  Context. High contrast imaging has thoroughly combed through the limited
search space accessible with first-generation ground-based adaptive optics
instruments and the Hubble Space Telescope. Only a few objects were discovered,
and many non-detections reported and statistically interpreted. The field is
now in need of a technological breakthrough.
  Aim. Our aim is to open a new search space with first-generation systems such
as NACO at the Very Large Telescope, by providing ground-breaking inner working
angle (IWA) capabilities in the L' band. The L' band is a sweet spot for high
contrast coronagraphy since the planet-to-star brightness ratio is favorable,
while the Strehl ratio is naturally higher.
  Methods. An annular groove phase mask (AGPM) vector vortex coronagraph
optimized for the L' band, made from diamond subwavelength gratings was
manufactured and qualified in the lab. The AGPM enables high contrast imaging
at very small IWA, potentially being the key to unexplored discovery space.
  Results. Here we present the installation and successful on-sky tests of an
L'-band AGPM coronagraph on NACO. Using angular differential imaging, which is
well suited to the rotational symmetry of the AGPM, we demonstrated a \Delta L'
> 7.5 mag contrast from an IWA ~ 0".09 onwards, during average seeing
conditions, and for total integration times of a few hundred seconds.
