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Attosecond and Strong-Field Physics: Principles and Applications

With the generation of attosecond pulses, probing and controlling electrons and nuclei in matter at the attosecond timescale became possible, which revolutionizes our understanding of atomic structure and molecular processes. This book provides an intuitive approach to this emerging field, utilizing simplified models to develop a clear understanding of how matter interacts with attosecond pulses of light. An introductory chapter outlines the structure of atoms and molecules and the properties of a focused laser beam. Detailed discussion of the fundamental theory of attosecond and strong-field physics follows, including the molecular tunnelling ionization model (MO-ADK theory), the quantitative rescattering (QRS) model, and the laser induced electronic diffraction (LIED) theory for probing the change of atomic configurations in a molecule. Highlighting the cutting-edge developments in attosecond and strong field physics, and identifying future opportunities and challenges, this self-contained text is invaluable for students and researchers in the field.

 

 

 

Recent progress on the studies of high-harmonic spectroscopy

High-order harmonic generation (HHG) is a nonlinear up-conversion process resulting from the interaction of an intense femtosecond infrared laser pulse with a gaseous, liquid, or solid target. It offers a unique coherent tabletop light source ranging from the extreme ultraviolet (XUV) to X-rays with sub-femtosecond or attosecond durations. Due to its exceptional coherence property in space and time, it has been employed extensively in many ultrafast experiments, such as in attosecond science, nanoscale structure imaging, assisted free-electron lasers, and HHG spectroscopy.

In some applications, it is desirable to control the spectral structure of the generated HHG spectrum. Prof. Jin suggested a simple and novel experimental scheme purely relying on the phase mismatch for selectively controlling soft X-ray tabletop light sources without adopting the filters for applications in a recent article [Photonics Research 6, 434-442 (2018), https://doi.org/10.1364/PRJ.6.000434]. They showed that harmonic suppression can occur at the proper combination of the propagation distance and gas pressure. The physical mechanism behind is the phase mismatch between “short”-trajectory harmonics generated at the early and later times through the interplay of geometric phase, dispersion, and plasma effects.

 

 

 

How to efficiently extend the cutoff photon energy of HHG spectrum set by the “critical ionization” has been a long-standing problem. Recently, Prof. Jin collaborated with Prof. Ming-Chang Chen at National Tsing Hua University in Taiwan and Prof. C. D. Lin at Kansas State University in the US, and demonstrated a scheme to overcome the limit of conventional harmonic cutoff for different pulse durations, laser wavelengths, and gas targets. By tuning the truncation of incident laser beam, they showed that the defocusing-assisted phase matching (DAPM) can be achieved in a tightly focused beam and highly ionized short gas cell, and can be used to effectively extend the harmonic cutoff energy and optimize its yield [Optica 4, 976-981(2017), https://doi.org/10.1364/OPTICA.4.000976; Optics Letters 43, 4433-4436 (2018), https://doi.org/10.1364/OL.43.004433 ].