Resonantly enhanced filamentation in gases

Femtosecond filamentation is a self-organization phenomenon during which an ultrashort high-power laser stays confined in a very small channel over very long distances. Ultimately, however, the finite energy contained in a filament is dissipated because of losses originated from ionization, limiting thereby the filament length. In other words, ionization represents a fundamental limitation in remote applications where long-ranged filaments are required. In this paper, a low-loss Kerr-driven optical filament in krypton gas is experimentally reported in the ultraviolet. A three-photon resonantly enhanced quintic nonlinearity is identified as the underlying physical mechanism responsible for intensity saturation during the filamentation process, while ionization plays only a minor role. The resonant nature of the process creates also conducive conditions, i.e., a significant population inversion, for forward and backward infrared lasing. Preliminary experimental results suggest that such lasing emission takes place. The reported resonantly enhanced filaments are one order of magnitude longer than their off-resonant counterparts. The resonance is also accompanied by a large decrease of both ionization and nonlinear optical losses. The experimental findings are supported by ab initio quantum calculations describing the atomic optical response. Beyond its theoretical interest, resonantly enhanced filamentation could benefit all applications deriving from the filamentation process. For instance, the extension of this work to molecular gases such as oxygen and nitrogen could lead to numerous atmospheric applications such as nonlinear spectroscopy, remote sensing, and lightning protection, in which the transport of high energies over long distances is of prime importance.