Optical propagation in molecular gases undergoing filamentation-assisted field-free alignment

An intense and ultrashort laser pulse undergoing filamentation in a molecular gas induces impulsive rotational excitation of the medium. The changes produced in the gas optical properties can dramatically affect a delayed probe pulse propagating in the wake of the pump one. We observe remarkable spatial and spectral reshaping of the probe beam occurring simultaneously at suitable pump-probe delays. These effects are experimentally demonstrated in several molecular gases, with minor changes related to the nature of the medium. A theoretical investigation shows that spatial and temporal domains are strictly related and cannot be disentangled like in experiments performed in a hollow waveguide.