The optimal control and dynamical analysis of high-order harmonic generation from atom

Abstract

High-order harmonic generation (HHG) is a frequency up-conversion process in which an ultrafast femtosecond laser pulse at high intensity interacts with atom or molecule. HHG covers the wider wavelength spectral regions of extreme ultraviolet (XUV), soft X-ray and even hard X-ray. With the high photon flux, the HHG promises to be a attractive broadband tabletop light source, and a number of interesting applications of HHG have been demonstrated in many research fields. In this paper, we review some common methods and results on the optimal control of HHG to improve its low photon flux. HHG can now be controlled for different purposes, i.e., extending the cutoff energy, increasing the efficiency of the high-harmonic conversion process, and selecting single or range of harmonics, by combining the technologies of femtosecond temporal and spatial laser pulse shaping, waveform synthesizing and evolutionary algorithm, such as genetic algorithm and optimal control theory. Selective generation of a single harmonic can now be achieved both in hollow fiber, long gas cell and gas jet. We show that by using the newly developed waveform synthesis with two-color laser fields, the harmonic yield can be enhanced by more than one order without increasing the total laser energy, or the harmonic cutoff can be extended about two times without losing harmonic yield, even after considering the propagation effects. These progresses will exert a far-reaching impact on strong field physics. To optimize the HHG processes, a detailed understanding of the corresponding dynamics is essential. We also introduce a new time-frequency (TF) analysis technique, the synchrosqueezing transform (SST), which is used to reveal the quantum dynamics of HHG. Compared with the classical type of TF methods, such as the Gabor transform, the Morlet wavelet transform, the SST can be applied to explore the dynamical origin of near- and below-threshold harmonic emission.