The time scale of electron dynamics in matter is represented by the atomic unit of time, which is 24 attoseconds. Although isolated attosecond photon pulses were demonstrated in 2001, few laboratories have been able to own the new light source because the generation technique is very difficult. Furthermore, the existing attosecond source is not strong enough for performing attosecond pump/attosecond probe experiments. In the last five years, Chang's group has developed a technique, Double Optical Gating (DOG), which can generate isolated attosecond pulses rather easily. It also allows the up-scaling of attosecond pulse energy. With the current support from the National Science Foundation, Chang's experimental group and Hu's theoretical group work together to understand and control ultrafast electron-electron interaction in the time domain by taking advantage of the attosecond source based on the DOG scheme.
We focus on the autoionization of atoms as it is dominated by the electron correlation. As the starting point, the two-electron system, helium, is chosen as the target. Unlike spectral-domain experiments performed in the past using synchrotron light, helium atoms are pumped from the ground state into either continuum states or doubly-excited states by isolated attosecond XUV pulses. The instantly-initiated electron-correlations in the doubly-excited states, their subsequent fast decay and the interference with the continuum states, are then probed either by another time-delayed attosecond XUV pulse or by an intense few-cycle near infrared (NIR) laser pulse. The attosecond probe pulse would "freeze" the motion of the two electrons and "map-out" in real time the ultrafast electron-correlation dynamics. The strong NIR field can modify the electron-electron interactions within a fraction of an optical cycle in order to manipulate and control the ultrafast electron correlations. The experiment-theory collaboration allows experimental benchmarking of the quantum-mechanical ab initio calculations, laying a foundation for studying electron dynamics in more complex atoms and molecules.
Electron-electron interactions play an essential role in a wide range of fundamentally important many-body phenomena in modern chemistry, physics and biology. The broad impacts of this program are two-fold. First of all, developing tools for observing electron dynamics with an unprecedented time resolution will lead to new insights into the fundamental questions of how electron correlation plays a role in molecular structure formation. Such insights will particularly help chemists to understand electron correlations in chemical reactions better. Secondly, finding techniques to control electron dynamics with external fields can significantly advance the technologies for manipulating complex systems and chemical reactions at the fundamental electronic level. Moreover, the new attosecond light source under development can spur a revolution in ultrafast free-space communication and biomolecule imaging.
The program supports three students to work at one of the most exciting forefronts of the physics. They are trained to become leaders in this new research field and experts of the next generation technologies. The experiments are conducted at the Florida Attosecond Science and Technology (FAST) laboratory, which was newly established at the University of Central Florida. A course on attosecond optics and another one on attosecond physics are offered by Chang for undergraduate and graduate students who use the attosecond facility for lab demonstrations.