Journal of Low Temperature Physics

, Volume 176, Issue 5–6, pp 650–656 | Cite as

Multi-Chroic Dual-Polarization Bolometric Detectors for Studies of the Cosmic Microwave Background

  • A. Suzuki
  • K. Arnold
  • J. Edwards
  • G. Engargiola
  • W. Holzapfel
  • B. Keating
  • A. T. Lee
  • X. F. Meng
  • M. J. Myers
  • R. O’Brient
  • E. Quealy
  • G. Rebeiz
  • P. L. Richards
  • D. Rosen
  • P. Siritanasak
Article

Abstract

We are developing multi-chroic antenna-coupled Transition Edge Sensor (TES) bolometer detectors for Cosmic Microwave Background (CMB) polarimetry. Multi-chroic detectors increase focal plane area efficiency, and thus the mapping speed per focal plane area, and provide greater discrimination against polarized galactic foregrounds with no increase in weight or cryogenic cost. In each pixel, a silicon lens-coupled dual-polarized sinuous antenna collects photons over a two-octave frequency band. The antenna couples the broadband millimeter wave signal into microstrip transmission lines, and on-chip filter banks split the broadband signal into multiple frequency bands. Separate TES bolometers detect the power in each frequency band and linear polarization state. We will describe the design and performance of these devices and present optical data taken. Our measurements of dual-polarization pixels in multiple frequency bands show beams with percent-level ellipticity, and percent-level cross-polarization leakage. We will also describe the development of large arrays of these multi-chroic pixels. Finally, we will describe kilo-pixel arrays of these detectors planned for the future CMB experiments that will achieve unprecedented mapping speed.

Keywords

Cosmic Microwave Background B-mode Broadband  Multichroic Polarization Anti-Reflectoin Coating 

1 Introduction

Characterization of the Cosmic Microwave Background (CMB) B-mode polarization signal will test models of inflationary cosmology, as well as constrain the sum of the neutrino masses and other cosmological parameters. The low intensity of the B-mode signal combined with the need to remove polarized galactic foregrounds requires a sensitive millimeter receiver and effective methods of foreground removal. Current bolometric detector technology is reaching the sensitivity limit set by photon noise. Thus, we need to increase the optical throughput to increase an experiment’s sensitivity. To increase the throughput without increasing the focal plane size, we can increase the frequency coverage of each pixel. Increased frequency coverage per pixel has the additional advantage that we can split the signal into frequency bands to obtain spectral information. The detection of multiple frequency bands allows for removal of the polarized foreground emission from synchrotron radiation and thermal dust emission, by utilizing its spectral dependence. Traditionally, spectral information has been captured with a multi-chroic focal plane consisting of a heterogeneous mix of single-color pixels. To maximize the efficiency of the focal plane area, we developed a multi-chroic pixel. This increases the number of pixels per frequency with the same focal plane area. For an earlier discussion of this work, please refer to O’Brient et al. [8].

2 Pixel Design

Figure 1 shows our prototype pixel. We used a silicon hemisphere with a silicon extension to form a synthesized elliptical lens to increase the gain of the antenna [3, 4]. The choice of the extension length was optimized with HFSS, a 3D electromagnetic (EM) simulator. The simulated model is shown in Fig. 1. We chose the extension length \( = 0.40\times \mathrm {radius}\) to maximize the gain. The silicon half-space also increases the front-to-back transmission ratio to approximately 10:1 due to its high dielectric constant, and it makes the antenna more efficient.
Fig. 1

(Left) Cross section of a CAD. In the simulation, the ratio of the silicon extension length (L) to the radius of the silicon lens (R) was optimized. Two layer anti-reflection coatings have dielectric constants of \(\varepsilon _r = 2\) and \(\varepsilon _r = 5\). Thickness of each layer is quarter-wavelength at 120 GHz. Material for the lens is silicon (\(\varepsilon _r = 11.7\)). A sinuous antenna is placed at the location indicated by the blue dot. (Right) Photograph of a single detector. The sinuous antenna, a broadband antenna, is in the middle of the photograph. Diplexer filters surround the antenna. The four dark rectangles are TES bolometers (Color figure online)

The high dielectric constant of the silicon lens (\(\varepsilon _r = 11.7\)) requires a multi-layer AR coating to suppress a reflection over a wide range of frequencies. We chose to use mixtures of various types of epoxies (Stycast 1090, Stycast 1266, Stycast 2850 FT) and filler (\(\mathrm {SrTiO_3}\)) which can be molded into appropriate AR coating layers. We successfully obtained epoxy with an intermediate dielectric constant by mixing two types of epoxy as shown in Fig. 2. We were able to fabricate materials with a dielectric constant between 2.0 and 7.5. To test the multi-layer AR coating, we applied two layers of coating made from Stycast 1090 (\(\varepsilon _r = 2.05\)) and Stycast 2850 FT (\(\varepsilon _r = 4.95\)) on flat 50 mm alumina (\(\varepsilon _r = 9.60\)) and measured the transmission using a Fourier Transform Spectrometer (FTS). As shown in Fig. 2, we obtained high transmission over a 70 % fractional bandwidth. Also, we were able to show that transmission was nearly perfect once loss is removed from the material at cryogenic temperature. To make a lens coating with \(25\,\upmu \mathrm {m}\) accuracy, we designed a mold that leaves a thin gap between lens and a mold. We calculated \(25\, \upmu \mathrm {m}\) accuracy translates to transmission for our observation bands to changes by less than 0.5 %. We thermally cycled coated lenses without a problem [9].
Fig. 2

