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There has been an ever-lasting pursuit for ever-faster physical processes and ever-shorter time scales. In the 19th century, the invention of high-speed photography solved the debate over whether there is a moment all four legs of a galloping horse are in the air. In the 1980s, femtosecond (1fs = 10^{-15}s) lasers started to develop, enabling the production of the movie of a chemical reaction. Nowadays, laser sources with even shorter pulse length have matured, whose duration has been down to the attosecond (1as = 10^{-18}s) level, making possible the observation of electronic processes and the formation and breakage of chemical bonds, which are the fastest resolvable physical processes up to date. Strong-field ultrafast optics studies the physical properties of materials under an unprecedented intense field on an unprecedented fine temporal resolution, which is a growing discipline across the globe. In this talk, we briefly overview a few attosecond-scale ultrafast processes we have studied so far and assess if they come with a time delay with respect to the initiating laser pulse: photoionization time delay, two-photon ionization time delay, single-photon double ionization time delay, tunneling ionization time delay, orbital deformation induced tunneling time delay, and photon momentum transfer time delay.
Prof. Hongcheng Ni obtained his PhD in Physics degree with Prof. Andreas Becker from University of Colorado in 2014. Subsequently, he worked as a Visiting Scientist, Humboldt Fellow at Max-Planck-Institut fur Physik komplexer Systeme from 2014 to 2018. Then he moved to Technische Universitat Wien as a Project Assistant, Project Leader, Meitner Fellow. Since September 2019, he has joined East China Normal University as a Junior Research Professor, Zijiang Youth Scholar. Prof. Ni’s research interests include Ultrafast Optics, Attosecond Physics, Strong-Field Physics and Theoretical AMO Physics.