The VORTEX project finds its origin in a seminal paper by Mawet et al. (2005), who first proposed to use an optical vortex phase mask as a coronagraph at the focal plane of a telescope. Coronagraphy is a technique that aims to attenuate the blinding glare of nearby stars to reveal their neighbourhood, including planetary systems (exoplanets, circumstellar dust disks). A possible way to generate a vortex phase mask, referred to as the vector vortex phase mask, is to use a spatially rotating half wave plate. Using the form birefringence of sub-wavelength gratings can, among others, generate such a wave plate. Sub-wavelength gratings (SG) have periods smaller than the wavelength of the light, and only transmit the zero-order (non-diffracted) mode. They are therefore also referred to as zero-order gratings (ZOG).
Historically, the concept of the sub-wavelength grating vortex coronagraph (SG-VC) results from an attempt to lift the two major limitations of the four-quadrant phase mask, namely the chromaticity and the presence of blind regions due to the transitions between quadrants. Concentric SG not only remove the blinds spots, thereby giving access to a 360° discovery zone, but can also be tuned to make the half-wave plate achromatic over a wide band. The vortex coronagraph is nowadays considered as on of the most promising concepts for high contrast imaging.
In 2009, we started etching such SGs into diamond substrates in collaboration with Uppsala Universitet. These vortex phase masks, referred to as “Annular Groove Phase Masks” (AGPMs) due to their grating geometry, have first been designed for operation at the N band, centered around a wavelength of 10µm, and then for operation in the L band, centered around 3.8µm. The L-band AGPMs were tested on the YACADIRE coronagraphic testbench in Paris-Meudon, showing a peak attenuation up to 500:1 (Delacroix et al. 2013).
Satisfied with this very high performance, we proposed to install an AGPM on several infrared camera located at the focus of 10-m class telescopes. In 2012, an L-band AGPM was installed on VLT/NACO, and an N-band AGPM on VLT/VISIR. The former got its first light in late 2012 (Mawet et al. 2013), and underwent science verification in early 2013 (Absil et a. 2013). Since then, several teams have used it for various astrophysical programs. The N-band AGPM on VISIR will only undergo commissioning in mid-2015, due to the prolonged unavailability of VISIR in 2013-2014.
In 2013, the VORTEX project was seriously boosted by two major grants: a Starting Grant from the European Research Council (PI: O. Absil) and an ARC grant from the University of Liège (PI: J. Surdej). The project now employs several graduate students and postdocs. Its main goal is to develop, test (in the lab and on sky), and validate the optical elements and observing techniques that could enable the characterization of Earth-like planets in the habitable zone of nearby stars, either with extremely large ground-based telescopes or dedicated space missions. Check out the Research link for more information on the current activities within the VORTEX project.