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Open Access Publications from the University of California

Scholarly Works (18 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2023)

With the physics of coherently upconvert ultrafast driving laser photon, the high harmonic generation process enables a tabletop laser source that is able to cover EUV to X-ray with a fully coherent attosecond burst. Its spectroscopic feature and extreme spatial and temporal resolution made it emerge as a novel tool for scientific research over the last three decades. The main limitation of this source to be is the brightness which is highly related to the phase-matching condition and the efficiency of optics after generation.The first observation of high harmonic generation is with a UV excimer laser with 248 nm over 30 years ago. This thesis presents works performed with the modern engineering design of high harmonic beamlines and a better understanding of phase-matching and physical models that were developed nearly thirty years. Chapter 3 is the selected experimental result of post-laser manipulation of laser parameters to a shorter pulse duration for more efficient high harmonic emission. Chapter 4 is a summary of our modern design of the beamline with hollow waveguide manufacturing and novel EUV to X-ray spectrometer design in both transmission and reflection mode with calculation and optimization where record-high efficiency spectrometer is designed and tested that can capture the most sensitive signal with physical significance that possibly ignored in the past. Chapter 5 presents an investigation of the feasibility of high harmonic spectrum tunability by shifting the spectrum of the driving laser in both directions to enable the spectroscopic study of metal resonance absorption edge. Finally, Chapters 6 and 7 present experimental results of the 400nm UV laser driving high harmonic with the robust parameter space for bright emission in the UV and X-ray regime for future application. The stable and robust harmonic emission is used as a platform for the spectroscopic study of the possible correlation effect between electrons and ions. For the first time, a double electron recombination process with an extended high harmonic cutoff in two-electron atoms has been experimentally observed which could pave the way for a better understanding of the few electron systems and contribute to further completing the theory of high harmonic generation.

