Coronagraph focal-plane phase masks based on photonic crystal technology: recent progress and observational strategy

Abstract

Photonic crystal, an artificial periodic nanostructure of refractive indices, is one of the attractive technologies for coronagraph focal-plane masks aiming at direct imaging and characterization of terrestrial extrasolar planets. We manufactured the eight-octant phase mask (8OPM) and the vector vortex coronagraph (VVC) mask very precisely using the photonic crystal technology. Fully achromatic phase-mask coronagraphs can be realized by applying appropriate polarization filters to the masks. We carried out laboratory experiments of the polarization-filtered 8OPM coronagraph using the High-Contrast Imaging Testbed (HCIT), a state-of-the-art coronagraph simulator at the Jet Propulsion Laboratory (JPL). We report the experimental results of 10 -8 -level contrast across several wavelengths over 10% bandwidth around 800nm. In addition, we present future prospects and observational strategy for the photonic-crystal mask coronagraphs combined with differential imaging techniques to reach higher contrast. We proposed to apply the polarization-differential imaging (PDI) technique to the VVC, in which we built a two-channel coronagraph using polarizing beam splitters to avoid a loss of intensity due to the polarization filters. We also proposed to apply the angular-differential imaging (ADI) technique to the 8OPM coronagraph. The 8OPM/ADI mode mitigates an intensity loss due to a phase transition of the mask and provides a full field of view around central stars. We present results of preliminary laboratory demonstrations of the PDI and ADI observational modes with the phase-mask coronagraphs.

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

“…A nonzero, even value of l is required for ideal starlight suppression with a circular aperture. Various techniques are available to fabricate achromatic phase masks (Bomzon et al 2001;Marrucci et al 2006;Murakami et al 2012). The scalar field immediately after the VPM owing to an on-axis point source may be written…”

Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics

“…The first effective version was the four quadrant phase mask (FQPM) [96][97][98] followed by designs with increased the number of sectors. 99,100 Masks that use continuouse azimuthal phase ramps are the known as vortex coronagraphs, which avoid dead zones between the mask segments. 101,102 It was noted early on that sector-based and vortex coronagraph masks can all be expressed as linear combinations of azimuthal Fourier modes with even topological charge.…”

Optimized focal and pupil plane masks for vortex coronagraphs on telescopes with obstructed apertures

“…Achromatic vortex phase masks have been manufactured using subwavelength gratings, 9,32 liquid crystals, 33,34 and photonic crystals. 35,36 One example is the annular groove phase mask (AGPM), which is an l = 2 vortex phase masks that operates in the mid-infrared 9, 37 (also see Absil et al, these proceedings). AGPMs provide achromatic phase-only transmission via form birefringence.…”