Ultrafast pump-probe interferometry of femtosecond laser microplasma of multiple ionized gases

The results of experimental studies of formation and evolution of a multiply ionized (multicharged) laser microplasma produced in gases (air, helium, nitrogen and argon) with high intensity ( I ~ 1017 W/cm2 ) ultrashort (τ ~ 100 fs ) 800-nm laser pulses tightly focused in a region of diameter ~ 2-3 μm are presented. The measuring technique and experimental setup for generation and precise optical diagnostics of laser-induced plasma are described. The spatiotemporal distribution of the refractive index and the free electron density was studied by the interferometric pump-probe method with a spatial resolution of ~ 1.5 μm and a temporal resolution of ~ 100 fs. An almost complete ionization of the initial gas (down to the nuclei) was shown to occur even in the initial stage of gas plasma formation.