  We report on the development of scalable prototype microwave kinetic
inductance detector (MKID) arrays tailored for future multi-kilo-pixel
experiments that are designed to simultaneously characterize the polarization
properties of both the cosmic microwave background (CMB) and Galactic dust
emission. These modular arrays are composed of horn-coupled,
polarization-sensitive MKIDs, and each pixel has four detectors: two
polarizations in two spectral bands between 125 and 280 GHz. A horn is used to
feed each array element, and a planar orthomode transducer, composed of two
waveguide probe pairs, separates the incoming light into two linear
polarizations. Diplexers composed of resonant-stub band-pass filters separate
the radiation into 125 to 170 GHz and 190 to 280 GHz pass bands. The
millimeter-wave power is ultimately coupled to a hybrid co-planar waveguide
microwave kinetic inductance detector using a novel, broadband circuit
developed by our collaboration. Electromagnetic simulations show the expected
absorption efficiency of the detector is approximately 90%. Array fabrication
will begin in the summer of 2016.
