  An ionization-induced plasma grating can be formed by spatially selective
ionization of gases by the interference of two intersecting ultra-short laser
pulses. The density modulation of a plasma grating can approach unity since the
plasma is produced only where the two pulses constructively interfere and
ionization does not occur in destructive interference regions. Such a large
density modulation leads to efficient Thomson scattering of a second
ultra-short probe pulse once the Bragg condition is satisfied. By measuring the
scattering efficiency, it is possible to determine the absolute electron
density in the plasma grating and thereby deduce the ionization degree for a
given neutral gas density. In this paper, we demonstrate the usefulness of this
concept by showing two applications: ionization degree measurement of
strong-field ionization of atoms and molecules and characterization of
extremely low-density gas jets. The former application is of particular
interest for ionization physics studies in dense gases where the collision of
the ionized electron with neighboring neutrals may become important-sometimes
referred to as many-body ionization, and the latter is useful for plasma-based
acceleration that requires extremely low-density plasmas.
