  CONTEXT.Large field-of-view imaging/polarimetry instruments operating at
millimeter and submm wavelengths are fundamental tools to understand the role
of magnetic fields (MF) in channeling filament material into prestellar cores
providing a unique insight in the physics of galactic star-forming regions.
Among other topics, at extra-galactic scales, polarization observations of AGNs
will allow us to constrain the possible physical conditions of the emitting
plasma from the jets and/or exploring the physics of dust inside supernova
remnants. The kilo-pixel NIKA2 camera, installed at the IRAM 30-m telescope,
represents today one of the best tools available to the astronomers to produce
simultaneous intensity/polarimetry maps over large fields at 260 GHz (1.15 mm).
AIMS.The polarization measurement, in NIKA and NIKA2, is achieved by rapidly
modulating the total incoming polarization. This allows in the end to safely
isolate the small science signal from the large, un-polarized and strongly
variable, atmospheric background. METHODS.The polarization 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 exceeding 250 mm. The modulation of the polarized
signal, at 12 Hz, requires also 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.(abridged)
