A mid-infrared vector vortex coronagraph for METIS: the AGPM project

One of the main science cases of a diffraction-limited mid-infrared imager on the E-ELT will be the discovery and characterisation of extrasolar planets through direct imaging. The instrument will therefore need to provide high-dynamic range imaging at angular separations as small as possible from the target stars. Up to now, little attention has been paid to coronagraphic applications in the mid-infrared regime, because the dynamic range of 10-m class telescopes is generally limited more by background noise than by residual stellar light in direct images. This will not be the case any longer when using the collecting area of a 40-m class telescope, so that specific coronagraphs providing both a deep star light rejection on a wide band and a small inner working angle need to be designed and tested. Since 2005, we have been developing a new type of vector vortex coronagraph, called the Annular Groove Phase Mask (AGPM), based on concentric sub-wavelength gratings. This design inherently provides a small inner working angle and a 360∞ discovery space, while being adapted to broadband operation. Indeed, the grating parameters can be tuned to make the phase shift quasi-achromatic over a wide band. During the past few years, we have developped nano-lithography techniques on CVD diamond substrates to produce subwavelength gratings for the mid-infrared domain, and a first generation of AGPMs has recently been manufactured for the L and N bands. In this talk, I will review the coronagraphic performance that we measured when testing these AGPMs in the lab and on the sky, including their recent first light on VLT/NACO and VLT/VISIR. These results will be discussed in the E-ELT context, and the perspectives for applications on METIS will be described.