Electrothermal characterization of AlMn transition-edge sensor bolometers for advanced ACTPol

Abstract

Current and future Cosmic Microwave Background (CMB) polarization anisotropy measurements demand large (104-105) numbers of high-performing detectors in order to unveil minute signals. At the forefront of the detector options available to this field are transition-edge sensor (TES) bolometers, which use superconducting thin films as extremely sensitive thermistors. Understanding the behavior of these TESes is critical to leveraging their full potential and identifying trade-offs between robustness and sensitivity in their design. In this work, we report on measurements of TES bolometers designed for the Advanced ACTPol (AdvACT) upgrade to the Atacama Cosmology Telescope (ACT). Using a three-stage SQUID amplifier chain, time-division multiplexing electronics, and an external signal generator, we characterize the impedance and noise properties of a set of test TES bolometers measured in the laboratory, encompassing various designs, over a wide signal band. Using the impedance data, we are able to explore deviations from the assumed bolometer model in use for TES design and instrument sensitivity forecasts. Modeling these deviations as the result of decoupling of bolometer elements, we extract parameters describing this extended model. We are then able to compare the noise spectra of the devices to forward modeling based on the TES parameters in the extended model. We explore how excess noise varies, and conclude with a discussion of how our results can be useful for designing future TES arrays based on the expected impact of the measured effects on instrumental sensitivities, and the relevance of these results to bolometer designs for upcoming CMB experiments like Simons Observatory and CMB-S4.