We report on the development of a polarization-sensitive dichroic (150/220 GHz) detector array for the Cosmology Large Angular Scale Surveyor (CLASS) delivered to the telescope site in June 2019. In concert with existing 40 and 90 GHz telescopes, the 150/220 GHz telescope will make observations of the cosmic microwave background over large angular scales aimed at measuring the primordial B-mode signal, the optical depth to reionization, and other fundamental physics and cosmology. The 150/220 GHz focal plane array consists of three detector modules with 1020 transition edge sensor (TES) bolometers in total. Each dual-polarization pixel on the focal plane contains four bolometers to measure the two linear polarization states at 150 and 220 GHz. Light is coupled through a planar orthomode transducer (OMT) fed by a smooth-walled feedhorn array made from an aluminum-silicon alloy (CE7). In this work, we discuss the design, assembly, and in-lab characterization of the 150/220 GHz detector array. The detectors are photon-noise limited, and we estimate the total array noise-equivalent power (NEP) to be 2.5 and 4 aW √ s for 150 and 220 GHz arrays, respectively.

We report a study of the frequency response of AT and BT cut Quartz Crystal Microbalances (QCMs) to laser illumination. AT and BT cut QCMs have a similar response to mass adsorption but the opposite frequency response to stress levels, allowing the individual contributions to stress and mass desorption induced by the laser light to be quantitatively investigated. A detailed analysis of the transient thermal behaviors in the quartz crystals has also been performed, which yields stress levels that are consistent with the observed results. Our results quantitatively confirm previous suggestions that radial compressive stress is a dominant contributor to the response of the QCM to laser illumination. They also establish a theoretical framework for the analysis of the transient phenomenon to infer temperature shifts and radial stress levels in the system irrespective of whether both AT and BT cut crystals are utilized.

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