  Context. The Annular Groove Phase Mask (AGPM) is one possible implementation
of the vector vortex coronagraph, where the helical phase ramp is produced by a
concentric subwavelength grating. For several years, we have been manufacturing
AGPMs by etching gratings into synthetic diamond substrates using inductively
coupled plasma etching. Aims. We aim to design, fabricate, optimize, and
evaluate new L-band AGPMs that reach the highest possible coronagraphic
performance, for applications in current and forthcoming infrared high-contrast
imagers. Methods. Rigorous coupled wave analysis (RCWA) is used for designing
the subwavelength grating of the phase mask. Coronagraphic performance
evaluation is performed on a dedicated optical test bench. The experimental
results of the performance evaluation are then used to accurately determine the
actual profile of the fabricated gratings, based on RCWA modeling. Results. The
AGPM coronagraphic performance is very sensitive to small errors in etch depth
and grating profile. Most of the fabricated components therefore show moderate
performance in terms of starlight rejection (a few 100:1 in the best cases).
Here we present new processes for re-etching the fabricated components in order
to optimize the parameters of the grating and hence significantly increase
their coronagraphic performance. Starlight rejection up to 1000:1 is
demonstrated in a broadband L filter on the coronagraphic test bench, which
corresponds to a raw contrast of about 1e-5 at two resolution elements from the
star for a perfect input wave front on a circular, unobstructed aperture.
Conclusions. Thanks to their exquisite performance, our latest L-band AGPMs are
good candidates for installation in state-of-the-art and future high-contrast
thermal infrared imagers, such as METIS for the E-ELT.
