrodynamic and atomic physics processes to simulate their behavior. The computer models developed as part of this work consist of hydrodynamic equations, coupled with an atomic model, radiation transport, and a ray propagation equation. The models solve the equations in a 1.5D or 2D approximation, and predict the spatio-temporal plasma variation of the parameter s, including the electron density and temperature, and the ion populations, which are then used to compute the population inversion and the resulting laser gain. A 3D post processor ray trace code was developed to simulate the amplification of stimulated emission along the plasma column length including saturation effects. This allows for the direct calculation of the soft x-ray laser output and its characteristics. Simulation results were compared with experiments conducted at Colorado State University. The general behavior of the plasma and the soft x-ray laser are well described by the model. A specific comparison of the model results with experimental measurements is presented for the case of a collisionally excited 13.2 nm wavelength Ni-like cadmium laser. The model predicts that an optical laser pulse of 1 J energy and 8 ps duration impinging at 23 degrees grazing incidence into a pre-created laser plasma can rapidly heat it to temperatures above 600 eV at a density of 2 x 1020 electrons/cm3. This results in a computed peak small signal gain coefficient of 150 cm-1 in the 4d 1S0 to 4p 1P1 transition of Ni-like Cd at 13.2 nm. The model indicates that the amplified beam reaches the gain-saturated regime after 2.5 mm of propagation in the plasma, in agreement with the experimental observation of saturated behavior for propagation lengths of 2.5-3.0 mm. The computed soft x-ray laser pulse width of 5-9 ps moderately exceeds the experimental value of 5 ps and is the result of a stronger saturation broadening in the simulation. The simulated laser output energy of the order of 1 μJ is also in agreement with experiments. Simulations of injection-seeded Ne-like Ti and Ni-like Ag amplifiers that show very good agreement with the experimental results are presented. A direct comparison of the pulsewidth and the near and far-field beam profiles is made. Finally, the results of a simulation of a plasma created by irradiation of solid targets with a 46.9 nm soft x-ray laser, in which single photon photoionization is the dominant energy absorption mechanism are presented. Low absorption (silicon, Z=14) and high absorption (chromium, Z=24) targets were heated by ~1 ns duration soft x-ray laser pulses. The experimental spectra agree with 1 ½ D simulations in showing that the Si plasmas are significantly colder and less ionized than the Cr plasma, confirming that in contrast to plasmas created by visible wavelength lasers the plasma properties are largely determined by the absorption coefficient of the target material.
Open Access
Quantum dot and polymer sensitization of single crystal titanium dioxide electrodes
(Colorado State University. Libraries, 2011) Sambur, Justin, author; Parkinson, Bruce A., advisor; Maciel, Gary E., committee member; Elliott, C. Michael, committee member; Van Orden, Alan K., committee member; Marconi, Mario C., committee member
The morphology of semiconductor nanocrystals or quantum dots (QDs) and conjugated polymers at the interface of TiO2 is expected to play an important role in the electron injection efficiency of mesoporous sensitized solar cells (SSCs). Atomic force microscopy (AFM) and photocurrent spectroscopy were employed to correlate the interfacial morphology of QDs and polymers with the sensitized photocurrent yields on planar TiO2 single crystal electrodes. QDs prepared by the ex situ ligand exchange method, whereby 3-mercaptopropionic acid (MPA)-capped QDs were synthesized and directly adsorbed onto bare TiO2 single crystals, resulted in both reproducible sensitized photocurrents and predominantly single layer surface coverages. Photoluminescence (PL) and photocurrent measurement techniques were simultaneously employed to detect electron injection from QDs to TiO2 for a variety of long and short alkyl chain capping ligands. Quenching of the PL lifetime, often interpreted as a spectroscopic signature for electron transfer, was observed for QDs capped with long chain ligands that do not produce sensitized photocurrent. The ex situ ligand exchange procedure was also utilized to adsorb single layers of MPA-capped CdSe/ZnS core/shell (CS) and PbS QDs onto single crystal TiO2 electrodes. Despite a potential energy barrier for photo-excited carriers in the CdSe core imposed by the wide band gap ZnS shell, type-I CS QDs effectively sensitized single crystal TiO2 electrodes and continued to operate in a regenerative mode in an aerated, corrosive iodide electrolyte for more than 20 h. PbS quantum dots adsorbed on TiO2 single crystals exhibited for the first time hot electron injection from higher QD excited states and absorbed photon-to-current efficiencies greater than 100% due to multiple exciton collection. The nanoscale morphology and photoactivity of conjugated polyelectrolytes (CPEs) deposited from different solvents onto single crystal TiO2 was investigated with atomic force microscopy (AFM) and photocurrent spectroscopy. Absorbed photon-to-current efficiencies approaching 50% were measured for CPE layers as thick as 4 nm on TiO2. The research herein suggests that controlling surface morphology of QD and polymer sensitizers may lead to the development of inexpensive, high-efficiency sensitized solar cells.
