Daniel Younis

The propagation of intense, partially spatially coherent laser beams in a medium with instantaneous third-order susceptibility is studied analytically and numerically. For sufficiently high power relative to that required for nonlinear self-focusing, the propagation initially proceeds in two stages. In the first stage, spatial coherence builds up, and in the second stage, the number of speckles reduces. Once the degree of coherence is sufficiently high, whole-beam self-focusing occurs. The beam power is mostly confined within the initial spot radius. Two analytical approaches for describing the evolution of the beam are presented. The method of moments leads to an analytical solution for the rms spot radius that is in excellent agreement with simulations. This method does not require any knowledge of the field statistics beyond the initial conditions and provides no information about the evolution of the individual speckles. The other approach employs a self-similar solution for the second-order coherence function of the field and assumes that the fourth-order coherence function is factorizable and obeys complex circular Gaussian random statistics. The latter method also leads to an analytical expression for the spot radius, but its predictions for the qualitative evolution of the speckles disagree with wave-optics simulations.

The major goal of the project was to investigate the nonsequential ionization dynamics of atomic systems with two active electrons under intense laser irradiation. In order to gain insights into such nonlinear dynamical systems, one must resort to clever numerical methods due to the poor scaling of computational memory and time. Accordingly, we have extended the virtual detector method of Feuerstein and Thumm [J. Phys. B 36, 707 (2003)] by incorporating quasi-classical “virtual” particles that evolve alongside the Schrödinger wavefunction. A major and recent part of this research effort was to further extend the virtual-detector method to model a two-active-electron atomic system.

An energy equilibrium arises when an electron-positron pair plasma is generated by the nonlinear Breit-Wheeler interaction of a γ -ray bunch with laser photons and becomes radiatively confined. Ordinarily, ponderomotive scattering prevents leptons from accessing the high-field regions where quantum electrodynamic (QED) effects dominate, but if quantum (stochastic) radiation reaction is sufficiently strong, a balance of acceleration and recoil forces emerges. This unique scenario enables leptons to continuously mediate the transformation of laser photons into directed γ -rays via inverse Compton scattering as they copropagate with the field. Importantly, it is observed that the lepton energy stabilizes while the γ energy increases in direct proportion to laser energy depletion. The proposed scheme enables fundamental tests of QED, and is investigated here using the particle-in-cell method in full dimensionality, in addition to test-particle calculations. The per-species energy conversion efficiency is analyzed as a function of the initial laser and γ -beam properties.

In this work, we employ singular-value decomposition to generate two types of partially coherent phase screens, exhibiting either polynomial or Gaussian decay of correlation. Ensembles of 10,000 phase screens are realized at a kHz rate using a deformable mirror unit, and it is demonstrated using a Shack–Hartmann wavefront sensor and beam quality measurements that the statistics are distinguishable in their correlation decay. The goodness-of-fit (R²) difference between the ideal and empirical correlation matrices is of order 10%. An analytical expression for the effective beam quality due to a Gaussian phase screen is presented and found to be in excellent agreement with empirical data. This study extends existing capabilities and can enable applications of partially coherent beam shaping, such as turbulence mitigation and incoherent soliton generation in nonlinear media.

Efficient helicity transfer from Poincaré fields to electrons of hydrogenic ions is revealed for the first time by four-dimensional relativistic simulations. The magnetic multipole class of Poincaré fields is chosen due to its fundamental role in light-matter spin coupling, and the calculation is demonstrated for Ne^{9+} ion irradiated by single and multimode x-ray pulses. Photoelectrons of both helicities emerge synchronously from the ion ensemble, and their directionality is controllable through the radiation mode numbers. The helicity density distributions display novel structures composed of jets, spirals, and rings, among others, that are unique to the combination of atomic and field parameters. Our approach to generate spin-polarized leptons using Poincaré fields may provide a new platform for helicity characterization based on advanced numerical capabilities.

Genuine multipartite entanglement is crucial for quantum information and related technologies, but quantifying it has been a long-standing challenge. Most proposed measures do not meet the “genuine” requirement, making them unsuitable for many applications. In this work, we propose a journey toward addressing this issue by introducing an unexpected relation between multipartite entanglement and hypervolume of geometric simplices, leading to a tetrahedron measure of quadripartite entanglement. By comparing the entanglement ranking of two highly entangled four-qubit states, we show that the tetrahedron measure relies on the degree of permutation invariance among parties within the quantum system. We demonstrate potential future applications of our measure in the context of quantum information scrambling within many-body systems.

Entanglement is known to decay even in isolated systems, an effect attributed to spontaneous emission. This fragility of entanglement can be exacerbated by entanglement sudden death (ESD), where entanglement drops to zero abruptly within a finite time. It is natural to assume that multipartite entanglement is more vulnerable to ESD, as it involves more parties experiencing spontaneous emission. In this work, we challenge this assumption and present a contrasting observation, which provides compelling evidence that multipartite entanglement demonstrates increased robustness against ESD from spontaneous emission.

Evolutionary computation is a powerful optimization method that is well-suited for high-dimensional parameter spaces. In this work, we apply it to an integral task in optical physics, namely, to the problem of programming a deformable mirror to generate laser beams that are partially coherent in space. This involves precise tuning of (on the order of) tens of thousands of phase values in the interval [-π, π) such that they exhibit defined second-order statistical characteristics. We discuss the implementation of two primary methods, the evolution of phase values and of a correlating linear transformation, and provide results based on a Gaussian covariance function. We show that the first-order statistical properties are fixed by the method of choice, which we suggest may be overcome by a multi-objective evolutionary calculation involving symbolic transformations. This is a largely unexplored domain in which stochastic processes, such as genetic algorithms, are superior. Lastly, we discuss the inherently physical limitations that prevent certain types of phase statistics from being realized, even with the genetic algorithm approach.