  Phase-mask coronagraphs are known to provide high contrast imaging
capabilities while preserving a small inner working angle, which allows
searching for exoplanets or circumstellar disks with smaller telescopes or at
longer wavelengths. The AGPM (Annular Groove Phase Mask, Mawet et al. 2005) is
an optical vectorial vortex coronagraph (or vector vortex) induced by a
rotationally symmetric subwavelength grating (i.e. with a period smaller than
{\lambda}/n, {\lambda} being the observed wavelength and n the refractive index
of the grating substrate). In this paper, we present our first mid- infrared
AGPM prototypes imprinted on a diamond substrate. We firstly give an
extrapolation of the expected coronagraph performances in the N-band (~10
{\mu}m), and prospects for down-scaling the technology to the most wanted L-
band (~3.5 {\mu}m). We then present the manufacturing and measurement results,
using diamond-optimized microfabrication techniques such as nano-imprint
lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength
grating profile metrology combines surface metrology (scanning electron
microscopy, atomic force microscopy, white light interferometry) with
diffractometry on an optical polarimetric bench and cross correlation with
theoretical simulations using rigorous coupled wave analysis (RCWA).
