Shannon M. Duff

Galaxies with substantial star formation rates often exhibit large-scale winds composed of material flowing out of the galaxy. The galactic fountain model posits that some fraction of this material reaches a large distance from the galaxy, cools, and then recondenses and falls back onto the galaxy. However, details of the condensing material’s density, ionization state, and temperature are not known. M82 exhibits an archetypal multi-phase galactic wind comprising a mixture of hot plasma, dust, and neutral gas. Far-infrared spectral observations of the wind and cooling region around the galaxy can elucidate the physical characteristics and time scales of the fountain mechanism. Here we calculate the expected intensity of this signal under simple assumptions, and demonstrate that a suitably designed sub-orbital experiment could search for direct emission from the cooling material comprising the galactic fountain in M82. We also study notional instrument designs for a sounding rocket and for a balloon payload, and determine their relative performance in the presence of realistic photon loading.

LiteBIRD is a planned space mission to observe the polarized cosmic microwave background in the frequency range 34 GHz to 448 GHz, with the goal of measuring or constraining the tensor-to-scalar ratio r. We have developed silicon platelet feedhorns for LiteBIRD, consisting of stacked metalized silicon wafers with deep-etched holes to create an array of feedhorns with profiled tapers and a coefficient of thermal contraction that is matched to the detector wafer. We present beam pattern measurements for silicon platelet feedhorn prototypes designed for 166-322 GHz, 200-388 GHz, and 356-448 GHz, corresponding to each of the planned LiteBIRD high-frequency pixel types. The measurements were carried out on a room-temperature millimeter-wave vector network analyzer and are in excellent agreement with simulation.

The Simons Observatory (SO) is a cosmic microwave background experiment with over 67,000 polarization-sensitive transition-edge sensor (TES) detectors currently installed for use in observations and plans to increase the total detector count to $\sim$98,000 detectors with the Advanced SO upgrade. The TES arrays are packaged into Universal Focal-Plane Modules (UFMs), which also contain the multiplexing readout circuit. Within a readout module, a DC/RF routing wafer provides a cold interface between the detectors and the readout multiplexing chips. Each routing wafer hosts twelve bias lines, which contain the $\sim$400 $\mu \Omega$ shunt resistors that are part of the TES bias circuitry. More than 70 routing wafers have been fabricated and tested both at room temperature and 100 mK before integration into UFMs. The lab measurements for all screened wafers have been compiled to show the distribution of measured average shunt resistance $R_{sh}$ for each bias line, both across bias lines on a single routing wafer and across all routing wafers. The mean average shunt resistance for all wafers was found to be 396 $\mu \Omega$ with a standard deviation of 16 $\mu \Omega$, or $\sim$$4 \%$. For each wafer, we note good uniformity of average $R_{sh}$ between bias lines, with a slight downward trend with increasing distance from the center of the wafer. The fabrication data collected at room temperature shows agreement with the cryogenic measurements of $R_{sh}$ distribution.

The Josephson junction radiator (JJR) is a thin-film superconducting device that converts DC voltage into coherent millimeter-wave (mm-wave) radiation. The source is frequency-tunable, instantaneously narrow-band, compact, lightweight, has no moving parts, and may share the same cryogenic and vacuum space as the device under test. With these favorable characteristics, the JJR may find use as a general purpose calibration tool for low temperature mm-wave detectors. We show that the JJR unambiguously emits mm-wave radiation and couples to a mm-wave detector. We use the JJR to determine the spectral response of a superconducting mm-wave detector that operates in two large frequency passbands centered at 150 GHz and 220 GHz. A comparison to measurements with a conventional Fourier Transform Spectrometer is presented, and the use of the JJR as a precision spectral calibration tool is discussed.

We present updated forecasts for the scientific performance of the degree-scale (0.5 deg FWHM at 93 GHz), deep-field survey to be conducted by the Simons Observatory (SO). By 2027, the SO Small Aperture Telescope (SAT) complement will be doubled from three to six telescopes, including a doubling of the detector count in the 93 GHz and 145 GHz channels to 48,160 detectors. Combined with a planned extension of the survey duration to 2035, this expansion will significantly enhance SO's search for a B -mode signal in the polarisation of the cosmic microwave background, a potential signature of gravitational waves produced in the very early Universe. Assuming a 1/ f noise model with knee multipole ℓ _knee = 50 and a moderately complex model for Galactic foregrounds, we forecast a 1 σ (or 68% confidence level) constraint on the tensor-to-scalar ratio r of σ _r = 1.2 × 10 ^-3 , assuming no primordial B-modes are present. This forecast assumes that 70% of the B -mode lensing signal can ultimately be removed using high resolution observations from the SO Large Aperture Telescope (LAT) and overlapping large-scale structure surveys. For more optimistic assumptions regarding foregrounds and noise, and assuming the same level of delensing, this forecast constraint improves to σ _r = 7 × 10 ^-4 . These forecasts represent a major improvement in SO's constraining power, being a factor of around 2.5 times better than what could be achieved with the originally planned campaign, which assumed the existing three SATs would conduct a five-year survey.

