Bahman Hafizi

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.

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.

The evolution of ultrashort laser pulses in dispersive, turbulent, nonlinear, and dissipative media is discussed in connection with nonlinear self-focusing collapse and the onset of laser filamentation. In quiescent air, a laser pulse propagating with a peak power greater than a critical power for self-focusing will undergo a catastrophic, transverse collapse until the intensity is large enough for photoionization. At this point, self-focusing is arrested and balanced by plasma refraction, forming a laser filament. By applying an appropriate chirp, the dispersive properties of the medium can be used to enhance this process and control its onset, and to counter dissipative effects such as molecular absorption and atmospheric scattering. This paper presents an analysis of the effect of atmospheric turbulence on the propagation of nonlinear pulses with dispersion compensation (chirp). The analytical results are compared with wave optics simulations and found to be in reasonable agreement as long as the pulse maintains a near-Gaussian spatiotemporal profile.

Naturally occurring self-lasing of a confined plasma discharge is used as a plasma diagnostic. Together with other readily measurable parameters such as discharge voltage and current, the laser radiation provides the necessary constraints for fitting the parameters of a plasma chemistry model. The model determines the plasma density, electron temperature and excited-state populations as functions of time and space and shows excellent agreement with experiments performed in a nitrogen-filled discharge tube. Plasma self-lasing has been observed in a form of a ring and has a plasma density profile that can be employed for optical guiding.

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.

We report on proton acceleration from intense CO2 laser-irradiated hydrogen plasmas at near-critical densities, with the density gradient steepened by Nd:YAG laser ablation-driven hydrodynamic shocks. While the experimental results, such as the quasi-monoenergetic proton spectra and their scaling with respect to the laser energy, are generally in agreement with the simulations, certain laser shots produced significantly higher proton energies than anticipated during the experiment. The increased proton energy may be linked to nonlinear propagation effects in the steepened plasma density ramp before the critical surface, including relativistic self-focusing and, for the case of temporally-structured laser pulses observed in the experiment, focusing of the trailing pulse through the plasma channel formed by the leading pulse 25 ps ahead. The occurrence of channel focusing in the underdense hydrogen plasma is supported by a subsequent pump-probe experiment with a dark-field imaging technique, where the formation of ion channels was observed after the passage of an intense CO2 laser pulse.

This paper investigates the spin characteristics of photoelectrons when hydrogen-like ions are centro-symmetrically irradiated with converging vector waves—a non-paraxial form of structured light. For a photon with given total angular momentum and third component thereof, photoelectrons with both helicities are obtained—in contrast to the fixed helicities produced by left- or right-circularly polarized light. The angular distribution of photoelectrons is broadly tunable through the radiation mode numbers, and opposite helicities can be extracted in synchronism.