Excited atoms density buildup in long-wavelength filamentation in atmospheric pressure gases*

When free electrons are driven by the strong oscillating electric field of a femtosecond laser pulse, the amplitude of their oscillations (the ponderomotive radius) scales as the square of the laser carrier wavelengths. For a long-wavelength (~4 microns) laser pulse interacting with a gas at atmospheric pressure, the ponderomotive radius is comparable with the interatomic spacing. In this situation, the driven electrons collide effectively with the neighboring neutral atoms, and this allows the electrons to gain energy via inverse Bremsstrahlung on neighboring atoms. Moreover, these collisions can either ionize the neighbor atoms or promote them to an excited state, leading to a considerable build-up of the excited-atom density. Such a buildup of excited atoms leads to a number of transient optical effects in the wake of the laser pulse, especially to the hallmark effect of Rabi sideband generation. Addressing the case of atmospheric-pressure argon, we investigate the production of excited atoms and ions, as well as the control of these processes by the shape of the long-wavelength femtosecond laser pulse. We further obtain the characteristic patterns of Rabi sideband emission from the filament wake channels.