The Impact of Atmospheric Fluctuations on Degree-scale Imaging of the Cosmic Microwave Background

Fluctuations in the brightness of the Earth's atmosphere originating from water vapor are an important source of noise for ground-based instruments attempting to measure anisotropy in the Cosmic Microwave Background. This paper presents a model for the atmospheric fluctuations and derives simple expressions to predict the contribution of the atmosphere to experimental measurements. Data from the South Pole and from the Atacama Desert in Chile, two of the driest places on Earth, are used to assess the level of fluctuations at each site.Comment: 29 pages, 7 figures, 1 table, appears in The Astrophysical Journa

Development of at 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

Horn Coupled Multichroic Polarimeters for the Atacama Cosmology Telescope Polarization Experiment

Multichroic polarization sensitive detectors enable increased sensitivity and spectral coverage for observations of the Cosmic Microwave Background (CMB). An array optimized for dual frequency detectors can provide 1.7 times gain in sensitivity compared to a single frequency array. We present the design and measurements of horn coupled multichroic polarimeters encompassing the 90 and 150 GHz frequency bands and discuss our plans to field an array of these detectors as part of the ACTPol project

Planar Silicon Metamaterial Lenslet Arrays for Millimeter-wavelength Imaging

Large imaging arrays of detectors at millimeter and submillimeter wavelengths have applications that include measurements of the faint polarization signal in the Cosmic Microwave Background (CMB), and submillimeter astrophysics. We are developing planar lenslet arrays for millimeter-wavelength imaging using metamaterials microlithically fabricated using silicon wafers. This metamaterial technology has many potential advantages compared to conventional hemispherical lenslet arrays, including high precision and homogeneity, planar integrated anti-reflection layers, and a coefficient of thermal expansion matched to the silicon detector wafer. Here we describe the design process for a gradient-index (GRIN) metamaterial lenslet using metal-mesh patterned on silicon and a combination of metal-mesh and etched-hole metamaterial anti-reflection layers. We optimize the design using a bulk-material model to rapidly simulate and iterate on the lenslet design. We fabricated prototype GRIN metamaterial lenslet array and mounted it on a Polarbear/Simons Array 90/150~GHz band transition edge sensor (TES) bolometer detector array with sinuous planar antennas. Beam measurements of a prototype lenslet array agree reasonably well with the model simulations. We plan to further optimize the design and combine it with a broadband anti-reflection coating to achieve operation over 70--350~GHz bandwidth.Comment: Presented at SPIE Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy X, December 13-18, 202

Design and experimental investigation of a planar metamaterial Silicon based lenslet

The next generations of ground-based cosmic microwave background experiments will require polarisation sensitive, multichroic pixels of large focal planes comprising several thousand detectors operating at the photon noise limit. One approach to achieve this goal is to couple light from the telescope to a polarisation sensitive antenna structure connected to a superconducting diplexer network where the desired frequency bands are filtered before being fed to individual ultra-sensitive detectors such as Transition Edge Sensors. Traditionally, arrays constituted of horn antennas, planar phased antennas or anti-reflection coated micro-lenses have been placed in front of planar antenna structures to achieve the gain required to couple efficiently to the telescope optics. In this paper are presented the design concept and a preliminary analysis of the measured performances of a phase-engineered metamaterial flat-lenslet. The flat lens design is inherently matched to free space, avoiding the necessity of an anti-reflection coating layer. It can be fabricated lithographically, making scaling to large format arrays relatively simple. Furthermore, this technology is compatible with the fabrication process required for the production of large-format lumped element kinetic inductance detector arrays which have already demonstrated the required sensitivity along with multiplexing ratios of order 1000 detectors/channel

Development of the Low Frequency Telescope Focal Plane Detector Modules for LiteBIRD

LiteBIRD is a JAXA-led strategic large-class satellite mission designed to measure the polarization of the cosmic microwave background and Galactic foregrounds from 34 to 448 GHz across the entire sky from L2 in the late 2020s. The scientific payload includes three telescopes which are called the low-, mid-, and high-frequency telescopes each with their own receiver that covers a portion of the mission's frequency range. The low frequency telescope will map synchrotron radiation from the Galactic foreground and the cosmic microwave background. We discuss the design, fabrication, and characterization of the low-frequency focal plane modules for low-frequency telescope, which has a total bandwidth ranging from 34 to 161 GHz. There will be a total of 4 different pixel types with 8 overlapping bands to cover the full frequency range. These modules are housed in a single low-frequency focal plane unit which provides thermal isolation, mechanical support, and radiative baffling for the detectors. The module design implements multi-chroic lenslet-coupled sinuous antenna arrays coupled to transition edge sensor bolometers read out with frequency-domain mulitplexing. While this technology has strong heritage in ground-based cosmic microwave background experiments, the broad frequency coverage, low optical loading conditions, and the high cosmic ray background of the space environment require further development of this technology to be suitable for LiteBIRD. In these proceedings, we discuss the optical and bolometeric characterization of a triplexing prototype pixel with bands centered on 78, 100, and 140 GHz.Comment: SPIE Astronomical Telescope + Instrumentation (AS22

CMB-S4: Forecasting Constraints on Primordial Gravitational Waves

CMB-S4---the next-generation ground-based cosmic microwave background (CMB) experiment---is set to significantly advance the sensitivity of CMB measurements and enhance our understanding of the origin and evolution of the Universe, from the highest energies at the dawn of time through the growth of structure to the present day. Among the science cases pursued with CMB-S4, the quest for detecting primordial gravitational waves is a central driver of the experimental design. This work details the development of a forecasting framework that includes a power-spectrum-based semi-analytic projection tool, targeted explicitly towards optimizing constraints on the tensor-to-scalar ratio, $r$, in the presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is unique in its direct use of information from the achieved performance of current Stage 2--3 CMB experiments to robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments given a desired scientific goal. To form a closed-loop process, we couple this semi-analytic tool with map-based validation studies, which allow for the injection of additional complexity and verification of our forecasts with several independent analysis methods. We document multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of the current reference design of the primordial gravitational-wave component of the Stage-4 experiment, optimized to achieve our science goals of detecting primordial gravitational waves for $r > 0.003$ at greater than $5\sigma$, or, in the absence of a detection, of reaching an upper limit of $r < 0.001$ at $95\%$ CL.Comment: 24 pages, 8 figures, 9 tables, submitted to ApJ. arXiv admin note: text overlap with arXiv:1907.0447

Design and characterization of the SPT-3G receiver

The SPT-3G receiver was commissioned in early 2017 on the 10-meter South Pole Telescope (SPT) to map anisotropies in the cosmic microwave background (CMB). New optics, detector, and readout technologies have yielded a multichroic, high-resolution, low-noise camera with impressive throughput and sensitivity, offering the potential to improve our understanding of inflationary physics, astroparticle physics, and growth of structure. We highlight several key features and design principles of the new receiver, and summarize its performance to date