The PROBIES diagnostic is a new, highly flexible, imaging and energy spectrometer designed for laser-accelerated protons. The diagnostic can detect low-mode spatial variations in the proton beam profile while resolving multiple energies on a single detector or more. When a radiochromic film stack is employed for “single-shot mode,” the energy resolution of the stack can be greatly increased while reducing the need for large numbers of films; for example, a recently deployed version allowed for 180 unique energy measurements spanning ∼3 to 75 MeV with <0.4 MeV resolution using just 20 films vs 180 for a comparable traditional film and filter stack. When utilized with a scintillator, the diagnostic can be run in high-rep-rate (>Hz rate) mode to recover nine proton energy bins. We also demonstrate a deep learning-based method to analyze data from synthetic PROBIES images with greater than 95% accuracy on sub-millisecond timescales and retrained with experimental data to analyze real-world images on sub-millisecond time-scales with comparable accuracy.

We present the development of a compact Thomson parabola ion spectrometer capable of characterizing the energy spectra of various ion species of multi-MeV ion beams from >1020W/cm2 laser produced plasmas at rates commensurate with the highest available from any of the current and near-future PW-class laser facilities. This diagnostic makes use of a polyvinyl toluene based fast plastic scintillator (EJ-260), and the emitted light is collected using an optical imaging system coupled to a thermoelectrically cooled scientific complementary metal-oxide-semiconductor camera. This offers a robust solution for data acquisition at a high repetition rate, while avoiding the added complications and nonlinearities of micro-channel plate based systems. Different ion energy ranges can be probed using a modular magnet setup, a variable electric field, and a varying drift-distance. We have demonstrated operation and data collection with this system at up to 0.2 Hz from plasmas created by irradiating a solid target, limited only by the targeting system. With the appropriate software, on-the-fly ion spectral analysis will be possible, enabling real-time experimental control at multi-Hz repetition rates.

We examine the electromagnetic emission from two photo-illuminated linear arrays composed of inductively charged superconducting ring elements. The arrays are illuminated by an ultrafast infrared laser that triggers microwave broadband emission detected in the 1–26 GHz range. Based on constructive interference from the arrays a narrowing of the forward radiation lobe is observed with increasing element count and frequency demonstrating directed GHz emission. Results suggest that higher frequencies and a larger number of elements are achievable leading to a unique pulsed array emitter concept that can span frequencies from the microwave to the terahertz (THz) regime.

We present in this work the development of an ultra-compact, multi-channel x-ray spectrometer (UCXS). This diagnostic has been specially built and adapted to perform at high-repetition-rate (>1 Hz) for high-intensity, short-pulse laser plasma experiments. X-ray filters of varying materials and thicknesses are chosen to provide spectral resolution up to ΔE ≈ 1 keV over the x-ray energy range of 1-30 keV. These filters are distributed over a total of 25 channels, where each x-ray filter is coupled to a single scintillator. The UCXS is designed to detect and resolve a large variety of laser-driven x-ray sources such as low energy bremsstrahlung emission, fluorescence, and betatron radiation (up to 30 keV). Preliminary results from commissioning experiments at the ABL laser facility at Colorado State University are provided.

We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV = m. The guide was formed via self-waveguiding of < 15 J, 45 fs ( < ∼ 300 TW) pulses over 20 cm in a low-density hydrogen gas jet, with accelerated electron bunches driven up to 5 GeV in quasimonoenergetic peaks of relative energy width as narrow as ∼ 15 %, with divergence down to ∼ 1 mrad and charge up to tens of picocoulombs. Energy gain is inversely correlated with on-axis waveguide density in the range N e 0 ¼ ð 1 . 3 – 3 . 2 Þ × 10 17 cm − 3 . We find that shot-to-shot stability of bunch spectra and charge are strongly dependent on the pointing of the injected laser pulse and gas jet uniformity. We also observe evidence of pump depletion-induced dephasing, a consequence of the long optical guiding distance.

