  Circularly-polarized extreme UV and X-ray radiation provides valuable access
to the structural, electronic and magnetic properties of materials. To date,
this capability was available only at large-scale X-ray facilities such as
synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV
circularly-polarized high harmonics and use this new light source for magnetic
circular dichroism measurements at the M-shell absorption edges of Co. We show
that phase matching of circularly-polarized harmonics is unique and robust,
producing a photon flux comparable to the linearly polarized high harmonic
sources that have been used very successfully for ultrafast element-selective
magneto-optic experiments. This work thus represents a critical advance that
makes possible element-specific imaging and spectroscopy of multiple elements
simultaneously in magnetic and other chiral media with very high spatial and
temporal resolution, using tabletop-scale setups.
