Refereed Scientific/Technical Papers

  • The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
    The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
    Sumit Dahal, Mandana Amiri, John W. Appel, Charles L. Bennett, Lance Corbett, Rahul Datta, Kevin Denis, Thomas Essinger-Hileman, Mark Halpern, Kyle Helson, Gene Hilton, Johannes Hubmayr, Benjamin Keller, Tobias Marriage, Carolina Nunez, Matthew Petroff, Carl Reintsema, Karwan Rostem, Kongpop U-Yen, Edward Wollack
    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 aWs√ for 150 and 220 GHz arrays, respectively.
    • Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
      Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
      Zhilei Xu, Michael K. Brewer, Pedro Fluxa, Yunyang Li, Keisuke Osumi, Bastian Pradenas, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Rahul Datta, Kevin Denis, Rolando Dunner, Joseph Eimer, Thomas Essinger-Hileman, Kathleen Harrington, Jeffrey Iuliano, Tobias A. Marriage, Nathan Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Duncan J. Watts, Janet Weiland, Edward J. Wollack
      The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale (1∘≲θ⩽90∘) CMB polarization to constrain the tensor-to-scalar ratio at the r∼0.01 level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from September 2016 to February 2018. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is <0.023∘ (<1.6% of the beam’s full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has a FWHM of 1.579∘±.001∘ and a solid angle of 838±6 μsr, consistent with physical optics simulations. The corresponding beam window function has sub-percent error per multipole at ℓ<200. An extended 90∘ beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperature-to-polarization leakage fraction is <10−4 (95% C.L.). We find that the Moon-based results are consistent with measurements of M42, RCW 38, and Tau A from CLASS’s CMB survey data. In particular, Tau A measurements establish degree-level precision for instrument polarization angles.
    • Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q Band
      Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q Band
      Matthew A. Petroff, Joseph R. Eimer, Kathleen Harrington, Aamir Ali, John W. Appel, Charles L. Bennett, Michael K. Brewer, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Denes Couto, Sumit Dahal, Rolando Dünner, Thomas Essinger-Hileman, Pedro Fluxá Rojas, Jeffrey Iuliano, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Ivan L. Padilla, Lucas Parker, Rodrigo Reeves, Karwan Rostem, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
      The Earth’s magnetic field induces Zeeman splitting of the magnetic dipole transitions of molecular oxygen in the atmosphere, which produces polarized emission in the millimeter-wave regime. This polarized emission is primarily circularly polarized and manifests as a foreground with a dipole-shaped sky pattern for polarization-sensitive ground-based cosmic microwave background (CMB) experiments, such as the Cosmology Large Angular Scale Surveyor (CLASS), which is uniquely capable of measuring large angular scale circular polarization. Using atmospheric emission theory and radiative transfer formalisms, we model the expected amplitude and spatial distribution of this signal and evaluate the model for the CLASS observing site in the Atacama Desert of northern Chile. Then, using two years of observations near 40 GHz from the CLASS Q-band telescope, we present a detection of this signal and compare the observed signal to that predicted by the model. We recover an angle between magnetic north and true north of (−5.5±0.6)∘, which is consistent with the expectation of −5.9∘ for the CLASS observing site. When comparing dipole sky patterns fit to both simulated and data-derived sky maps, the dipole directions match to within a degree, and the measured amplitudes match to within ∼20%.
    • Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz
      Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz
      Ivan L. Padilla, Joseph R. Eimer, Yunyang Li, Graeme E. Addison, Aamir Ali, John W. Appel, Charles L. Bennett, Ricardo Bustos, Michael K. Brewer, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna Couto, Sumit Dahal, Kevin Denis, Rolando Dünner, Thomas Essinger-Hileman, Pedro Fluxá, Saianeesh K. Haridas, Kathleen Harrington, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, Nathan J. Miller, Carolina Núñez, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Karwan Rostem, Robert W. Stevens, Deniz Augusto Nunes Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
      We report circular polarization measurements from the first two years of observation with the 40 GHz polarimeter of the Cosmology Large Angular Scale Surveyor (CLASS). CLASS is conducting a multi-frequency survey covering 75% of the sky from the Atacama Desert designed to measure the cosmic microwave background (CMB) linear E and B polarization on angular scales 1∘≲θ≤90∘, corresponding to a multipole range of 2≤ℓ≲200. The modulation technology enabling measurements of linear polarization at the largest angular scales from the ground, the Variable-delay Polarization Modulator, is uniquely designed to provide explicit sensitivity to circular polarization (Stokes V). We present a first detection of circularly polarized atmospheric emission at 40 GHz that is well described by a dipole with an amplitude of 124±4μK when observed at an elevation of 45∘, and discuss its potential impact as a foreground to CMB experiments. Filtering the atmospheric component, CLASS places a 95% C.L. upper limit of 0.4μK2 to 13.5μK2 on ℓ(ℓ+1)CVVℓ/(2π) between 1≤ℓ≤120, representing a two-orders-of-magnitude improvement over previous limits.
