We present a practical and accurate technique for retrieving the amplitude and the phase of ultrashort pulses from a nonlinear (second-order) intensity cross correlation and the spectrum that overcomes shortcomings of previous attempts. We apply the algorithm to theoretical and experimental data and compare it with frequency-resolved optical gating.
Focusing of ultrashort light pulses with single lenses is analyzed by taking into account the unavoidable interplay between chromatic and spherical aberration simultaneously for the first time to our knowledge. The spatial intensity distribution is mainly affected by spherical aberration, whereas the temporal distribution is determined by both aberrations. The impact on second-harmonic generation for femtosecond pulse measurements is discussed. For example, the presence of spherical aberration allows one to record the correct autocorrelation of a 10-fs pulse even if chromatic aberration alone would cause a half-width of the autocorrelation function of 40 fs.
A model for the multiple-pulse laser-induced breakdown behavior of dielectrics is presented. It is based on a critical conduction band (CB) electron density leading to dielectric breakdown. The evolution of the CB electron density during the pulse train is calculated using rate equations involving transitions between band and mid-gap states (native and laser-induced). Using realistic estimations for the trap density and ionization cross-section, the model is able to reproduce the experimentally observed drop in the multiple-pulse damage threshold relative to the single-pulse value, as long as the CB electron density is controlled primarily by avalanche ionization seeded by multiphoton ionization of the traps and the valence band. The model shows that at long pulse duration, the breakdown threshold becomes more sensitive to presence of traps close (within one photon energy) to the CB. The effect of native and laser-induced defects can be distinguished by their saturation behavior. Finally, measurements of the multiple-pulse damage threshold of hafnium oxide films are used to illustrate the application of the model.
An abrupt (less than 100 fs) decrease in the second-harmonic intensity reflected from the surface of a GaAs (110) wafer has been observed experimentally. The linear reflectivity was found to increase on a time scale of ~1 ps. Thus the concept of fast atomic disorder induced by electronic excitation within a relatively cold lattice is given new experimental support.
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