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ItemOn-shot Spatiotemporal Laser Wavefront Characterization via Wavelength-Multiplexed Holography for Precision Control of High-Intensity Laser Plasma Interactions(Georgia Institute of Technology, 2022-04-14) Grace, Elizabeth S.Short-pulse (<10 ps), high-intensity (>10^18 W/cm^2) laser systems can be used to generate and probe extremely high energy and density conditions of matter. The plasmas produced by these high intensity laser systems can act as compact radiation sources, can emulate astrophysical phenomena like supernova and centers of giant planets, and even allow access to fusion regimes. Besides fundamental plasma physics, these ultraintense laser-matter interactions also lend themselves to impactful scientific applications, including renewable energy and state-of-the-art medical techniques. A continuing fundamental need in the field of laser-driven High Energy Density (HED) and plasma physics is the accurate and precise spatial and temporal characterization of the laser pulse, which would provide valuable insight into the foundational physics that drive these interactions. Laser-plasma interactions are complex, rapidly evolving, and highly sensitive to shot-to-shot variations in laser parameters, such as laser peak intensity, pulse duration, pre-pulse, and focal spot, and/or thermal instabilities. Even under nominally identical laser conditions, small variations can drastically influence outcomes. However, in typical HED experiments currently, the complexity of the 4-D laser electric field E(x,y,z,t) can mean that the wavefront is not often characterized, or that it is characterized in a surrogate shot. This thesis discusses the development of a single-frame laser characterization diagnostic for novel use on high-intensity, low repetition rate laser systems, its adaptation to high-repetition rate (>Hz) laser systems, its use in diagnosing and optimizing an ultraintense laser system, and its role in developing a more complete understanding of the underlying laser-plasma interaction physics. Breakthroughs in these measurement capabilities can unlock an entirely new regime of experimental measurement, deliver a novel capacity to determine and assess pivotal factors that limit more precise control of laser-driven radiation sources, and serve as a necessary tool to improve laser-plasma interaction predictive capability.
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ItemCOMPLETE TEMPORAL MEASUREMENT OF LOW-INTENSITY AND HIGH-FREQUENCY ULTRASHORT LASER PULSES(Georgia Institute of Technology, 2020-12-08) Jones, Travis N.Two important frontiers in the field of ultrashort pulse measurement are the complete temporal measurement of low-intensity picosecond pulses in the near-infrared (NIR) and intense ultrashort pulses in the ultraviolet (UV) and extreme ultraviolet (EUV). The former are projected to find great use in the field of optical telecommunications, while the latter are the result of the relatively recent development of bright, coherent light sources in this wavelength range. The challenge in measuring weak pulses in the NIR is that many measurement techniques require expensive electronics and/or are complicated and difficult to align. UV pulse measurement on the other hand, due to the higher photon energies involved, is primarily hindered by slow light-matter interactions such as absorption or photoionization. In this thesis, we develop and present two novel pulse-measurement techniques based on the widely-used method of Frequency-Resolved Optical Gating (FROG) which are aimed at addressing these challenges. The first technique, called Collinear GRENOUILLE, is an experimentally-simple and sensitive device which tests the sensitivity of second harmonic generation to measure low-intensity, picosecond pulses in the NIR. The measurement capabilities of Collinear GRENOUILLE are experimentally demonstrated by the successful measurement moderately complex pulses with femtojoule pulse energies at 800 nm. A similar measurement is also presented at 1030 nm where more efficient nonlinear crystals exist. The second technique, known as Induced-Grating Cross-correlation FROG, is designed to measure intense laser pulses in the UV and EUV. To demonstrate this technique, we first perform measurements of chirped 400 nm pulses in a fast-responding nonlinear medium. We show that the resulting traces contain the complete electric field of the UV pulse we intended to measure. We further confirm these measurement by developing a modified phase-retrieval algorithm to reconstruct the pulse from the measured traces. Next, we performed similar measurements in a slowly-responding medium. FROG typically requires a fast nonlinear-optical processes to measure pulses, however once the response of the medium in accounted for, the measurements made using the IG XFROG technique indicate that accurate measurements of the pulse can still be made using a slow light-matter interaction. In the case of slow media, the IG XFROG technique is first demonstrated using absorption from amplified pulses at 400 nm and 267 nm. After establishing feasibility at these wavelengths, we applied this technique to EUV laser pulses from the FERMI free-electron laser at 31.3 nm, for which the dominant light-matter interaction is photoionization.
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ItemReliable Pulse-Retrieval for Frequency-Resolved-Optical-Gating(Georgia Institute of Technology, 2020-08-19) Jafari, RanaFrequency-resolved-optical-gating (FROG) measures the full temporal information of arbitrary ultrashort laser pulses' fields using two-dimensional data in the time-frequency domain. By benefiting from the properties of the acquired 2D data, FROG not only measures stable pulses accurately, but also it indicates the presence of pulse-shape instability in the pulse train by a discrepancy between the measured and retrieved FROG traces using its pulse-retrieval algorithm. However, this discrepancy could also arise when the pulse-retrieval algorithm stagnates. When the current best algorithm, generalized projections (which is known to stagnate as much as half the time) is used to retrieve the field from a trace of unstable pulse trains, the retrieved fields may vary significantly from one retrieval to the other, while never returning a pulse that matches the measured trace. So, it is difficult to distinguish the presence of instability from the stagnation of the algorithm. In this dissertation, the Retrieved-Amplitude N-grid Algorithmic (RANA) approach is introduced for the 100% reliable pulse-retrieval from FROG traces of stable pulses with even high complexities and significant noise for the commonly used FROG geometries. Further, the performance of the RANA approach for traces contaminated with artifacts due to averaging over a pulse train with unstable pulse shapes is investigated. The results indicate that the RANA approach returns pulses that yield the lowest achievable discrepancy between the measured and retrieved traces for the traces of unstable trains. Thus RANA approach convincingly indicates the stability or instability of the pulse train, solving the problem. It also yields the correct approximate pulse length, spectral width, and time-bandwidth product, including some structure when present.