| The experiments described in this thesis, illustrate effects that are observable only in the femtosecond pulse regime. Moderately high peak intensity (>1015 W/cm2) is essential for these experiments, and this is set by the threshold for optical field ionization of atoms.; We first examine the ionization scattering instability. This instability results directly from the very strong dependence of the optical field ionization rate on the laser electric field amplitude. Pulse propagation is accompanied by strong filamentary modulations in the electron density, which scatter the pulse leading to further modulations. Eventually, transverse beam breakup can occur. This is a universal effect that occurs for all pulses in excess of the field ionization threshold intensity. It is for shorter pulses of moderate intensity that the effect is most evident, where the instability can be present for the full pulse duration. Accompanying the modulations, we measure significant second harmonic generation.; We next study the propagation of femtosecond pulses in clustered gases produced by the adiabatic expansion and cooling of ordinary gases in high-pressure nozzle flow into vacuum. In the intense laser heating of these clusters, we have discovered a macroscopic effect on the heating beam. If the laser pulse is still on before the clusters have fully exploded, the beam can self-focus. This stands in strong contrast to the beam spreading effect observed in unclustered gases. The self-focusing effect is explained in terms of the explosive dynamics of the individual clusters induced by the laser pulse.; Finally, we used our femtosecond time resolving diagnostics to explore a phenomenon predicted earlier: the measurement of superluminal ionization fronts induced by intense Bessel beams. Using our femtosecond laser system, the superluminal group velocity of an ultrashort optical Bessel beam pulse was measured for its entire depth of field, corresponding to ∼2 × 104 optical wavelengths. To do this, we measured speed of the ionization front induced by the laser pulse, which travels at the Bessel beam pulse group velocity. Our experiment shows that pulse envelope can travel at superluminal speed and can generate physically observable phenomenon. |