    • The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
      The CLASS 150/220 GHz Polarimeter Array: Design, Assembly, and Characterization
      Sumit Dahal, Mandana Amiri, John W. Appel, Charles L. Bennett, Lance Corbett, Rahul Datta, Kevin Denis, Thomas Essinger-Hileman, Mark Halpern, Kyle Helson, Gene Hilton, Johannes Hubmayr, Benjamin Keller, Tobias Marriage, Carolina Nunez, Matthew Petroff, Carl Reintsema, Karwan Rostem, Kongpop U-Yen, Edward Wollack
      Video Link
      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 aWs√ for 150 and 220 GHz arrays, respectively.
      arXiv: 1908.00480
    • On-sky performance of the CLASS Q-band telescope
      On-sky performance of the CLASS Q-band telescope
      Appel, John W.; Xu, Zhilei; Padilla, Ivan L.; Harrington, Kathleen; Pradenas Marquez, Bastián et al.
      The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the Cosmic Microwave Background (CMB) at large angular scales (2<ℓ≲200) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of r≈0.01. The same dataset will provide a near sample-variance-limited measurement of the optical depth to reionization. Between June 2016 and March 2018, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31 000~square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature (NET) sensitivity of 32 μKcmbs√. We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 aWs√) match the expected noise model dominated by photon bunching noise. We derive a 13.1±0.3 K/pW calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48±0.04 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 315±19 Jy at 38.8±1.0 GHz, consistent with the WMAP Tau A time-dependent spectral flux density model.
      arXiv:1811.08287
    • A 3D-printed broadband millimeter wave absorber

      A 3D-printed broadband millimeter wave absorber  

      Petroff, Matthew; Appel, John; Rostem, Karwan et al.
      We present the design, manufacturing technique, and characterization of a 3D-printed broadband graded index millimeter wave absorber. The absorber is additively manufactured using a fused filament fabrication (FFF) 3D printer out of a carbon-loaded high impact polystyrene (HIPS) filament and is designed using a space-filling curve to optimize manufacturability using said process. The absorber’s reflectivity is measured from 63 GHz to 115 GHz and from 140 GHz to 215 GHz and is compared to electromagnetic simulations. The intended application is for terminating stray light in Cosmic Microwave Background (CMB) telescopes, and the absorber has been shown to survive cryogenic thermal cycling.
      arXiv:1808.00820

    • A Projected Estimate of the Reionization Optical Depth Using the CLASS Experiment’s Sample-Variance Limited E-Mode Measurement
      A Projected Estimate of the Reionization Optical Depth Using the CLASS Experiment’s Sample-Variance Limited E-Mode Measurement
      Watts, Duncan J.; Wang, Bingjie et al.
      We analyze simulated maps of the Cosmology Large Angular Scale Surveyor (CLASS) experiment and recover a nearly cosmic variance limited estimate of the reionization optical depth τ. We use a power spectrum-based likelihood to simultaneously clean foregrounds and estimate cosmological parameters in multipole space. Using software specifically designed to constrain τ, the amplitude of scalar fluctuations A s , and the tensor-to-scalar ratio r, we demonstrate that the CLASS experiment will be able to estimate τ within a factor of two of the cosmic variance limit allowed by full-sky cosmic microwave background polarization measurements. Additionally, we discuss the role of CLASS’s τ constraint in conjunction with gravitational lensing of the CMB on obtaining a ≳4σ measurement of the sum of the neutrino masses.
      arXiv:1801.01481
    • Cosmology Large Angular Scale Surveyor (CLASS) Focal Plane Development
      Cosmology Large Angular Scale Surveyor (CLASS) Focal Plane Development
      Chuss, D. T. et al.
      The Cosmology Large Angular Scale Surveyor (CLASS) will measure the polarization of the Cosmic Microwave Background to search for and characterize the polarized signature of inflation. CLASS will operate from the Atacama Desert and observe ~70% of the sky. A variable-delay polarization modulator provides modulation of the polarization at ~10 Hz to suppress the 1/ f noise of the atmosphere and enable the measurement of the large angular scale polarization modes. The measurement of the inflationary signal across angular scales that spans both the recombination and reionization features allows a test of the predicted shape of the polarized angular power spectra in addition to a measurement of the energy scale of inflation. CLASS is an array of telescopes covering frequencies of 38, 93, 148, and 217 GHz. These frequencies straddle the foreground minimum and thus allow the extraction of foregrounds from the primordial signal. Each focal plane contains feedhorn-coupled transition-edge sensors that simultaneously detect two orthogonal linear polarizations. The use of single-crystal silicon as the dielectric for the on-chip transmission lines enables both high efficiency and uniformity in fabrication. Integrated band definition has been implemented that both controls the bandpass of the single-mode transmission on the chip and prevents stray light from coupling to the detectors.
      arXiv151104414C [astro-ph.CO] Accepted by the Journal of Low Temperature Physics (2015).
