Atomic and molecular processes initiated and controlled by intense, ultrashort laser pulses are a subject of steadily growing interest in plasma science and related technology. The laser-induced ionization and excitation of individual atoms and molecules beget in turn various nonlinear optical phenomena in the plasma medium. One such phenomenon, laser filamentation, is enabled by self-focusing of the laser beam, which results in a drastic contraction of the laser pulse and generation of an extended filament at a controllable stand-off distance. Partially ionized plasma is left in the wake of the filamenting laser pulse; it can be probed by subsequent laser pulses and is open to various practical applications. This theoretical research addresses a new realm of transient nonlinear optics in evolving filament-wake channels; it explores longer-lasting optical consequences of strong-field ionization occurring in a relatively dense plasma. The research will advance broader understanding of means to control strong-field light-matter interaction on a microscopic scale. It also directly addresses recent developments of filament-based, multiplex methods of remote detection of complex mixtures, which is a challenge with many technological applications. It is related to control of filamentation dynamics with the aim of remote lasing in the atmosphere, and to developing new sources of attosecond and X-ray pulses.
This research concerns the interrelated dynamics of ions, energetic free electrons, and excited neutrals in wake channels of femtosecond laser filaments, as well as the interaction of the evolving system with probe laser pulses. The expected outcomes will form a basis for effective control of filament-wake nonlinear optical effects. The task will be addressed using a combination of ab initio calculations for nonlinear response of individual ions, kinetic modeling of inhomogeneous ionized-gas medium evolution, and density matrix calculations for probe laser coupling with the medium. The objectives include: 1) developing predictive models of the evolution of ion density profiles and molecular/atomic excitation in filament wake channels and ionization gratings, which is driven by competing processes of impact ionization, impact electronic excitation of neutrals and ions, dissociative recombination, vibrational excitation, and elastic scattering; 2) calculating dynamic polarizability and hyperpolarizability coefficients of ions in the wake of the ionizing laser pulse (when perturbative approaches become applicable) by implementing the auxiliary-field approach and ab initio calculations, and applying the results to obtain evolving spatial profiles of dynamic quadratic and quartic nonlinear refractive indices in filament wake channels; and 3) predicting outcomes of nonlinear interactions of probe laser pulses with the wake channels, including two-beam coupling mediated by ionization gratings, ionic rotational revivals, and controllable spatial-spectral patterns of dynamic Rabi sideband emission. The expected results will be applied to experiments on ionization-related polarization rotation, rotational-revival spectroscopy, and dynamic Rabi sidebands in filament wake channels at the Center for Advanced Photonics Research (CAPR) at Temple University. Further, the ionization-specific nonlinear effects are related to recent experiments on bright higher harmonic generation (up to X-rays) in highly ionized gases and on igniter-heater processes in filament wake channels.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