We report on design and construction of a high nanosecond and picosecond contrast kHz dual chirp pulse amplification (DCPA) system with >15 mJ/pulse, pulse duration < 35 fs, for developing an efficient MeV ion acceleration and fast neutron source.

Mg x Zn 1−x O thin films were grown as metastable alloys via a sputtering technique in order to achieve single-phase wurtzite alloys with deep-UV optical bandgaps. As-grown alloys with Mg composition range 0–72% resulted in optical bandgaps spanning the UV-range of 3.3–4.4 eV. The thermal stability of the alloys was studied via post-growth controlled annealing experiments up to 900 °C. Alloys with low Mg up to 34% were found to be highly stable and retained their optical and material properties; however, alloys with higher Mg, up to 72%, were found to be unstable and were phase separated into wurtzite and cubic structural phases with respective optical bandgaps at ~ 3.5 and 6.0 eV. Both the as-grown and annealed alloys were studied using X-ray diffraction for structural identification, transmission spectroscopy for bandgap analysis, and Raman scattering for mapping the phonon mode-behavior. The experimental value for the solubility limit was found to be ~ 30%. A straightforward model calculation based on the Raman-mode saturation behavior yielded a similar value for the solubility limit of the alloys. The results are discussed in terms of available phase-diagrams for stable-state ceramics alloys that were grown under thermodynamics equilibrium conditions.

Time resolved dynamics of a sub-micron liquid sheet interacting with a 1019 Wcm-2 kHz laser focus at 780 nm wavelength, accelerating MeV electrons and protons, was captured using shadowgraphy with synchronized 80 fs pulses.

MgZnO is emerging as a vital semiconductor-alloy system with desirable optical properties that can span a large range of the UV spectrum. Due to its benign chemical character, MgZnO is considered to be an environmentally friendly material. This paper presents studies on annealing as a useful and straightforward approach for the enhancement of the optical and crystal quality of Mg 0.17 Zn 0.83 O nanocrystalline films grown via DC sputtering. The alloys were studied via several imaging and optical techniques. It was found that high-temperature annealing, ~900 °C, in Argon atmosphere, significantly improves the solubility of the alloy. This temperature range is consistent with the thermal diffusion temperature of Mg needed for the creation of a soluble alloy. Moreover, the annealing process was found to minimize the undesirable visible luminescence, attributed to Mg and Zn interstitials, while significantly enhancing the bandgap sharpness and the efficiency of the UV-luminescence at ~3.5 eV. The analysis indicated that these optical attributes were achieved due to the combined effects of good solubility, an improved morphology, and a reduction of native defects. The annealing was also proven to be beneficial for the reduction of the compressive stress in the alloy: a relaxation ~1.8 GPa was calculated via Raman scattering. The inherent stress was inferred to originate mainly from the granular morphology of the alloys.

We present how chamber background pressure affects energetic proton acceleration from an ultra-intense laser incident on a thin liquid target. A high-repetition-rate (100 Hz), 3.5 mJ laser with peak intensity of 8 × 10^18 Wcm^-2 impinged on a 450 nm sheet of flowing liquid ethylene glycol. For these parameters, we experimentally demonstrate a threshold in laser-to-proton conversion efficiency at background pressures < 8 Torr , wherein the overall energy in ions >1 MeV increases by an order of magnitude. Proton acceleration becomes increasingly efficient at lower background pressures and laser-to-proton conversion efficiency approaches a constant as the vacuum pressure decreases. We present two-dimensional particle-in-cell simulations and a charge neutralization model to support our experimental findings. Our experiment demonstrates that high vacuum is not required for energetic ion acceleration, which relaxes target debris requirements and facilitates applications of high-repetition rate laser-based proton accelerators.

