Propagation of ultrashort laser pulses in optically ionized gases

Propagation of 800 nm, 120 fs laser pulses with intensities of 4x10(16) W/cm(2) in supersonic gas jets of N-2 and H-2 is studied using a shear-type interferometer. The plasma density distribution resulting from photoionization is resolved in space and time with simultaneously measured initial neutral density distribution. A distinct difference in laser beam propagation distance is observed when comparing propagation in jets of H-2 and N-2. This is interpreted in terms of ionization induced refraction, which is stronger when electrons are produced from states of higher ionization potential. Three dimensional particle-in-cell simulations, based on directly solving the Maxwell-Lorentz system of equations, show the roles played by the forward Raman and ionization scattering instabilities, which further affect the propagation distance.