Development and application of metamaterial-based half-wave plates for the NIKA and NIKA2 polarimeters

Abstract

Context. Large field-of-view imaging and polarimetry instruments operating at millimetre and sub-millimetre wavelengths are fundamental tools to understand the role of magnetic fields in channelling filament material into prestellar cores, providing unique insight in the physics of galactic star-forming regions. Among other topics, at extra-galactic scales, polarisation observations of Active Galactic Nuclei (AGNs) will allow us to constrain the possible physical conditions of the emitting plasma from the jets and/or explore the physics of dust inside supernova remnants. The kilo-pixel New IRAM KIDs Array 2 (NIKA2) camera, installed today at the Institut de Radioastronomie Millimétrique (IRAM) 30-m telescope, represents one of the best tools available to astronomers to produce simultaneous intensity and polarimetry maps over large fields at 260 GHz (1.15 mm). Aims. The polarisation measurement, in NIKA and NIKA2, is achieved by rapidly modulating the total incoming polarisation. In the end, this allows one to safely isolate the small science signal from the large, un-polarised, and strongly variable, atmospheric background. Methods. The polarisation modulation is achieved by inserting a fast rotating half-wave plate (HWP) in the optical beam. In order to allow wide field-of-view observations, the plate has to be large, with a diameter of 250 mm. The modulation of the polarised signal at 12 Hz also requires the waveplate to be sufficiently light. In addition, this key optical element has to exhibit optimal electromagnetic characteristics in terms of transmission and differential phase-shift. For this purpose, three metamaterial HWPs have been developed using the mesh-filter technology. The knowledge acquired in developing the first two single-band HWPs was used to achieve the more challenging performance requirements of the last dual-band HWP. The first and the third waveplates met the requirements for both the NIKA and NIKA2 instruments. Results. We first illustrate the design, the technical developments, the fabrication, and laboratory characterisation of the three mesh-HWPs. The deployment of two such elements in the NIKA and NIKA2 instruments at the 30-metre telescope is then described. We conclude with representative examples of astrophysical maps integrating polarimetry.

Detailed study of HWP non-idealities and their impact on future measurements of CMB polarization anisotropies from space

“…We have adopted simulated frequency profiles for the efficiency h and the non-ideal phase shift β in the MFT frequency bands, provided by finite-element simulations of the MHFT LiteBIRD MHWPs. The waveplate designs are based on previous developments and realisations (Pisano et al 2020). The profiles for ζ have always been fixed to a realistic (Pisano et al 2012) level of 0.01 in all the bands, as this parameter was not included in the suite of simulations at our disposal.…”

“…The waveplate designs are based on previous developments and realisations (Pisano et al 2020). The profiles for ζ have always been fixed to a realistic (Pisano et al 2012) level of 0.01 in all the bands, as this parameter was not included in the suite of simulations at our disposal. In the LFT (also not included in the simulation suite) and HFT bands , we have used constant profiles also for h and β.…”

“…First of all, we allow for HWP matrix elements to be frequency dependent: we assume a specific LiteBIRD MHWP design with certain frequency profiles within each band. The MHWP performance was computed using realistic models developed for previous waveplate applications (Pisano et al 2020). Secondly, we include a frequency-dependent foreground component in the input sky maps.…”

“…In general, HWP parameters show a non trivial dependence on the frequency of the incident signal due to fabrication details. Finite-element modelling and laboratory characterisation of the HWP devices allow to reconstruct the expected profiles of the HWP matrix elements (Pisano et al 2014(Pisano et al , 2020(Pisano et al , 2012. In this analysis, we adopt a model of the HWP derived for the MFT channels (see Figures 5, 6) (Montier et al 2020;Lamagna et al 2021).…”

Detailed study of HWP non-idealities and their impact on future measurements of CMB polarization anisotropies from space

“…First of all, we allowed for HWP matrix elements to be frequency-dependent: we assumed a specific LiteBIRD MHWP design with certain frequency pro-files within each band. The MHWP performance was computed using realistic models developed for previous waveplate applications (Pisano et al 2022).…”