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

Abstract

In this paper, we address two challenges of subwavelength grating manufacturing: the need for grating with a smaller feature size to produce components working at shorter wavelengths, and the more complex grating design to achieve higher topological charge masks. First, we demonstrate the feasibility of topological charge 2 components (so-called annular groove phase masks) in diamond to be used in the IR region at the K- (1.95–2.35 µm) and H-band (1.48–1.77 µm). The manufactured phase masks are optically evaluated using a coronagraph test bench and a polarization/phase shift test bench to determine peak attenuation, phase shift and local fast axis orientation. Secondly, in preparation for the upcoming generation of extremely large telescopes, vector vortex phase masks for coronagraphy with topological charge 4 are realized in diamond for the L-band (3.4-4.1 µm). These phase masks, consisting of discrete subwavelength gratings, should reduce sensitivity to low-order aberrations compared to topological charge 2 components that we have previously demonstrated for the N- (11-13.2 µm) and L-band.