Laboratory demonstration of a mid-infrared AGPM vector vortex coronagraph

Abstract

Context. Coronagraphy is a powerful technique to achieve high contrast imaging, hence to image faint companions around bright targets. Various concepts have been used in the visible and near-infrared regimes, while coronagraphic applications in the mid-infrared nowadays remain largely unexplored. Vector vortex phase masks based on concentric subwavelength gratings show great promise for such applications. Aims. We aim at producing and validating the first high-performance broadband focal plane phase mask coronagraphs for applications in the mid-infrared regime, and in particular the L band with a fractional bandwidth of ∼16% (3.5-4.1 μm). Methods. Based on rigorous coupled wave analysis, we designed an annular groove phase mask (AGPM) producing a vortex effect in the L band, and etched it onto a series of diamond substrates. The grating parameters were measured by means of scanning electron microscopy. The resulting components were then tested on a mid-infrared coronagraphic test bench. Results. A broadband raw null depth of 2 × 10 −3 was obtained for our best L-band AGPM after only a few iterations between design and manufacturing. This corresponds to a raw contrast of about 6 × 10 −5 (10.5 mag) at 2λ/D. This result is fully in line with our projections based on rigorous coupled wave analysis modelling, using the measured grating parameters. The sensitivity to tilt and focus has also been evaluated. Conclusions. After years of technological developments, mid-infrared vector vortex coronagraphs have finally become a reality and live up to our expectations. Based on their measured performance, our L-band AGPMs are now ready to open a new parameter space in exoplanet imaging at major ground-based observatories.

Lyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph

“…The VC suppresses the light from a star, allowing direct detection of dim companions, exoplanets, and circumstellar disks. Imaging objects with a VC, which are otherwise buried in the noise associated with the bright host star, has been demonstrated in laboratory (e.g., Mawet et al 2009;Delacroix et al 2013) and on-sky observations (e.g., Swartzlander et al 2008;Mawet et al 2010;Serabyn et al 2010;Absil et al 2013;Defrère et al 2014).…”

Keck Planet Imager and Characterizer: concept and phased implementation

“…The new NIRC2 vortex mask is a phase-based coronagraph made from a concentric subwavelength grating etched in a diamond substrate. 14,15 It offers well-demonstrated high contrast performance at a very small IWA, 16 L-band coronagraphy is the perfect complement to near-IR wavefront sensing for cooler M-stars because their red colors make them particularly bright in the L-band with respect to the background so that a coronagraph can yield its best starlight suppression factor. The dynamic range enabled by the L-band coronagraph allows reaching the planetary regime much faster around M-stars than earlier types.…”

Polarization holography for vortex retarders recording: laboratory demonstration

“…• to enhance the contrast, • to stay close to the case of coronagraphic applications where half-wave plates are used [3,[6][7][8],…”

“…After such a component, the transmitted beam is characterized by a phase dislocation following exp(ilθ) where θ is the azimuthal angle and l is the topological charge [2][3][4][5]. These particular retarders lead to several applications, such as coronagraphy [3,[6][7][8], optical tweezing [9,10], plasmonic excitation with a focused beam [10,11], focusing into a spot smaller than the one obtained with a linearly polarized beam [10,12,13], etc.…”