Experimental characterization of a planar phase-engineered metamaterial lenslet for millimeter astronomy.

To unveil presently inscrutable details of the origins of our universe imprinted in the cosmic microwave background, future experiments in the millimeter and submillimeter range are focusing on the detection of fine features, which necessitate large and sensitive detector arrays to enable multichroic mapping of the sky. Currently, various approaches for coupling light to such detectors are under investigation, namely, coherently summed hierarchical arrays, platelet horns, and antenna-coupled planar lenslets. The last option offers increased bandwidth and a simpler fabrication while maintaining the desired optical performance. In this work, the design, fabrication, and experimental characterization of a prototype planar metamaterial phase-engineered lenslet operating in W-band [75 GHz; 110 GHz] is presented. Its radiated field, initially modeled and measured on a systematics-limited optical bench, is compared against a simulated hyperhemispherical lenslet, a more established technology. It is reported here that our device reaches the cosmic microwave background (CMB) specification for the next stages of experiments, demonstrating power coupling above 95% and beam Gaussicity above 97% while maintaining ellipticity below 10% and a cross-polarization level below -21d B through its operating bandwidth. Such results underline the potential advantages our lenslet can offer as focal optics for future CMB experiments.

SLAC microresonator RF (SMuRF) electronics: A tone-tracking readout system for superconducting microwave resonator arrays.

We describe the newest generation of the SLAC Microresonator RF (SMuRF) electronics, a warm digital control and readout system for microwave-frequency resonator-based cryogenic detector and multiplexer systems, such as microwave superconducting quantum interference device multiplexers (µmux) or microwave kinetic inductance detectors. Ultra-sensitive measurements in particle physics and astronomy increasingly rely on large arrays of cryogenic sensors, which in turn necessitate highly multiplexed readout and accompanying room-temperature electronics. Microwave-frequency resonators are a popular tool for cryogenic multiplexing, with the potential to multiplex thousands of detector channels on one readout line. The SMuRF system provides the capability for reading out up to 3328 channels across a 4-8 GHz bandwidth. Notably, the SMuRF system is unique in its implementation of a closed-loop tone-tracking algorithm that minimizes RF power transmitted to the cold amplifier, substantially relaxing system linearity requirements and effective noise from intermodulation products. Here, we present a description of the hardware, firmware, and software systems of the SMuRF electronics, comparing achieved performance with science-driven design requirements. In particular, we focus on the case of large-channel-count, low-bandwidth applications, but the system has been easily reconfigured for high-bandwidth applications. The system described here has been successfully deployed in lab settings and field sites around the world and is baselined for use on upcoming large-scale observatories.

Development of Flat Silicon-Based Mesh Lens Arrays for Millimeter and Sub-millimeter Wave Astronomy.

The high sensitivity requirements set by future cosmic microwave background instruments are pushing the current technologies to produce highly performant focal plane arrays with thousands of detectors. The coupling of the detectors to the telescope optics is a challenging task. Current implemented solutions include phased-array antenna-coupled detectors, platelet horn arrays, and lenslet-coupled planar antennas. There are also recent developments of flat graded-index lenses based on etched silicon. However, there are strong requirements in terms of electromagnetic performance, such as coupling efficiency and bandwidth, as well as requirements in terms of easy manufacturing and scalability, and it is very challenging to meet all these requirements with one of the above solutions. Here, we present a novel approach for producing flat metal-mesh lenslet arrays based on devices previously realized using the mesh-filter technology. We have now adapted the polypropylene-based mesh lens design to silicon substrates, thus providing a good mechanical match to the silicon-based detector arrays. The measured performance of prototype pixels operating at millimeter wavelengths is presented.

Tile-and-trim micro-resonator array fabrication optimized for high multiplexing factors.

We present a superconducting micro-resonator array fabrication method that is scalable and reconfigurable and has been optimized for high multiplexing factors. The method uses uniformly sized tiles patterned on stepper photolithography reticles as the building blocks of an array. We demonstrate this technique on a 101-element microwave kinetic inductance detector (MKID) array made from a titanium-nitride superconducting film. Characterization reveals 1.5% maximum fractional frequency spacing deviations caused primarily by material parameters that vary smoothly across the wafer. However, local deviations exhibit a Gaussian distribution in fractional frequency spacing with a standard deviation of 2.7 × 10-3. We exploit this finding to increase the yield of the BLAST-TNG 250 µm production wafer by placing resonators in the array close in both physical and frequency space. This array consists of 1836 polarization-sensitive MKIDs wired in three multiplexing groups. We present the array design and show that the achieved yield is consistent with our model of frequency collisions and is comparable to what has been achieved in other low temperature detector technologies.

Millimeter-wave achromatic half-wave plate.

We have constructed an achromatic half-wave plate (AHWP) suitable for the millimeter wavelength band. The AHWP was made from a stack of three sapphire a-cut birefringent plates with the opticalaxes of the middle plate rotated by 50.5 deg with respect to the aligned axes of the other plates. The measured modulation efficiency of the AHWP at 110 GHz was 96 +/- 1.5%. In contrast, the modulation efficiency of a single sapphire plate of the same thickness was 43 +/- 4%. Both results are in close agreement with theoretical predictions. The modulation efficiency of the AHWP was constant as a function of incidence angles between 0 and 15 deg. We discuss design parameters of an AHWP in the context of astrophysical broadband polarimetry at the millimeter wavelength band.