Plasma-induced self-phase and cross-phase modulation of femtosecond laser pulses

The spectral, temporal, and spatial characteristics of plasma-induced self-phase and cross-phase modulation in rare gases have been investigated using a femtosecond KrF excimer laser focused to peak intensities of 10{dollar}sp{lcub}14{rcub}{dollar}-10{dollar}sp{lcub}15{rcub}{dollar} W cm{dollar}sp{lcub}-2{rcub}.{dollar} The quiver energy of a free electron under these conditions is less than the ionization potential of all rare gases, ensuring that ionization occurs only by optical field-induced processes. Spectral blueshifts of up to 2 nm have been observed, and the blueshifted spectra show an oscillatory structure. The blueshifted spectra are shown to be the result of plasma-induced self-phase modulation and can be modeled by assuming tunneling ionization and one dimensional pulse propagation. The newly discovered oscillatory structure in the spectra is related to that observed in earlier experiments on self-phase modulation in optical fibers.; To investigate the temporal behavior of the field ionization process, pump-probe experiments have been performed with a 100 fs probe pulse at 497 nm and a 400 fs pump pulse at 248 nm. Under conditions of weak ionization (Z {dollar}ll{dollar} 1), pump-probe experiments and theoretical calculations show that the ionization rate of the field ionized gas is maximum at the peak of the laser pulse and that the degree of ionization changes over a time equal to about half of the pump pulse width. By observing changes in the transmission of the probe pulse caused by plasma absorption, the electron temperature of a field ionized rare gas is determined to be on the order of 1 eV. The time varying electron density in the pump-probe experiments also causes plasma-induced cross-phase modulation, or spectral blueshifting of the probe pulse spectrum of up to 15 nm.; The pump-probe experiments show that plasma defocusing causes the spectral blueshifting to be spatially dependent. Experimental results and a two dimensional pulse propagation model indicate that the most defocused beam components also show the maximum spectral blueshift. Plasma-induced cross-phase modulation has also been used to characterize the amplitude and phase of a 1 ps chirped pulse at 497 nm and the pulse width of a 400 fs pulse at 147 nm generated by four wave frequency mixing in xenon.