Giampaolo Pisano Radioastronomy Technology Group Jodrell Bank Centre for Astrophysics, University of Manchester, UK B-Pol Meeting - Paris, July 2010 B-Pol Satellite experiment: HWP technologies comparison The University of Manchester

Half-Wave Plates: Requirements - In sensitive multi-pixel array applications there are many demanding requirements for the HWP characteristics, let’s mention the most general ones: 2 Robust and light device: mechanical rotation needed; should not vibrate. 1 Large dimensions: (up to 30 cm in diameter) to achieve meaningful sensitivities. 3 Anti-reflection coatings (ARCs): to achieve broadband performance. 4 Low absorption losses: to minimise the thermal emission seen by the detectors (  also very low differential losses between fast/slow axes). 5 Polarisation systematic effects introduced by the device: deep understanding and good control both needed.

Sapphire Achromatic HWPs: Pancharatnam designs G. Pisano et al., Applied Optics v45, n26 (2006) G. Savini et al., Applied Optics v45, n35 (2006) ~10cm - Recipes based on birefringent plates:  Low absorption (2-4%) - Can be achieved by cooling at cryogenic temperatures but : differential loss axes ~0.1% (not reducible) 4  Very robust but heavy 2  Limited maximum diameters  Very expensive plates (Quartz Ø ~110mm; Sapphire Ø ~280 mm) 1  New ARCs to be synthesised - For sapphire (n~3.4)  new materials needed (n~1.8) 3 (Ex: 3-plates sapphire, no ARC)

Results Sapphire Achromatic HWPs: Modelling & FTS Tests G. Pisano et al., Applied Optics v45, n26 (2006) G. Savini et al., Applied Optics v45, n35 (2006) Extrapolated Minimum Cross Polarisation Measured Cross-Polarisation Absorption calculation X-pol ~-25/-30dB Results Fast axis Transmission 5  We have excellent understanding of the polarisation systematics

Sapphire Achromatic HWPs: Quasi-Optical Tests 5  We can characterise/control the polarisation systematics with high accuracy Co-Pol & Cross-Pol Beams Horn-OMT pixel + HWP Averaged Cross-Pol: -29dB - VNA measurements: (Across beam and frequency band)

Mesh Half-Wave Plate: Air-gap design G. Pisano et al., Applied Optics v47, n33 (2008) Capacitive Stack Inductive Stack   ARCs not required ! 3  Absorption can be very low at room T (1.5%)  Differential losses (0.6%) could be equalised 4  Limits in diameter ~100mm 1  It is very fragile and can vibrate ! 2

Mesh HWP : Air-gap design results Fast Axis Transmission Slow Axis Transmission Differential Phase-Shift Cross-Polarisation G. Pisano et al., Applied Optics v47, n33 (2008) X-pol ~ -25dB 5  We have very good understanding of the polarisation systematics

Mesh HWP: Dielectrically embedded design (v.1) G. Pisano et al., to be submitted to Appl. Opt. Pol 1 Pol 2 Capacitive Stack Inductive Stack   ARC commercially available 3  Present diameter ~200mm (soon 300mm !) 1  Very robust & light 2  Absorption should be low at room T  Differential loss could be equalised 4

Mesh HWP: Dielectrically embedded design (v.2) G. Pisano et al., to be submitted to Appl. Opt. Recipe with Inductive and Capacitive lines on the same grid  One C/L stack only  Grids # reduced by a factor 2  Grids easier to align  should have lower losses 20cm

Conclusions - HWP studies: - Birefringent vs Mesh options - We know how to model and build both - Mesh HWP the most promising  We should be able to meet all the requirements