(Left) Transmission of a two-layer AR coated alumina at room temperature (black), 2-layer AR coated alumina at cryogenic temperature (140 Kelvin) (red) and uncoated alumina sample (purple dotted) measured with an FTS. Fabry–Perot fringes disappear over 70 % of the bandwidth for the AR coated sample indicating low reflection. (Right) Plot of measured dielectric constant of the material and various epoxy and filler mixtures. Percentage on the X-axis is the percentage of mass of the second material in the mix (Color figure online)

For the antenna, we used a sinuous antenna. The sinuous antenna is a broadband antenna that has log-periodic and self-complementary structure and provides a stable input impedance over a wide frequency range [2]. We chose the inner and outer radii of the antenna to be 15 and 1,500 \(\upmu \mathrm {m}\) respectively. A four-arm, self-complementarity sinuous antenna on silicon radiates at 360 \(\upmu \mathrm {m}\) radius for 70 GHz, the lowest frequency we use. From the results of 3D EM simulation with HFSS, we found out that a larger outer radius was required to obtain stable input impedance and round beam shape at our operating frequencies. Experimental tests with antennas of different sizes agreed with the simulation.

Power from the antenna is coupled to microstrip lines to allow RF frequency-selection with on-chip filters prior to detection at the bolometers. The metalized arms of the sinuous antenna are used as the ground plane of the microstrip line so that we can bring the microstrip line to the center of the antenna without interfering with the antenna. We used lumped element diplexers and channelizers with seven frequency bands to partition the signal. Filter designs were based on a 0.5 dB ripple three-pole Chebyshev band pass filter, as a compromise between loss due to the filter elements and roll-off [1]. We designed an inductor with short lengths of a coplanar waveguide. For a capacitor, we designed a parallel plate capacitor with two layers of niobium separated by silicon dioxide. For calculating the required values of inductor and capacitor, we followed O’Brient et al. and Kumar et al. [6, 7]. The filter elements were optimized with the Sonnet EM simulator until \(-\)20 dB reflection in the passband and \(-\)20 dB isolation between frequency bands were obtained. In the channelizer, lumped band-pass filters are spaced in a log-periodic frequency spacing, and attached to the transmission line from high to low frequency. Within the filter’s resonant band-pass, the signal is transmitted through the filter to its associated bolometer, while other frequencies continue on the main transmission line [5]. For a space observation where atmospheric lines are not a problem, a channelizer design can efficiently extract spectral information. The partitioned signals are detected by superconducting Transition Edge Sensor (TES) bolometers. For details on fabrication of these pixels, please refer to Suzuki et al. [9].

3 Result

Fabricated pixels were tested in an 8 inch IR Labs dewar. We modified the dewar to receive millimeter waves using a Zotefoam window and expanded teflon and metal mesh filters. The pixel stage is isolated from the liquid helium buffer with thin walled vespel tubes. The stage is cooled to 0.25 Kelvin with a homemade \(^3\mathrm {He}\) adsorption fridge. The TES bolometer is DC voltage biased with \(0.02\, \mathrm {\Omega }\) of shunt resistance in parallel with the bolometer. Current through the bolometer is read-out by a commercially available laboratory DC SQUID from Quantum Design with its input inductor coil in series with the bolometer.

A proto type chip was mounted on a 14 mm diameter silicon synthesized elliptical lens. Alignment of the proto-type pixel to the lens was done manually under a microscope. The proto-type pixel was fixed to the lens by GE varnish. We were able to achieve accuracy of approximately \(10~\upmu \mathrm {m}\).

We produced beam maps of the pixel by scanning a temperature-modulated source on an X-Y stage in front of the dewar. We measured the response of the pixel to a linearly polarized source by rotating a wire grid polarizer between the pixel and the temperature-modulated source. We measured spectra of the device using a Michaelson Fourier Transform Spectrometer (FTS). The efficiency of the device was measured by comparing the difference in received power from a beam-filling temperature-modulated source and \(\mathrm {Power} = k_B\Delta T \Delta \nu \), where \(\Delta \nu \) is the bandwidth of the detector.

