  We have developed a bolometric detector that is intrinsically sensitive to
linear polarization which is optimized for making measurements of the
polarization of the cosmic microwave background radiation. The receiver
consists of a pair of co-located silicon nitride micromesh absorbers which
couple anisotropically to linearly polarized radiation through a corrugated
waveguide structure. This system allows simultaneous background limited
measurements of the Stokes I and Q parameters over ~ 30% bandwidths at
frequencies from ~ 60 to 600 GHz. Since both linear polarizations traverse
identical optical paths from the sky to the point of detection, the
susceptibility to systematic effects is minimized. The amount of uncorrelated
noise between the two polarization senses is limited to the quantum limit of
thermal and photon shot noise, while drifts in the relative responsivity to
orthogonal polarizations are limited to the effect of non-uniformity in the
thin film deposition of the leads and the intrinsic thermistor properties.
Devices using NTD Ge thermistors have achieved NEPs of 2 x 10^{-17} W/sqrt{Hz}
with a 1/f knee below 100 mHz at a base temperature of 270 mK. Numerical
modelling of the structures has been used to optimize the bolometer geometry
and coupling to optics. Comparisons of numerical results and experimental data
are made. A description of how the quantities measured by the device can be
interpreted in terms of the Stokes parameters is presented. The receiver
developed for the Boomerang and Planck HFI focal planes is presented in detail.
