  We describe a novel multi-layered metal mesh achromatic half wave plate for
use in astronomical polarimetric instruments. The half wave plate is designed
to operate across the frequency range from 125-250 GHz. The wave plate is
manufactured from 12-layers of thin film metallic inductive and capacitive
grids patterned onto polypropylene sheets, which are then bonded together using
a hot pressing technique. Transmission line modelling and 3-D electromagnetic
simulations are used to optimize the parameters of the metal-mesh patterns and
to evaluate their optical properties. A prototype half wave plate has been
fabricated and its performance characterized in a polarizing Fourier transform
spectrometer. The device performance is consistent with the modelling although
the measured differential phase shift for two orthogonal polarizations is lower
than expected. This difference is likely to result from imperfect patterning of
individual layers and misalignment of the grids during manufacture.