The beam maps from a lumped diplexer pixel are shown in Fig. 3. We characterize the beam properties by fitting a 2-D Gaussian, and define ellipticity as \(e = (a-b)/(a+b)\), where \(a\) and \(b\) are the spread of two orthogonal Gaussians. Ellipticities were measured to be \(e = 1.1\) and \(e = 1.3\) % for the 95 and 150 GHz bands, respectively. At this level, we were limited by measurement systematics. We also compared spread of beam to the HFSS simulation that was simulated with a smaller (6.35 mm) lens for computing time issue. When beam spread was adjusted linear by ratio of lens size, measured beam spread agrees with the simulation. A polarization measurement from the same device is shown in Fig. 3. We measured polarization leakage of 0.3 and 1.3 % for the 95 and 150 GHz bands, respectively. We expect the wire grid to have approximately 1 % leakage, thus we are limited by systematics for the polarization leakage measurement. The spectra from the lumped filter diplexer and channelizer are shown in Fig. 4. Peaks of the spectra were normalized to a measured optical efficiency of each band. The results show that we successfully partitioned a broadband signal into multiple bands. Spectra were presented in O’Brient et al. [8]. As article states, loss as function of frequency is not instrinsic to the channelizer design, but we suspect it comes from other factors such as dielectric loss in microstrip line and dewar filter loss.
Fig. 3

Beam map measurement from a lumped diplexer. 95 GHz band beam (left) and 150 GHz band beam (center). Plots were peak normalized. (right) The detector responce to a rotating wiregrid in front of a thermally modulated source (Color figure online)

Fig. 4

The spectra from the lumped filter diplexer and channelizer. Peaks of the spectra were normalized to a measured optical efficiency of each band that includes all optical elements (Color figure online)

4 Detector Array

To bring the prototype multi-chroic pixels closer to readiness for future CMB experiments, we fabricated arrays of multi-chroic detectors. We fabricated these on 150 mm wafers. The array has a side-to-side length of 13 cm, containing 271 pixels. Each pixel is a dual-polarized diplexer (95 GHz and 150 GHz bands). We are in the process of building a setup to check detector uniformity across the array. We took an opportunity of fabricating an array of detectors to implement a polarization wobble (rotation of polarization axis as a function of frequency) mitigation. The sinuous antenna is favored over other log-periodic antennas for its small polarization wobble amplitude of 5 degrees [3]. We realized that having two kinds of pixel where one is a mirror image of the other allows us to cancel the wobble effect. The two types of pixels’s polarization axis would rotate in opposite directions, thus we can recover the true polarization angle of the signal by combining data from two pixels.

5 Conclusion

The next generation CMB B-mode experiments such as the Polarbear-2, the Simons Array, the South Pole Telescope-3G (SPT-3G) and the LiteBIRD are planning to use these multi-chroic detectors to increase their mapping speed. Polarbear-2 will use seven of these 13 cm hexagonal arrays in its focal plane. The total number of bolometers will be 7,588. The Simons Array is a project that will depoy three Polarbear-2 type receivers to observe 95 Ghz, 150 Ghz and 220 GHz bands with 22,764 detectors. The SPT-3G will use a triplexer pixel. SPT-3G will deploy with 15,234 detectors. LiteBIRD a compact satellite CMB B-mode experiment, will observe with 2,022 detectors. We have successfully demonstrated multi-chroic detection with single pixels. We produced a detector with a circular beam and low cross-polarization performance. We have also presented a solution for broadband AR coatings, and have fabricated arrays of multi-chroic detectors which will be used in future CMB experiments.

Notes

Acknowledgments

We acknowledge support from the NASA, NASA grant NNG06GJ08G. Detectors were fabricated at Berkeley nanofabrication laboratory. Praween Siritanasak is supported by the Royal Thai Government fellowship.

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  • A. Suzuki
    • 1
  • K. Arnold
    • 2
  • J. Edwards
    • 3
  • G. Engargiola
    • 4
  • W. Holzapfel
    • 1
  • B. Keating
    • 2
  • A. T. Lee
    • 1
  • X. F. Meng
    • 5
  • M. J. Myers
    • 1
  • R. O’Brient
    • 6
  • E. Quealy
    • 1
  • G. Rebeiz
    • 3
  • P. L. Richards
    • 1
  • D. Rosen
    • 1
  • P. Siritanasak
    • 2
  1. 1.Department of PhysicsUniversity of California, BerkeleyBerkeleyUSA
  2. 2.Department of PhysicsUniversity of California, San DiegoLa JollaUSA
  3. 3.Department of Electrical and Computer EngineeringUniversity of California, San DiegoLa JollaUSA
  4. 4.Lawrence Berkeley National LaboratoryBerkeleyUSA
  5. 5.Department of Electrical EngineeringUniversity of California, BerkeleyBerkeleyUSA
  6. 6.Department of PhysicsCalifornia Institute of TechnologyPasadenaUSA

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