Optimization Of High-Order Harmonic Generation & Optimization Of Attosecond Pulses

This dissertation is divided into two parts: the optimization of high-order harmonic generation and the optimization of attosecond pulses.First, we introduce the generation, the development and the application of high-order harmonics and those of attosecond pulses. Since both of them are achieved by genetic algorithm optimization of the chirp and the initial phase of the excitation pulse, the concept and the principles of genetic algorithm are also presented in Chapter I.In Chapter II, we discuss the optimization of high-order harmonic generation. In terms of the semi-classical model including the propagation effects, the brightness of a particular harmonic is optimized for the chirp and initial phase of the laser pulse by genetic algorithm. It is shown that the intensity of a specific harmonic can be enhanced by at least one order of magnitude over the one before optimization and the cutoff of harmonic spectrum can be tuned by adjusting the optimized harmonic order. On the other hand, the influences of the chirp and initial phase of the excitation pulse on the harmonic spectrum are discussed. The results indicate that the harmonic spectrum and its cutoff have strong dependence on the chirp of the laser pulse, but comparatively slight on its initial phase.The optimization of attosecond pulses synthesized by harmonics is shown in Chapter III. By genetic algorithm optimization of the chirp and initial phase of 5fs laser pulses, the peak intensity of single attosecond x-ray pulses is enhanced by one or two orders of magnitude and the pulse duration is greatly compressed as well. Furthermore, it is shown that the optimal propagation distance is shifted after optimization as a consequence of the changed coherence length of the synthesized attosecond pulse. However, we find that as the laser intensity increases, the efficiency of optimization decreases in the enhancement and compression of the generated attosecond pulses. This can be explained by the fact that more efficient nonadiabatic self-phase-matching (NSPM) can lead to the dramatically enhanced harmonic yield even before optimization.At last, the main work of the dissertation is summarized in the last chapter andthe possible future development is prospected.