Watching extreme lasers at work – Frame-by-frame

Observations of the ionization of argon atoms under extremely
bright and energetic illumination could prove a boon to research
Under extremely intense illumination materials may exhibit so-called
nonlinear optical properties such as ceasing to absorb light beyond
a certain brightness, or becoming highly ionized. Yasumasa Hikosaka,
Mitsuru Nagasono and colleagues at RIKEN and several other
Japanese research institutes have now described the details of
this ionization process by using very short bursts of bright laser
light. Their finding is relevant to a broad range of pure and applied
research, including x-ray imaging of biological molecules, ultrafast
optical switches, fusion and astrophysics.
The researchers focused on the behavior of argon atoms, which is
easy to handle and well-characterized, under illumination by laser
light about one hundred trillion times brighter than the noonday
sun, and containing about seven times more energy per photon
than the bluest light visible to the human eye. Previous work by
other researchers showed that such intense, energetic light removes
multiple electrons from target atoms, resulting in highly charged
ions. While the mechanism of the ionization process was partially
understood from observations of the yields and momenta of these
ions, important details were missing.
Hikosaka, Nagasono and colleagues chose to observe the electrons
emitted during the ionization process (Fig. 1), instead of the ions
themselves. Not only do these electrons carry unique information
about the ionization process, but they can be measured after each
ultra-short laser pulse. Since the laser spectrum and power are
constantly fluctuating, the fine details of the ionization process
are averaged or ‘smeared’ during a continuous measurement.
A shot-by-shot measurement, however, can account for laser
The experiment showed that the dominant ionization pathway of
the argon atoms has two steps: first, a single laser photon is absorbed
to create singly-ionized argon, and then two more photons are
absorbed to create doubly-ionized argon. The researchers also
found that the intermediate argon ion states had energy levels,
or energy resonances, that induced this pathway.
The research leverages the recent development of free electron
lasers, which are uniquely capable of producing very bright, energetic
and short pulses of radiation. The work also illustrates that energy
resonances are key to multi-photon, multiple ionization processes,
a finding that is likely to be relevant to a variety of research programs.
Hikosaka says that the research team will continue to focus on the
basic science, as well as applications: “Our goal is to develop and
leverage a deep understanding of the mechanism and dynamics
of non-linear processes in order to manipulate or control these
processes and their final products.”