A Metal Mesh Achromatic Half Wave Plate G.Pisano 1 , G.Savini 2 , P.A.R. Ade 2 and V.Haynes 1 1 The University of Manchester, School of Physics and Astronomy - Alan Turing Building, Upper Brooke street, Manchester, M13 4PL (UK) 2 Cardiff University, School of Physics and Astronomy, Queens Buildings, The Parade, Cardiff, CF24 3AA (UK) giampaolo.pisano@manchester.ac.uk Abstract. Achromatic Half-Wave Plates can be realised using photolithographic techniques. Prototypes working at 150GHz have been designed, manufactured and tested and preliminary performance results are presented [1]. This type of device could replace the usual birefringent HWP used in mm/submm polarimetry experiments. 1. Introduction Birefringent material based HWPs are used in many present experiments dedicated to the detection of the polarisation of the Cosmic Microwave Background radiation. Accurate models [2,3] have been developed which enable accurate prediction the HWP performances and enable through design modifications a minimization of systematic effects such as the inherent instrumental and the cross polarization levels. However, the demanding requirements for the next generation of CMB instruments, which aim to detect the very low level B-Mode signature, necessitates a new HWP concept which can be utilized with the very large arrays needed to achieve a meaningful sensitivity limit. The costs and the availability of birefringent materials of increasing size is a known problem (crystalline quartz available in ~ 110mm diameter boules and crystalline sapphire available in ~280 mm diameter boules) so alternative solutions must be sought. The type of HWP proposed here has a very good chance of replacing the crystalline technology with one constructed from metal mesh components. The manufacture of metal mesh components is a well established technology and will not be discussed in detail here (see filter review of these proceedings). 2. Concept A Half Wave Plate introduces a phase shift of 180 degrees between two monochromatic electro- magnetic waves that have orthogonal polarizations and that are traveling through it. In an Achromatic HWP, this phase shift is maintained over a broader electromagnetic frequency band. When rotating, a HWP rotates any polarized incident vector at twice the angular velocity of the plate. Thus any linearly polarised incident radiation can be modulated and detected by using detectors sensitive to the polarisation (analyzers, OMTs, etc.) after the HWP. The phase shift between the orthogonal axes is normally achieved using the difference between the refractive indices of the ordinary and the extraordinary axes of a birefringent material. Hence , the plate thickness determines the frequency (or