    • Fabrication of Feedhorn-Coupled Transition Edge Sensor Arrays for Measurement of the Cosmic Microwave Background Polarization
      Fabrication of Feedhorn-Coupled Transition Edge Sensor Arrays for Measurement of the Cosmic Microwave Background Polarization
      Kevin Denis et al.
      Characterization of the minute cosmic microwave background polarization signature requires multi-frequency, high-throughput precision instrument systems. We have previously described the detector fabrication of a 40 GHz focal plane and now describe the fabrication of detector modules for measurement of the CMB at 90 GHz. The 90 GHz detectors are a scaled version of the 40 GHz architecture where, due to smaller size detectors, we have implemented a modular (wafer level) rather than the chip-level architecture. The new fabrication process utilizes the same design rules with the added challenge of increased wiring density to the 74 TES’s as well as a new wafer level hybridization procedure. The hexagonally shaped modules are tile-able, and as such can be used to form the large focal planes required for a space-based CMB polarimeter. The detectors described here will be deployed in two focal planes with seven modules each in the Johns Hopkins University led ground-based Cosmology Large Angular Scale Surveyor (CLASS) telescope.
      arXiv151105036D [astro-ph.CO] Presented at Low Temperature Detectors Conference Grenoble France (2015).
    • Recovery of Large Angular Scale CMB Polarization for Instruments Employing Variable-delay Polarization Modulators
      Recovery of Large Angular Scale CMB Polarization for Instruments Employing Variable-delay Polarization Modulators
      Nathan J. Miller; David T. Chuss; Tobias A. Marriage; Edward J. Wollack et al.
      Variable-delay Polarization Modulators (VPMs) are currently being implemented in experiments designed to measure the polarization of the cosmic microwave background on large angular scales because of their capability for providing rapid, front-end polarization modulation and control over systematic errors. Despite the advantages provided by the VPM, it is important to identify and mitigate any time-varying effects that leak into the synchronously modulated component of the signal. In this paper, the effect of emission from a 300 K VPM on the system performance is considered and addressed. Though instrument design can greatly reduce the influence of modulated VPM emission, some residual modulated signal is expected. VPM emission is treated in the presence of rotational misalignments and temperature variation. Simulations of time-ordered data are used to evaluate the effect of these residual errors on the power spectrum. The analysis and modeling in this paper guides experimentalists on the critical aspects of observations using VPMs as front-end modulators. By implementing the characterizations and controls as described, front-end VPM modulation can be very powerful for mitigating 1/f noise in large angular scale polarimetric surveys. None of the systematic errors studied fundamentally limit the detection and characterization of B-modes on large scales for a tensor-to-scalar ratio of r = 0.01. Indeed, r < 0.01 is achievable with commensurately improved characterizations and controls.
      arXiv1509.04628 [astro-ph.CO] Submitted to ApJ (2015).
    • Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky
      Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky
      Duncan J. Watts, David Larson, Tobias A. Marriage et al.
      We consider the effectiveness of foreground cleaning in the recovery of Cosmic Microwave Background (CMB) polarization sourced by gravitational waves for tensor-to-scalar ratios in the range 0<r<0.1. Using the planned survey area, frequency bands, and sensitivity of the Cosmology Large Angular Scale Surveyor (CLASS), we simulate maps of Stokes Q and U parameters at 40, 90, 150, and 220 GHz, including realistic models of the CMB, diffuse Galactic thermal dust and synchrotron foregrounds, and Gaussian white noise. We use linear combinations (LCs) of the simulated multifrequency data to obtain maximum likelihood estimates of r, the relative scalar amplitude s, and LC coefficients. We find that for 10,000 simulations of a CLASS-like experiment using only measurements of the reionization peak (ℓ≤23), there is a 95% C.L. upper limit of r<0.017 in the case of no primordial gravitational waves. For simulations with r=0.01, we recover at 68% C.L. r=0.012+0.011−0.006. The reionization peak corresponds to a fraction of the multipole moments probed by CLASS, and simulations including 30≤ℓ≤100 further improve our upper limits to r<0.008 at 95% C.L. (r=0.01+0.004−0.004 for primordial gravitational waves with r=0.01). In addition to decreasing the current upper bound on r by an order of magnitude, these foreground-cleaned low multipole data will achieve a cosmic variance limited measurement of the E-mode polarization’s reionization peak.
