Controllable transient excitation gratings in crossing-beams filament wake channels in a dense gas

When femtosecond laser filamentation occurs in a dense gas, electron-collisional processes lead to extensive excitation of the constituent atoms or molecules. This dense-gas situation corresponds to either high-pressure gases or longer carrier wavelengths. When filamentation takes place in an intersection area of two laser beams crossing at a small angle, the transient interference patterns predicate formation of ionization and excitation gratings, which can be controlled by temporal characteristics of the laser pulses. During the pulse, the strong-field ionization of constituent atoms/molecules competes with impact ionization and collisional excitation by energetic free electrons, which are driven by the oscillating laser field and gain considerable energy via inverse Bremsstrahlung process while scattering on neighboring neutral atoms. The complex interplay of these processes determines the transverse grating profiles of the ionic and excited-atom densities, as well as of the electron density and temperature at the end of the pulse. Using a kinetic model of these processes, we explore sensitivity of the ionization and excitation grating profiles in high-pressure argon gas to the envelope shape of the driving laser pulses. Evolution of such a grating in the filament wake is manifested in specific angular patterns of the Rabi sidebands emission associated with the excited states manifold.