Optimizing the subwavelength grating ofL-band annular groove phase masks for high coronagraphic performance

Abstract

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 10 −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.

Subwavelength diamond gratings for vortex coronagraphy: towards an annular groove phase mask for shorter wavelengths and topological charge 4 designs

“…Depending on the initial result it is possible to adjust the process parameters to reach a sidewall angle which gives the widest region of acceptable performances [6] for varying w and d. The variation in angle gives freedom for both w and d. The line width can both decrease or increase depending on the amount of residual resist layer left after the lithography step and on the anisotropy of the aluminum etch process as it can etch underneath the mask pattern. However, the diamond etching preserves the line width of the grating as long as the mask is sufficiently thick [7]. The line width is therefore set to about 50% of Λ in the electron beam printed master pattern as it still can produce a good component while the the line width is between 40 and 60% of Λ.…”

“…The substrates were circular with a diameter of 10 mm and thickness of 300 μm. For each produced component, two substrates were prepared in parallel: one of them leading to a functional coronagraphic element, while the second one (twin) was cracked for cross-sectional characterization, as there is no method yet for measuring the grating parameters without breaking the component [7]. Cleaning and pattern transfer steps as well as the aluminum and silicon etching are described in detail in [6], the main difference is in the lithography technique known as Solvent Assisted Microcontact Molding (SAMIM).…”

“…In 2005 D. Mawet et al proposed a vector phase mask known as the Annular Groove Phase Mask (AGPM) [4], comprising a circular subwavelength grating that induces form-birefringence. Since 2010 our group has been working to design, manufacture and optically evaluate AGPMs in diamond for the L-(3.4-4.1 µm) and N-bands (11-13.2 µm) [5][6][7]. A method for increasing or decreasing the depth of the grating for the L-band has been developed [7], which allows optimization of the phase mask rejection ratio.…”

“…Since 2010 our group has been working to design, manufacture and optically evaluate AGPMs in diamond for the L-(3.4-4.1 µm) and N-bands (11-13.2 µm) [5][6][7]. A method for increasing or decreasing the depth of the grating for the L-band has been developed [7], which allows optimization of the phase mask rejection ratio. The phase masks have been installed in infrared coronagraphic instruments (N-and L-band) on several 10-m class telescopes (VLT/NACO [8], VLT/VISIR [9], LBT/LMIRCam [10] and Keck/NIRC2 [1][2][3]) around the world.…”

Keck Planet Imager and Characterizer (KPIC): status update

“…The upgrades aim to enable unique exoplanet science. NIRC2 was upgraded with an L-band vortex coronagraph, [1][2][3][4][5] now offered in shared-risk mode. The NIRC2 L-band vortex enabled the discovery of new protoplanet candidates, 6, 7 disks, 8 and allowed us to constraint mass accretion rates in protoplanetary disks with gaps.…”