ZnO is an efficient luminescent material in the UV-range similar to 3.4 eV with a wide range of applications in optical technologies. Sputtering is a cost-effective and relatively straightforward growth technique for ZnO films; however, most as-grown films are observed to contain intrinsic defects which can significantly diminish the desirable UV-emission. In this research the defect dynamics and optical properties of ZnO sputtered films were studied via post-growth annealing in Ar or O-2 ambient, with X-ray diffraction (XRD), imaging, transmission and Urbach analysis, Raman scattering, and photoluminescence (PL). The imaging, XRD, Raman and Urbach analyses indicate significant improvement in crystal morphology and band-edge characteristics upon annealing, which is nearly independent of the annealing environment. The native defects specific to the as-grown films, which were analyzed via PI, are assigned to Zn-i related centers that luminesce at 2.8 eV. Their presence is attributed to the nature of the sputtering growth technique, which supports Zn-rich growth conditions. After annealing, in either environment the 2.8 eV center diminished accompanied by morphology improvement, and the desirable UV-PL significantly increased. The 02 ambient was found to introduce nominal O-i centers while the Ar ambient was found to be the ideal environment for the enhancement of the UV-light emission: an enhancement of similar to 40 times was achieved. The increase in the UV-PL is attributed to the reduction of Zn-i-related defects, the presence of which in ZnO provides a competing route to the UV emission. Also, the effect of the annealing was to decrease the compressive stress in the films. Finally, the dominant UV-PL at the cold temperature regime is attributed to luminescent centers not associated with the usual excitons of ZnO, but rather to structural defects. (C) 2016 The Authors. Published by Elsevier B.V.

Phonon decay channels and anharmonicities in embedded domains in MgZnO film were studied via selective resonant Raman scattering at the elevated temperature regime up to 870 K. The resonant conditions provided by excitation with different ultraviolet laser lines enabled the detection of longitudinal optical (LO) phonons from domains with the wurtzite structure, and domains with the cubic rocksalt structure which lack inversion symmetry due to alloying. The phonon behavior was modeled in terms of three- and four-phonon decay processes using Ridley and Klemens type decay processes. It is found that the wurtzite phase displays dominantly three-phonon decay with a small four-phonon component. In contrast, the cubic phase displays a higher degree of anharmonicity in which the four-phonon processes contribute significantly to the temperature dependent frequency shift. At the elevated temperature range, the LO frequency shift rate is ∼−2.6 × 10−2 cm−1/K for the wurtzite structure while that of the cubic exhibits a much larger shift rate of ∼−1.6 × 10−1 cm−1/K. The larger anharmonicity of the domains with the cubic structure is discussed in terms of strain and deformation effects.

High-order optical phonon modes in wurtzite ZnO and MgZnO nanocrystals were investigated using resonant Raman spectroscopy. The Raman modes in ZnO showed agreement with a harmonic model, consistent with prior work. MgZnO, in contrast, exhibited a large degree of anharmonic behavior. The strength of the anharmonicity increases upon application of hydrostatic pressure. These results suggest a harmonic-to-anharmonic phase transition, which could be related to the anisotropy ($c$/$a$ ratio) of the crystal structure.

The issue of phase segregation, inherent to the MgZnO alloy system, was investigated via selective resonant Raman scattering. We demonstrate that it is a highly sensitive technique for the detection of embedded structural inhomogeneities. MgZnO thin-films with bandgaps that span the UV-range of 3.2–5.7 eV were realized. Under resonant conditions facilitated via different laser excitation energies, the LO-phonon behavior indicated that the phase segregation is in the range of 35%–65% Mg, in which domains of hexagonal-wurtzite and cubic-NaCl structures coexist. The scattering of the forbidden LO-mode of the cubic phase is discussed in terms of inversion-symmetry relaxation due to alloying.

Experiments to generate neutrons from the 7Li(p,n)7Be reaction with 60 J, 180 fs laser pulses have been performed at the Texas Petawatt Laser Facility at the University of Texas at Austin. The protons were accelerated from the rear surface of a thin target membrane using the target-normal-sheath-acceleration mechanism. The neutrons were generated in nuclear reactions caused by the subsequent proton bombardment of a pure lithium foil of natural isotopic abundance. The neutron energy ranged up to 2.9 MeV. The total yield was estimated to be 1.6 × 107 neutrons per steradian. An extreme ultra-violet light camera, used to image the target rear surface, correlated variations in the proton yield and peak energy to target rear surface ablation. Calculations using the hydrodynamics code FLASH indicated that the ablation resulted from a laser pre-pulse of prolonged intensity. The ablation severely limited the proton acceleration and neutron yield.

