Rhombic gridiron excitation structures in filament wake channels in a dense gas: control and manifestations

When femtosecond laser filamentation takes place at a crossing of two laser beams, it is strongly affected by the transient interference pattern. The resulting intensity comb runs through the beam-crossing region and creates in the pulse wake a modulated distribution of the excited atoms in a form of rhombic gridiron, whose characteristics are controlled by the beam crossing angle, the phase delay between the two pulses, and the spatial and temporal pulse shapes. These modulated excitation patterns relult mainly form the electron-collisonal processes in a relatively dense gas, where the interatomic distances are comparable with the ponderomotive radius. They manifest themselves in a number of linear and nonlinear optical effects. In particular, they generate a broadband emission in the form of structured red-shifted and blue-shifted Rabi sidebands when probed by a picosecond laser pulse. The spectral and spatial interference of the emitted radiation leads to complicated spatial-spectral patterns that can be observed on a remote screen and that carry information of the excitation structure. We have studied numerically the excitation gridiron formation in argon gas at elevated pressures and laser pulses of 800-nm carrier wavelength, and also in argon gas at atmospheric pressure and laser pulses of 4 um carrier wavelength. The picosecond probe pulse is of 800 nm carrier wavelength couples to several transitions in the excited states manifold to produce the Rabi sideband patterns, which are deciphered to reveal the characteristics of the underlying excitation gridiron.