UC Berkeley

We describe the design, fabrication, and laboratory-demonstration of a novel dual-polarized multichroic antenna-coupled Transition Edge Sensor (TES) bolometer. Each pixel separates the incident millimeter radiation into two linear polarization channels as well as several frequency channels (bands). This technology enables us to realize bolometer arrays for Cosmic Microwave Background (CMB) polarimetry measurements that map the sky at multiple colors while simultaneously boosting the optical throughput over what would have been attained from arrays of single-frequency channel detectors. Observations at multiple frequency channels are important for differentiating polarized galactic foregrounds and atmospheric fluctuations from the CMB.

Each pixel couples free-traveling radiation onto lithographed microstrip transmission lines prior to the bolometers using a dual-polarized broadband antenna known as a sinuous antenna. The transmission lines are integrated onto the back of the antenna arms and the antennas are in direct contact with an extended-hemispherical lens. We show measurements of scale model (4-12GHz) and to-scale (80-240Hz) antennas to demonstrate high antenna-gain, low cross-polarization contamination, and efficient coupling over a 1-2 octave bandwidth.

We have developed microstrip circuits that divide the antenna's wide bandwidth into smaller channels. In one scheme, two or three frequency channels can be extracted from the antenna's received power using microstrip circuits known as diplexers and triplexers. These avoid atmospheric spectral lines and are well suited to terrestrial observations. We can also partition this bandwidth into contiguous bands using cochlear channelizers inspired by the physiology of the human ear; this design is most advantageous for satellite missions where there are no concerns about atmospheric contamination. We present design methodologies for these circuits and show measurements of prototypes coupled to TES bolometers to verify acceptable performance. We also describe the fabrication of a broadband anti-reflection coating for the contacting lenses and demonstrate that lens-coupled sinuous pixels receive more power with the coatings than without. Finally, we remark on the last un-resolved challenge of forming symmetric beams and balun designs that may help form patterns more useful for polarimetry.

This technology is a candidate for use in the Polarbear ground-based experiment. By packing more detectors into the focal-plane than can be done with monochromatic pixels, multichroic pixels will allow Polarbear to map the sky much faster. This technology is also candidate for future space-based missions as well, where multhchroic pixels will allow a less massive payload and hence a lower cost mission. Finally, we envision using arrays of similar pixels in sub-millimeter observations of high-redshift galaxy clusters as well (e.g.example Sunyaev-Zeldovich Effect measurements). However, we require more sophisticated lithography and etching techniques to shrink these pixels to a size suitable for such wavelengths.