Annular Groove Phase Mask coronagraph in diamond for mid-IR wavelengths: manufacturing assessment and performance analysis

TL;DR

This paper evaluates the manufacturing and performance of mid-infrared Annular Groove Phase Mask coronagraphs on diamond substrates, highlighting their potential for high-contrast exoplanet imaging at longer wavelengths.

Contribution

It introduces the first mid-infrared AGPM prototypes on diamond, assesses their manufacturing process, and analyzes their expected performance for exoplanet imaging.

Findings

Successful fabrication of diamond-based AGPM prototypes

Performance extrapolation for N-band (~10 μm)

Prospects for scaling to L-band (~3.5 μm)

Abstract

Phase-mask coronagraphs are known to provide high contrast imaging capabilities while preserving a small inner working angle, which allows searching for exoplanets or circumstellar disks with smaller telescopes or at longer wavelengths. The AGPM (Annular Groove Phase Mask, Mawet et al. 2005) is an optical vectorial vortex coronagraph (or vector vortex) induced by a rotationally symmetric subwavelength grating (i.e. with a period smaller than {\lambda}/n, {\lambda} being the observed wavelength and n the refractive index of the grating substrate). In this paper, we present our first mid- infrared AGPM prototypes imprinted on a diamond substrate. We firstly give an extrapolation of the expected coronagraph performances in the N-band (~10 {\mu}m), and prospects for down-scaling the technology to the most wanted L- band (~3.5 {\mu}m). We then present the manufacturing and measurement results, using diamond-optimized microfabrication techniques such as nano-imprint lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength grating profile metrology combines surface metrology (scanning electron microscopy, atomic force microscopy, white light interferometry) with diffractometry on an optical polarimetric bench and cross correlation with theoretical simulations using rigorous coupled wave analysis (RCWA).