  Exploration of a new ultrafast-ultrasmall frontier in atomic and molecular
physics has begun. Not only is is possible to control outer-shell electron
dynamics with intense ultrafast optical lasers, but now control of inner-shell
processes has become possible by combining intense infrared/optical lasers with
tunable sources of x-ray radiation. This marriage of strong-field laser and
x-ray physics has led to the discovery of methods to control reversibly
resonant x-ray absorption in atoms and molecules on ultrafast timescales. Using
a strong optical dressing field, resonant x-ray absorption in atoms can be
markedly suppressed, yielding an example of electromagnetically induced
transparency for x rays. Resonant x-ray absorption can also be controlled in
molecules using strong non-resonant, polarized laser fields to align the
framework of a molecule, and therefore its unoccupied molecular orbitals to
which resonant absorption occurs. At higher laser intensities, ultrafast field
ionization produces an irreversible change in x-ray absorption. Finally, the
advent of x-ray free electron lasers enables first exploration of non-linear
x-ray processes.
