Principle and progress of attosecond pulse generation

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<p indent=0mm>With the rapid development of laser technology, the vision of attosecond world has been unfolded. Since the first isolated attosecond pulses produced by high-order harmonic generation in 2001, the time-resolved spectroscopy has entered the attosecond domain and greatly strengthened our understanding of ultrafast electron dynamics in atoms, molecules and condensed matter. Since it approaches the time scale of inner shell electron motion, attosecond pulses become one of the most important tools to explore the electron dynamics. Therefore, developing techniques of attosecond pulse generation is not only essential in optical research field but also imperiously demanded in atomic and molecular physics, which has attracted great attention of researchers worldwide. After years of continuous innovation and breakthroughs, the shortest pulse of 43 attosecond has been achieved. Comparing with the ultrafast laser pulse generation techniques in picosecond or femtosecond regions, the generation of attosecond pulses is rather complex and challenging. From high-order harmonic generation to single attosecond pulse characterization, several crucial technical details need to be considered and investigated carefully, while attosecond gating is a vital link in the chain of technics required by attosecond pulse generation. Up to now, various gating methods have been developed, such as amplitude gating, which, with spectral filtering, takes advantage of the continuous spectrum near the cut-off. In addition, new gating techniques, such as polarization gating, double optical gating, interference polarization gating, polarization-assisted amplitude gating, and phase-matched time gating, also have been proposed or demonstrated to generate isolated attosecond pulses. Recently, spatial gating, which takes advantage of the variation on the driving laser wave front to produce isolated attosecond pulses, has also been developed. Meanwhile, as the intensity at a femtosecond laser focal point now easily exceeds <sc>10¹⁶ W/cm²,</sc> attosecond pulse generation via laser-plasma interaction has been investigated more intensively. Combined with atto-chirp compensation techniques, these methods are pushing the frontier of ultrafast optics toward shorter and stronger attosecond pulses, and achieving even better time resolution in a broader range of applications. In this article, starting with a general introduction of the principle of attosecond pulse generation, we review various gating techniques for attosecond pulse generation in gas targets, new development of attosecond pulses generated by laser-plasma interaction, as well as atto-chirp compensation for even shorter pulses, which are all crucial for the advancement of attosecond science.