Broad Overview
In the last decade or so, advances in producing new extreme ultraviolet (XUV) light sources with attosecond (1 as = 10-18 s) duration have created a new research field of ultrasfast dynamics, namely, attosecond physics. This progress in generation of ultrashort electromagnetic pulses made possible the regime of laser-atom interaction where the pulse duration is shorter than (i) the Kepler period of a classical electron revolving around the proton (about 150 as), (ii) the time scale associated with the dielectronic interaction or electron correlations in helium ground state (about 140 as), etc. As atoms and molecules are made of electrons and nuclei, it is worth noted that these electronic time scales are shorter than that associated with the rather heavier nucleus motion, which is a few of femtoeconds (1 fs = 10-15 s). With such new technological tools, exquisite studies of some of the most fundamental aspects of basic natural science (such as exploration of electron correlations, realization of electron movies, chemical dynamics processes including charge migration and chemical bond formation) can become a reality.
A main goal of attosecond physics is to control electron motion on its natural (attosecond) time scale. One of its ultimate goals is to carry out direct attosecond pump/attosecond probe measurements (dubbed the ``holy grail'' of attosecond physics), as an analogue to the routine femtosecond spectroscopy. Satisfying fully these goals requires the experimental production of isolated or a train of attosecond pulses arbitrarily-polarized with tunable and stable carrier-envelope phase (CEP). A milestone toward achieving such goals is the experimental realization of isolated single-cycle linearly-polarized attosecond pulses (with a probably stabilized CEP) with durations as short as 67 attoseconds. Presently, the intensity of the existing linearly-polarized attosecond pulses is still too weak to induce and control nonlinear processes by a single pulse, or to implement the ``holy grail'' of attosecond physics. But progress in increasing the attosecond pulse intensity is being made as this subject is actually a hot topic in strong-field physics.
Bright and phase-matched XUV high harmonics radiation with tunable polarization (from linear to circular) has been produced experimentally. But chiral (elliptically- or circularly-polarized) attosecond pulses are not yet a reality, this is in contrast to linearly-polarized attosecond pulses. However, different means to produce chiral attosecond pulses is a very active field of research. Such chiral pulses, if produced experimentally, would find numerous applications, e.g., in chiral-sensitive light-matter interactions such as chiral recognition via photoelectron circular dichroism, study of ultrafast chiral-specific dynamics in molecules, and x-ray magnetic circular dichroism spectroscopy, including time-resolved imaging of magnetic structures.
Some Open Questions
To completely understand how electrons and nuclei move in atoms and molecules (the building blocks of nature) when interrogating matter by attosecond laser pulses, the future experiments will need state-of-art theories for their interpretations and analysis. Thus ab initio theoretical works including both numerical and analytical developments are of high interest, as they can guide future experiments and predict new physics that could lead to scientific and technological breakthroughs. The development of these numerical and analytical tools is our expertise fields. Such development requires outstanding knowledge of high performance computing and solid backgrounds in atomic and molecular physics. In particular, our focus are on ultrafast highly correlated two-electron processes that can be controlled by the waveform, intensity, carrier frequency and polarization of the few-cycle attosecond pulse, or by the time delay between two attosecond pulses and their relative phase. The two-electron processes we considered so far are double ionization and single ionization (including or not excitation of the residual ion) of helium. Considering the other two-active-electron atomic or molecular targets is truly exciting and fascinating, but it is a very challenging problem that has to be solved.