Open Access
Relativistic plasma nano-photonics for ultra-high energy density physics
(Colorado State University. Libraries, 2014) Purvis, Michael Anthony, author; Rocca, Jorge J., advisor; Yalin, Azer P., committee member; Menoni, Carmen S., committee member; Marconi, Mario C., committee member
The trapping of femtosecond laser pulses of relativistic intensity deep within ordered nanowire arrays is shown to volumetrically heat near solid density matter transforming it into ultra-hot highly ionized plasmas. The plasmas were generated by focusing intense ~ 60 femtosecond duration ultra-high-contrast laser pulses onto targets consisting of arrays of densely packed vertically aligned nanowires 35-80 nm diameter. X-ray spectra are presented showing that irradiation of Ni and Au nanowire arrays heats a plasma volume several µm in depth to reach extraordinarily high degrees of ionization (i.e. 26 times ionized Ni , 52 times ionized Au), in the process generating gigabar level pressures. Electron densities nearly 100 times greater than the typical critical density and multi-keV temperatures are achieved using laser pulses of only 0.5 J energy. The large plasma volume and high electron density lead to an increased hydrodynamic-to-radiative lifetime ratio that results in a significant increase in X-ray yield. Measurements from a filtered photodiode array reveal a 100X increase in emission with respect to polished flat targets for photons with energies greater than 9keV. Scaling to higher laser intensities promises to create plasmas with temperatures and pressures approaching those in the center of the sun.
Open Access
Simultaneous trapping of 85Rb & 87Rb in a far off resonant trap
(Colorado State University. Libraries, 2010) Gorges, Anthony R., author; Roberts, Jacob Lyman, advisor; Leisure, Robert Glenn, 1938-, committee member; Eykholt, Richard Eric, 1956-, committee member; Marconi, Mario C., committee member
The experiments described in this thesis were focused on the physics of simultaneous trapping of 85Rb and 87Rb into a Far Off Resonant Trap (FORT), with a view towards the implementation of a non-evaporative cooling scheme. Atoms were first trapped in a Magneto Optical Trap (MOT) and from there loaded into the FORT. We investigated the effects of loading the FORT from a MOT vs. an optical molasses; observing that the molasses significantly improved the trapped atom number. The ultimate number of atoms trapped is determined by a balance between efficient laser cooling into the FORT and light-assisted collisional losses from the FORT. We have studied and measured the loss rates associated with light-assisted collisions for our FORT, measuring both heteronuclear and homonuclear collisions. It was discovered that induced long range dipole-dipole interactions between 85Rb and 87Rb have a significant impact on FORT loading. This interaction interferes with the loading into the trap and thus limits the number of atoms which can be trapped in the FORT under simultaneous load conditions. Despite this limitation, all required experimental parameters for our future measurements have been met. In addition to these FORT studies, we have found a technique which can successfully mitigate the effects of reabsorption in optically thick clouds, which is a limitation to the ultimate temperature an atom cloud will reach during light-based cooling. Planned future measurements for this project include the creation of a variable aspect ratio FORT; along with investigating collision assisted Zeeman cooling.