      Astrophysical Journal, Vol. 814, 103 (2015).
    • Fabrication of a Silicon Backshort Assembly for Waveguide-Coupled Superconducting Detectors
      Fabrication of a Silicon Backshort Assembly for Waveguide-Coupled Superconducting Detectors
      Erik J. Crowe et al.
      The Cosmology Large Angular Scale Surveyor (CLASS) is a ground-based instrument that will measure the polarization of the cosmic microwave background to search for evidence for gravitational waves from a posited epoch of inflation early in the Universe’s history. This measurement will require integration of superconducting transition-edge sensors with microwave waveguide inputs with excellent control of systematic errors, such as unwanted coupling to stray signals at frequencies outside of a precisely defined microwave band. To address these needs, we present work on the fabrication of micromachined silicon, producing conductive quarter-wave backshort assemblies for the CLASS 40 GHz focal plane. Each 40 GHz backshort assembly consists of three degeneratively doped silicon wafers. Two spacer wafers are micromachined with through-wafer vias to provide a 2.04-mm-long square waveguide delay section. The third wafer terminates the waveguide delay in a short. The three wafers are bonded at the wafer level by Au-Au thermal compression bonding then aligned and flip chip bonded to the CLASS detector at the chip level. The micromachining techniques used have been optimized to create high aspect ratio waveguides, silicon pillars, and relief trenches with the goal of providing improved out of band signal rejection. We will discuss the fabrication of integrated CLASS superconducting detector chips with the quarter-wave backshort assemblies.
      IEEE Transactions on Applied Superconductivity, vol. 23 , Issue, 3 (2013).
    • Properties of a variable-delay polarization modulator
      Properties of a variable-delay polarization modulator
      Chuss, David T.; Wollack, Edward J. et al.
      We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 microns and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device.
      Applied Optics, vol. 51, Issue 2, p. 197 (2012).
    • Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry
      Electromagnetic Design of Feedhorn-Coupled Transition-Edge Sensors for Cosmic Microwave Background Polarimetry
      Chuss, D. T. et al.
      Observations of the cosmic microwave background (CMB) provide a powerful tool for probing the evolution of the early universe. Specifically, precision measurement of the polarization of the CMB enables a direct test for cosmic inflation. A key technological element on the path to the measurement of this faint signal is the capability to produce large format arrays of background-limited detectors. We describe the electromagnetic design of feedhorn-coupled, TES-based sensors. Each linear orthogonal polarization from the feedhorn is coupled to a superconducting microstrip line via a symmetric planar orthomode transducer (OMT). The symmetric OMT design allows for highly-symmetric beams with low cross-polarization over a wide bandwidth. In addition, this architecture enables a single microstrip filter to define the passband for each polarization. Care has been taken in the design to eliminate stray coupling paths to the absorbers. These detectors will be fielded in the Cosmology Large Angular Scale Surveyor (CLASS).
      Journal of Low Temperature Physics, Volume 167, Issue 5-6, pp. 923-928 (2012).
    • Vector reflectometry in a beam waveguide
      Vector reflectometry in a beam waveguide
      Eimer, J. R. et al.
      We present a one-port calibration technique for characterization of beam waveguide components with a vector network analyzer. This technique involves using a set of known delays to separate the responses of the instrument and the device under test. We demonstrate this technique by measuring the reflected performance of a millimeter-wave variable-delay polarization modulator.
      Review of Scientific Instruments, Volume 82, Issue 8, pp. 086101-086101-3 (2011).
    • A Low Cross-Polarization Smooth-Walled Horn With Improved Bandwidth
      A Low Cross-Polarization Smooth-Walled Horn With Improved Bandwidth
      Zeng, Lingzhen et al.
      Corrugated feed horns offer excellent beam symmetry, main beam efficiency, and cross-polar response over wide bandwidths, but can be challenging to fabricate. An easier-to-manufacture smooth-walled feed is explored that approximates these properties over a finite bandwidth. The design, optimization and measurement of a monotonically-profiled, smooth-walled scalar feedhorn with a diffraction-limited ~14° FWHM beam is presented. The feed was demonstrated to have low cross polarization (< -30 dB) across the frequency range 33-45 GHz (30% fractional bandwidth). A power reflection below -28 dB was measured across the band.
      IEEE Transactions on Antennas and Propagation, vol. 58, Issue 4, pp. 1383-1387 (2010).