    Cover page: Bright Coherent EUV and X-ray Light driven by UV and Visible Lasers and Attosecond-to-femtosecond Electron-Electron Correlation Spectroscopy and Recombination Physics
    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2023)
    We demonstrate a high-energy, 1 kilohertz, Yb-based, femtosecond regenerative amplifier in a chirped pulse amplification (CPA) architecture by using a single disordered Yb:CALYO crystal, providing 125 fs pulses of 2.3 mJ energy per pulse at a central wavelength of 1039 nm. The amplified compressed pulses, with a spectral bandwidth of 13.6 nm, represent the shortest ultrafast pulse duration reported to date for any multi-millijoule class,Yb-crystalline classical CPA system without additional spectral broadening techniques. We have demonstrated an increase in the gain bandwidth proportionally to the ratio of the excited to total Yb3+ ion densities. A net wider spectrum of the amplified pulses is the result of the interplay between the increased gain bandwidth and the gain narrowing. Finally, our broadest amplified spectrum of 16.6 nm, corresponding to a 96 fs transform limited pulse, can be expanded further to support sub-100 fs pulse durations and 1-10 mJ energies at 1 kHz.
      Cover page: Multi-millijoule class, high repetition rate, Yb:CALYO regenerative amplifier with sub-130 fs pulses.Creative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2023)
        Cover page: High Performance Ytterbium Regenerative Amplifier Based on Yb:CALYO with High Energy 100 fs PulsesCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC San Diego Previously Published Works (2023)
          Cover page: Multipass Spectral Broadening of Spatially Chirped PulsesCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2016)
          We present a semi-classical study of the effects of the Lorentz force on electrons during high harmonic generation in the soft and hard X-ray regions driven by near- and mid-infrared lasers with wavelengths from 0.8 to 20 μm, and at intensities below 1015 W/cm2. The transverse extent of the longitudinal Lorentz drift is compared for both Gaussian focus and waveguide geometries. Both geometries exhibit a longitudinal electric field component that cancels the magnetic Lorentz drift in some regions of the focus, once each full optical cycle. We show that the Lorentz force contributes a super-Gaussian scaling which acts in addition to the dominant high harmonic flux scaling of λ-(5-6) due to quantum diffusion. We predict that the high harmonic yield will be reduced for driving wavelengths > 6 μm, and that the presence of dynamic spatial mode asymmetries results in the generation of both even and odd harmonic orders. Remarkably, we show that under realistic conditions, the recollision process can be controlled and does not shut off completely even for wavelengths >10 μm and recollision energies greater than 15 keV.
            Cover page: Lorentz drift compensation in high harmonic generation in the soft and hard X-ray regions of the spectrum.
            • Article
            • Peer Reviewed
            UC San Diego Previously Published Works (2018)
            We demonstrate numerically and experimentally that intense pulses propagating in gas-filled capillaries can undergo localization in space and time due to strong plasma defocusing. This phenomenon can occur below or above the self-focusing threshold Pcr as a result of ionization-induced refraction that excites higher-order modes. The constructive interference of higher-order modes leads to spatiotemporal localization and resurgence of the intensity. Simulations show that this confinement is more prominent at shorter wavelength pulses and for smaller capillary diameters. Experiments with ultraviolet pulses show evidence that this ionization-induced refocusing appears below Pcr and thus represents a mechanism for spatiotemporal confinement without self-focusing.
              Cover page: Ionization-assisted spatiotemporal localization in gas-filled capillaries.
              • Article
              • Peer Reviewed
              UC San Diego Previously Published Works (2022)
              We demonstrate a dual-optic imaging-spectrometer for high harmonic generation sources, ideal for coherent diffractive imaging in a tunable monochromatic mode. Our optimization combines record-high efficiency, high spectral resolution, low aberrations, broad spectral coverage, and polarization-maintaining.
                Cover page: Globally Optimized Monochromator for Coherent Diffractive Imaging with Tunable EUV Wavelength
                • Article
                • Peer Reviewed
                UC San Diego Previously Published Works (2016)
                Phase matching of circularly polarized high-order harmonics driven by counter-rotating bi-chromatic lasers was recently predicted theoretically and demonstrated experimentally. In that work, phase matching was analyzed by assuming that the total energy, spin angular momentum and linear momentum of the photons participating in the process are conserved. Here we propose a new perspective on phase matching of circularly polarized high harmonics. We derive an extended phase matching condition by requiring a new propagation matching condition between the classical vectorial bi-chromatic laser pump and harmonics fields. This allows us to include the influence of the laser pulse envelopes on phase matching. We find that the helicity dependent phase matching facilitates generation of high harmonics beams with a high degree of chirality. Indeed, we present an experimentally measured chiral spectrum that can support a train of attosecond pulses with a high degree of circular polarization. Moreover, while the degree of circularity of the most intense pulse approaches unity, all other pulses exhibit reduced circularity. This feature suggests the possibility of using a train of attosecond pulses as an isolated attosecond probe for chiral-sensitive experiments.
                  Cover page: Helicity-selective phase-matching and quasi-phase matching of circularly polarized high-order harmonics: towards chiral attosecond pulses
                  • Article
                  • Peer Reviewed
                  UC San Diego Previously Published Works (2014)
                  We demonstrate the highest flux tabletop source of coherent soft X-rays to date, driven by a single-stage 10 mJ Ti:sapphire regenerative amplifier at 1 kHz. We first down-convert the laser to 1.3 µm using a parametric amplifier, before up-converting it to soft X-rays using high harmonic generation in a high-pressure, phase matched, hollow waveguide geometry. The resulting optimally phase matched broadband spectrum extends to 200 eV, with a soft X-ray photon flux of > 10(6) photons/pulse/1% bandwidth at 1 kHz, corresponding to > 10(9) photons/s/1% bandwidth, or approximately a three order-of-magnitude increase compared with past work. Finally, using this broad bandwidth X-ray source, we demonstrate X-ray absorption spectroscopy of multiple elements and transitions in molecules in a single spectrum, with a spectral resolution of 0.25 eV, and with the ability to resolve the near edge fine structure.
                    Cover page: High flux coherent super-continuum soft X-ray source driven by a single-stage, 10mJ, Ti:sapphire amplifier-pumped OPA.
                    • Article
                    • Peer Reviewed
                    UC San Diego Previously Published Works (2017)
                    We investigate the plasma dynamics inside a femtosecond-pulse-induced filament generated in an argon gas for a wide range of pressures up to 60 bar. At higher pressures, we observe ionization immediately following a pulse, with up to a threefold increase in the electron density within 30 ps after the filamentary propagation of a femtosecond pulse. Our study suggests that this picosecond evolution can be attributed to collisional ionization including Penning and associative ionizations and electron-impact ionization of excited atoms generated during the pulse. The dominance of excited atoms over ionized atoms at the end of the pulse also indicates an intrapulse inhibition of avalanche ionization. This delayed ionization dynamics provides evidence for diagnosing atomic and molecular excitation and ionization in intense laser interaction with high-pressure gases.
                      Cover page: Picosecond ionization dynamics in femtosecond filaments at high pressures
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