  We describe the design and measurement of feedhorn-coupled, transition-edge
sensor (TES) polarimeters with two passbands centered at 220 GHz and 280 GHz,
intended for observations of the cosmic microwave background. Each pixel
couples polarized light in two linear polarizations by use of a planar
orthomode transducer and senses power via four TES bolometers, one for each
band in each linear polarization. Previous designs of this detector
architecture incorporated passbands from 27 GHz to 220 GHz; we now demonstrate
this technology at frequencies up to 315 GHz. Observational passbands are
defined with an on-chip diplexer, and Fourier-transform-spectrometer
measurements are in excellent agreement with simulations. We find coupling from
feedhorn to TES bolometer using a cryogenic, temperature-controlled thermal
source. We determine the optical efficiency of our device is $\eta$ =
77%$\pm$6% (75%$\pm$5%) for 220 (280) GHz, relative to the designed passband
shapes. Lastly, we compare two power-termination schemes commonly used in
wide-bandwidth millimeter-wave polarimeters and find equal performance in terms
of optical efficiency and passband shape.
