FPP Instrument: Review of quasi-optical Polarisation Modulators The University of Manchester FPP Instrument: Review of quasi-optical Polarisation Modulators Giampaolo Pisano Radioastronomy Technology Group Jodrell Bank Centre for Astrophysics, University of Manchester, UK FPP Workshop - Henri Poincaré Institute, Paris, 8th-9th October 2010

Polarisation modulator baseline: Reflecting Half-Wave Plate (RHWP) A bit challenging !

Polarisation modulator: Some of the present requirements 1 Very large dimensions   1.2 m !! 2 Broadband performance  Bandwidth ~180% !! 3 Robust and light device: mechanical rotation needed 4 Modulation efficiency:  80%? 5 Low absorption losses (also differential losses): thermal emissivity 6 Polarisation systematic effects: deep understanding / control needed 7 ...

RHWP: Bands and efficiency see G. Siringo et al., Laboca Experiment D - Phase shift between s & p pol - Modulation efficiency Freq [GHz] BW [%] 60 33 100 20 140 14 180 11 220 9 340 5.9 540 3.7 820 2.4 Bandwidth such that the averaged e~0.8  Dn=20GHz (independent on frequency) d=5.3mm, f=45  nn=20(2n+1)GHz Example  Cross-Pol issues to be solved

(50 cm diameter wire-grid example) RHWP: Feasibility D. Chuss (2008) (50 cm diameter wire-grid example) 500mm diameter wire grids has been built (see VPM - D.Chuss later)  Is it possible to go up to ~1.2m? 1 2  RHWP bandwidth needs to be improved 3  It would be very fragile, will the wires bend ?

RHWP: Bandwidth increase If we could use filters within the 30% bandwidth to select sub-bands where the average modulation efficiency is >80%:  Increase in effective bandwidth Example 540GHz channel: Freq [GHz] 60 100 140 180 220 340 540 820 BW [%] 33 20 14 11 9 5.9 3.7 2.4 BW+ [%] 33 20 14 11 12 17 15  Increase from 3.7% to 15% in BW

Other known polarisation modulators Variable Phase Delay modulators (VPM) Birefringent HWPs Mesh HWPs (Air-gap or dielectrically embedded) Note: we are not considering the following devices because they are relatively ‘narrow’ band (30-40%): Waveguide polarisation modulators/rotators: Faraday rotators, rotating waveguides Microstrip devices: MEMS switches, SC switches. Etc.

Similar polarisation modulator: Variable Phase Delay Modulator D. Chuss (2008) This type of modulator does not modulate Q and U at the same time  Can this apply in our case ?

(Example of 3-plate sapphire recipe, no ARC) Birefringent HWPs: Pancharatnam designs - Recipes based on birefringent plates: ~10cm (Example of 3-plate sapphire recipe, no ARC) Limits on maximum diameters available :  Quartz Ø ~110mm, Sapphire Ø ~280 mm 1 2 Bandwidth: 5-plate recipe ~100%

(Example of inductive stack) Mesh Half-Wave Plate: Air-gap design G. Pisano et al., Applied Optics v47, n33 (2008) - Recipes based on metal grids geometry/spacing: ~4cm (Example of inductive stack) Dimension in principle achievable but very thin substrates required  Present limits in diameter ~200mm 1 2  Present max bandwidth ~70% 3  Too fragile, it can vibrate

(Example of embedded mesh-HWP) Mesh HWP: Dielectrically embedded design Pol 1 Pol 2 20cm (Example of embedded mesh-HWP)  Present hot-pressing working up to 300mm (near future 500mm) Alternative ‘cold bonding’ for bigger diameters under study 1 2 Bandwidth similar to air-gap Very robust & light although it might bend with diameters >1m  Flatness problem 3

Other types and other possible solutions of RHWPs Dielectrically embedded RHWP Twist reflectors - Dielectrically embedded Mesh RHWP Hard & Soft surfaces Artificial surfaces

Modified RHWPs: Dielectrically embedded RHWP  Photolithographic Wire-grid Anti-Reflection Coating  Dielectric substrate   Mirror 1  Dimensions: should be feasible using photolithography (2 evaporated/etched substrates + cold bonding) * 2  Bandwidth: same as the free-standing one ? Very light & robust (held by a mirror) 3 (*) - 2 m diameter evaporator chambers available Possible to print masks on 2m width acetate Printer resolution will allow to build grids with 50um period and 25um strip:  Wire-grid efficiency still >90% at 1THz frequency

Other RHWPs: Twist reflectors - They are meant to provide 180º phase-shift and work off-axis a) b) c) K.C Hwang El.Lett. (2008) R.Kastner IEEE TAP (1982) K.C Hwang IEEE MWCL (2010) Corrugated metal surface Meander-grooved metal surface Meander-strips on dielectric/ metal surfaces 1 Dimensions:  Ok: depends on CNC machines, photolithography 2 Bandwidth:  a) ~10% , b) 15% , c) 24%  All too narrow

Other RHWPs: Dielectrically embedded Mesh-RHWP - Can we improve the bandwidth using multi-layered embedded grids ? Anti-Reflection Coating  Dielectric substrates   C/L grids  Mirror  Present hot-pressing working up to 300mm Alternative ‘cold bonding’ for bigger diameters not ready yet 1 2  Bandwidth: same as the free-standing one ? What about the off-axis behaviour ? Very light & robust (held by a mirror) 3

Other RHWPs: Hard & Soft surfaces - Corrugated surfaces are part of the family of Hard & Soft surfaces P.S. Kildal - Could we design a very broadband RHWPs using this kind of surfaces ?

Other RHWPs: Artificial surfaces (Metasurfaces) - Many more complex surfaces are used to control the propagation of waves at grazing incidence: P.S. Kildal (2009) - The surface impedance can be customised: Q. Wu (2010) Can we tailor the phase characteristics in order to design very broadband RHWPs?

RHWP: Improving efficiency D - Phase shift between s & p pol - D does not depend only on the path difference between s & p polarisations We are implicitly assuming the metallic reflection to give a phase-shift of p Artificial surface  Could we improve the RHWP performance (bandwidth and cross-pol) using a frequency dependent ‘artificial’ surface’ instead of a flat mirror?

In the view of the imminent proposal writing: Discussion.. In the view of the imminent proposal writing: - Can we keep the wire-grid RHWP as baseline with the present performance? - Can we improve the RHWP bandwidth ? - Shall we investigate the dielectrically embedded RHWP ? - How can we reduce the cross-pol effects ? Flatter efficiencies across bands. - Other ideas? - ...