Open Access
Single-shot flash imaging using a compact soft x-ray microscope
(Colorado State University. Libraries, 2012) Carbajo, Sergio, author; Menoni, Carmen S., advisor; Rocca, Jorge J., committee member; Marconi, Mario C., committee member; Krapf, Diego, committee member; Van Orden, Alan K., committee member
Microscopes extend the ability of our eyes to see objects at micro- and nanoscales. There are applications, however, for which a static image is not sufficient, and thus require information on the dynamics before a process can be understood and controlled. Therefore, the visualization of nanoscale dynamics in real-space can significantly contribute to the understanding of nanoscale processes and to accelerate the development of new nanodevices. Today, there is a need for practical microscopes capable of delivering nanometer spatial resolution and ultrafast temporal resolution in order to readily visualize any arbitrary nanoscale phenomenon. Conventional visible light microscopes can visualize ultrafast dynamics but are inherently limited in spatial resolution to about 200 nm. Alternatively, transmission electron microscopes can routinely provide atomic spatial resolutions of static samples. Probing dynamics is possible using stroboscopic schemes with nanosecond temporal resolution or scanning methods which can obtain femtosecond temporal resolution at the expense of hours-long image acquisition times. Soft x-rays (SXR) microscopes provide the ability to resolve at the nanoscale and at the same time image dynamics with nanosecond to picosecond time resolution. Pioneering work has been carried out using synchrotron illumination that has allowed to study repetitive phenomena in magnetic materials. There are however processes that are statistically reproducible but individually non-recurring that require SXR flash illumination to capture their dynamics. SXR flash imaging requires a large number of photons per pulse to illuminate the sample (about 10E12 photons per pulse). There are two types of SXR sources presently available which offer such high peak brightness: free electron lasers (FEL) and table-top SXR lasers. FELs have been used to probe dynamics using holographic and diffractive imaging configurations. This thesis describes the first demonstration of real-space flash imaging using a compact SXR laser operating at a wavelength of 46.9 nm. A sequence of flash images obtained with the full-field SXR microscope with a spatial resolution of 50 nm and temporal resolution of 1.5 ns captured the interaction dynamics of a rapidly oscillating magnetic tip in close proximity to a magnetized surface. The interaction of the tip and the stray magnetic fields led to changes in the amplitude of the tip oscillation as small as 30 nm. Modeling of the interaction assuming an undamped perturbed harmonic oscillator corroborate the experimental results. The use of compact plasma-based SXR lasers operating at wavelengths down to 10.9 nm will allow to capture flash images and render animations of picosecond phenomena with a few nanometers accuracy on a table-top.
Open Access
Soft x-ray laser interferometry of dense colliding plasmas and plasma jets
(Colorado State University. Libraries, 2010) Grava, Jonathan, author; Rocca, Jorge J., advisor; Menoni, Carmen S., committee member; Marconi, Mario C., committee member; Lundeen, Stephen R., committee member
This dissertation describes the study of dense plasmas created by laser irradiation of solid targets of various geometries using soft x-ray laser interferometry and hydrodynamic simulations. High contrast interferograms yielded electron density maps describing the evolution of several different plasmas that due to their high density gradients cannot be probed with optical lasers. In a first series of experiments, the evolution of dense aluminum plasmas produced by laser irradiation of 500 µm diameter semi-cylindrical targets was studied. Plasmas created heating the cavity walls with optical laser pulses of ~ 1×1012 W cm-2 peak intensity and 120 ps duration were observed to expand and converge on axis to form a localized high density plasma focus. Electron density maps were obtained using a 46.9 nm wavelength table-top capillary discharge soft x-ray laser probe in combination with an amplitude division interferometer based on diffraction gratings. The measurements showed that the plasma density on axis exceeds 1×1020 cm-3. The electron density profiles were compared with simulations conducted using the hydrodynamic code HYDRA, which showed that the abrupt density increase near the axis is caused by the convergence of plasma generated at the bottom of the groove and the side walls during laser irradiation. At late times in the plasma evolution, extreme ultraviolet radiation is emitted along a narrow arc outside the cavity. Complementary measurements of this lower electron density region were performed using optical interferometry. These measurements combined with two dimensional hydrodynamic simulations show that the emission results from a long lasting collisional shock that arises from the collision of counter-streaming plasmas originating from within the cavity and the surrounding flat walls of the target. The shock is sustained through tens of nanoseconds by the continuous arrival of plasma ablated by radiation emitted by the plasma itself. A second series of experiments was conducted to study collisional aluminum plasma jets created by optical laser irradiation of triangular grooves at an intensity of 1×1012 W cm-2. The dynamics and formation mechanism of dense plasma jets are of interest for basic plasma physics understanding and in some cases astrophysics, and motivate this series of experiments. Series of high-contrast soft x-ray laser interferograms obtained with the 46.9 nm laser mapped the plasma density evolution of an initially narrow plasma jet that expands along the symmetry plane and evolves into a broader plasma plume with significant side lobes. Simulations reveal that the jet formation is initiated by accelerated material ablated from the vertex of the cavity and is augmented by the continual sequential arrival of wall material along the symmetry plane, where it collides and is redirected outward. Radiative cooling is identified as an important process in maintaining the collimation of the jet. The last series of experiments demonstrates that high electron density (ne > 1.5×1021 cm-3) plasma jets can be generated by irradiation of cone targets with a laser pulse of only ~ 1 J energy. Two dimensional electron density maps of jets created by irradiation of conical Cu targets at an intensity of 6×1012 W cm-2 were obtained using soft x-ray laser interferometry. Radiation hydrodynamic simulations agree with experiments in showing that very high jet densities can be obtained with these relatively low laser pulse energies. The simulations revealed that radiation cooling of the plasma plays an important role in the jet evolution and collimation. These results further establish the use of soft x-ray laser interferometry as a powerful diagnostic tool for dense plasmas.