Optical efficiency of feedhorn-coupled TES polarimeters for next-generation CMB instruments

The next generation of Cosmic Microwave Background (CMB) experiments probing for signals of inflation and small angular scale polarization anisotropies require higher sensitivity and better control of systematics. We are developing monolithic arrays of orthomode transducer (OMT) coupled transition edge sensor (TES) polarimeters designed for operation at 150 GHz to address these requirements. OMT coupling allows for simultaneous and independent detection of two orthogonal linear polarization states incident on a single pixel. We present measurements of optical efficiencies η op of single pixels with on-chip band-defining filters, with η op = 57±4 stat±9 sys %. We also provide evidence for an out-of-band blue leak and address possible sources as well as mitigation techniques. Additionally, we discuss methods for increasing efficiency being implemented in the next generation of pixels, currently in fabrication. Still under development, these pixels are produced as monolithic polarimeter arrays and are slated for use in the Atacama Cosmology Telescope Polarization (ACTpol) and South Pole Telescope Polarization (SPTpol) experiments, while single-pixel polarimeters are to be deployed in the Atacama B-mode Search (ABS) experiment.