ZnO is emerging as one of the materials of choice for UV applications. It has a deep excitonic energy level and a direct bandgap of ∼3.4 eV. Alloying ZnO with certain atomic constituents adds new optical and electronic functionalities to ZnO. This paper presents research on MgxZn1-xO and ZnS1-xOx nanocrystalline flexible films, which enable tunable optical properties in the deep-UV and in the visible range. The ZnO and Mg0.3Zn0.7O films were found to have bandgaps at 3.35 and 4.02 eV, respectively. The photoluminescence of the Mg0.3Zn0.7O exhibited a bandedge emission at 3.95 eV, and at lower energy 3.38 eV due to the limited solubility inherent to these alloys. ZnS0.76O0.24 and ZnS0.16O0.84 were found to have bandgaps at 3.21 and 2.65 eV, respectively. The effect of nitrogen doping on ZnS0.16O0.84 is discussed in terms of the highly lattice mismatched nature of these alloys and the resulting valence-band modification.

MgxZn1−xO is a promising alloy system with UV-tunable bandgap. The alloy can have the hexagonal or cubic structure depending on the composition x and growth conditions. We present studies of the optical and material properties of Mg0.1Zn0.9O and Mg0.6Zn0.4O sintered ceramics. The rationale for choosing these compositions is that alloys of both the wurtzite and the cubic phases, respectively, can be investigated. To study the alloying dynamics for the optimization of light emission, the properties as a function of annealing temperature in the range of 600–1,100 °C were investigated via micro-photoluminescence, X-ray diffraction, and imaging techniques. For the Mg0.1Zn0.9O it was found that a threshold temperature of ~900 °C is required in order to initiate the formation of the solid solution with the wurtzite structure. At the elevated temperature regime, the photoluminescence energy for this ceramic sample shifted from 3.25 to 3.5 eV, while the ceramic retained the wurtzite structure. The Mg0.6Zn0.4O was found to have a sequence of phases: initially the alloy formed with the wurtzite structure, and then a transition into the NaCl cubic structure took place. Similar to the Mg0.1Zn0.9O, a threshold temperature of ~900 °C was required in order to initiate the alloying process. At that temperature regime an alloy was formed with the wurtzite structure, and its photoluminescence energy was ~3.25 eV. At ~1,100 °C the alloy was found to undergo a phase transition from wurtzite to cubic structure. The photoluminescence energy at that temperature was considerably shifted into the UV-range of 4 eV.

We report apparent robust doping of ZnO and MgxZn1-xO (x ~20%) nanoparticle films by annealing in hydrogen gas. The annealing was done at sequentially higher temperatures from about 20 °C to 140 °C. The effect of the annealing was determined by comparing current-voltage measurements of the samples at room-temperature and in vacuum after each annealing cycle. The nanoparticles were grown using an aqueous solution and heating process that created thin-films of ZnO or MgZnO nanoparticles with diameters of about 30 nm. When exposed to hydrogen gas at room-temperature or after annealing at temperatures up to about 100 °C, no measureable changes to the room-temperature vacuum conductivity of the films was observed. However, when the samples were annealed at temperatures above 100 °C, an appreciable robust increase in the room-temperature conductance in vacuum occurred. Annealing at the maximum temperature (~135-140 °C) resulted in about a factor of about twenty increase in the conductivity. Furthermore, the ratio of the conductance of the ZnO and MgZnO nanoparticle films while being annealed to their conductance at room-temperature were found to increase and then decrease for increasing annealing temperatures. Maximum changes of about five-fold and seven-fold for the MgZnO and ZnO samples, respectively, were found to occur at temperatures just below the annealing temperature threshold for the onset of the robust hydrogen gas doping. Comparisons of these results to other work on bulk ZnO and MgZnO films and reasons for this behavior will be discussed.