Aluminum doped with small concentrations of manganese (AlMn) is an important material for the fabrication of superconducting transition-edge sensor (TES) bolometers and microcalorimeters. AlMn is a versatile thin film material and has a tunable superconducting critical temperature T_c, which has been demonstrated to be repeatable and uniform across 150 mm wafers. In the last decade, large-format arrays of highly sensitive AlMn TES bolometers have been deployed to numerous cosmological experiments to study the cosmic microwave background (CMB) such as Advanced ACTPol on the Atacama Cosmology Telescope in Chile, the Simons Observatory in Chile, and the SPIDER balloon-borne instrument which has undergone two flights from the South Pole. Next generation CMB projects and future far-infrared space missions will rely on the demonstrated success of AlMn TESs. Understanding the long-term stability of this thin film material is critical to minimize risk for these future projects. We have characterized the effects of temperature and humidity on patterned and unpatterned AlMn thin films as an essential step for predicting performance limits and establishing failure criteria for future experiments utilizing AlMn TES detectors. Our findings show that AlMn exposed to elevated temperature (60°C and 85°C) and humidity (60% and 85% relative humidity) for prolonged time periods exhibits excellent stability, with small or negligible changes measured in superconducting critical temperature and normal resistance. We show that the presence or absence of a passivating dielectric layer does not alter these results. Additionally, we present the effects of re-annealing passivated and unpassivated AlMn TESs, which may aid in achieving a targeted T_c during the fabrication process. The sample preparation, environmental testing, and data collection and analysis are discussed.

The development of large-scale, high-resolution transition-edge sensor (TES) arrays requires precise, wafer-scale control of the superconducting critical temperature (T_c). For microcalorimeters, this is often achieved with a superconductor-normal metal bilayer where the thickness ratio between the two materials determines T_c through the proximity effect. Reliably achieving a specific T_c across multiple wafers with good uniformity remains challenging as the fabrication process is highly sensitive to substrate quality, deposition parameters, and deposited thin-film thickness. Missing a T_c target results in an array with degraded or unusable performance and wastes fabrication/measurement resources. To help solve these limitations with TES microcalorimeter fabrication, we have developed a method of measuring 76.2 mm wafers after bilayer deposition to screen for T_c accuracy and uniformity. Four-wire resistance measurements of test structures across the wafer provide rapid feedback early in the fabrication process at temperatures down to 20 mK. We leverage the hybrid additive-subtractive TES process to screen TES arrays with multiple T_c values on a single wafer comprised of a single bilayer. After measurement, the bilayer normal metal thickness can be increased with an additional evaporation step to lower T_c or decreased with an ion mill step to raise T_c as needed. We also present a design to expand measurement capability to 150 mm wafers to screen both larger microcalorimeter arrays as well as NIST cosmic microwave background TES bolometer arrays.

The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment designed to measure the polarization of the cosmic microwave background on large angular scales to probe cosmic reionization and search for the inflationary $B$-mode signal. CLASS is a multi-frequency ensemble of telescopes with bands centered at 40, 90, 150, and 220GHz. Each telescope has arrays of feedhorn-coupled transition edge sensor bolometers at the focal plane. The frequency response is primarily defined by the on-chip bandpass filter with additional contributions coming from the feedhorn, orthomode transducer, and 180-degree hybrid. In this study, we compare simulations and measurements of the frequency response of single pixel witness devices in the 90GHz band with and without the bandpass filter. For the first time, we can separate the effects of the bandpass filter from the other microwave components using Fourier transform spectroscopy and design splits of the pixel. The results show that the -3 dB band edges are 80GHz and 108 GHz. The measurements demonstrate a robust method for characterizing the spectral response of individual components, which is crucial for optimizing the performance of future detector arrays.