Feedhorn-Coupled TES Polarimeters for Next-GenerationCMB Instruments

K. W. Yoon∗, J. W. Appel†, J. E. Austermann∗∗, J. A. Beall∗, D. Becker∗, B. A.Benson‡, L. E. Bleem‡, J. Britton∗, C. L. Chang‡, J. E. Carlstrom‡, H.-M. Cho∗, A.T. Crites‡, T. Essinger-Hileman†, W. Everett‡, N. W. Halverson∗∗, J. W. Henning∗∗,G. C. Hilton∗, K. D. Irwin∗, J. McMahon‡, J. Mehl‡, S. S. Meyer‡, S. Moseley§, M.D. Niemack∗, L. P. Parker†, S. M. Simon∗∗, S. T. Staggs†, K. U-yen§, C. Visnjic†, E.

Wollack§ and Y. Zhao†

∗NIST Quantum Devices Group, 325 Broadway Mailcode 817.03, Boulder,CO, USA 80305, USA†Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, NJ, 08544, USA

∗∗Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences andDepartment of Physics, University of Colorado, Boulder, CO 80309, USA

‡Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637,USA

§NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA

Abstract. The next generation of cosmic microwave background (CMB) polarization experiments targeting the signaturesof inflation will require unprecedented sensitivities in addition to careful control of systematics. With existing detectortechnologies approaching the photon noise limit, improvements in system sensitivities must come from ever-larger focalplane arrays of millimeter-wave detectors. We report on the design and performance of microfabricated planar orthomodetransducer (OMT) coupled TES polarimeters and silicon micromachined platelet feedhorns optimized for scaling to largemonolithic arrays. Future versions of these detectors are targeted for deployment in a number of upcoming CMB experiments,including ABS, SPTpol, and ACTpol.

Keywords: cosmic microwave background, millimeter-wave, polarization, polarimetry, transition edge sensors, TES, bolometersPACS: 95.55.-n

INTRODUCTION

Detailed studies of the polarization of the cosmic mi-crowave background (CMB) promise rich new insightsinto the universe. Measurements of the E-mode polar-ization will augment CMB temperature power spectrummeasurements by helping to constrain cosmological pa-rameters and breaking model degeneracies. B-mode po-larization at degree angular scales is expected to be aunique signature of primordial gravitational waves fromthe epoch of inflation, and as such a window onto phys-ical processes at energy scales unattainable with parti-cle accelerators. At smaller angular scales, gravitationallensing of E-modes into B-modes would provide valu-able information about the distribution of matter betweenrecombination and the present universe.

A number of experiments in development are now tar-geting the CMB polarization at various angular scales topursue the above-mentioned science goals. One of themany challenges these new generation of experimentswill attempt to overcome is one of sheer sensitivity: be-cause existing millimeter-wave detector technologies al-

ready operate close to the background photon noise limit,further gains in total instrument sensitivity must comefrom the ability to field ever-larger arrays of denselypacked pixels.

Recently, BICEP[1] and QUAD[2] have made thefirst high signal-to-noise measurements of the first fouracoustic peaks in the E-mode spectrum, and have setunprecedented upper limits on the magnitude of the B-modes. While significantly improving on previous ef-forts, BICEP’s initial upper limits are an order of magni-tude above the expected levels for a tensor-to-scalar ratioof r = 0.1, and QUAD’s upper limits at smaller angu-lar scales are nearly two orders of magnitude above theexpected levels of gravitationally-lensed B-modes. Largegains in sensitivities are needed if the next-generation ex-periments are to achieve their science goals.

One of the challenges of building a large-format arrayof CMB polarimeters lies with the choice of radiation-coupling mechanism onto the bolometers. Experimentssuch as BICEP and QUAD used individually electro-formed Cu corrugated feedhorns. While they providegood control of sidelobes, wide-band performance and

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FIGURE 1. Prototype 145 GHz Polarimeter.

low cross-polarization, this method quickly becomes im-practical for implementing arrays of several hundredsto thousands of pixels. BICEP and QUAD also usedpolarization-sensitive micromesh bolometers that werehand-assembled into individual modules, which wouldalso be impractical for large-format arrays.

To address these issues, we propose a monolithic, all-silicon focal plane architecture using micromachined Siplatelet feedhorns and microfabricated planar orthomodetransducers (OMTs) that couple the incoming radiationto an array of multiplexed superconducting transitionedge sensors (TESs). This paper is intended as a brief anddescriptive overview of the design and measurementsof single-pixel prototype OMT-coupled polarimeters andstacked Si platelet waveguides. Companion papers else-where in this volume provide in-depth technical detail,and are referenced in relevant sections below.

DESIGN OVERVIEW

A feedhorn array consisting of stacked, metalized Siwafers—with each wafer defining a single corrugationalong the profile of a feedhorn—retains all of the advan-tages of traditional corrugated feedhorns while eliminat-ing many of the disadvantages. Such a monolithic arrayof feeds can be densely packed, and is significantly lower

in thermal mass than a comparable metal platelet array.It is also perfectly matched with a monolithic detectorarray wafer in thermal contraction, eliminating problemswith precision alignment.

The Si platelet feedhorns are integrated directly with amonolithic detector array consisting of OMTs and othersuperconducting planar transmission line componentsthat take full advantage of existing photolithographictechniques.

Figure 1 shows a 5-mm diameter single-pixel proto-type polarimeter fabricated at NIST and designed to cou-ple to a 1.6-mm diameter circular waveguide for opera-tion at 145 GHz. Preliminary testing was carried out bymounting the single pixels onto specially designed metalmodules that provide λ/4 backshorts and appropriate in-terfacing to existing metal feedhorns.

OMT & CPW-to-microstrip transition

The planar OMT is optimized for maximum couplingto the fundamental TE11 mode of the corresponding1.6 mm diameter circular waveguide. The OMT con-sists of four triangular Nb probes—with orthogonal pairscoupling to X and Y polarizations—that taper to 5 µmwide Nb lines at the waveguide wall and transition intoa coplanar waveguide (CPW). Simulations of the fabri-

cated design have shown greater than 96% coupling ofthe TE11 mode, with ∼ 2% reflected out and ∼ 1% radi-ated into the 50 µm airgaps above and below the relievedOMT membrane.

While the rest of the superconducting components onthe polarimeter make use of Nb microstrip lines, cou-pling from the circular waveguide to CPW is easier be-cause of the higher characteristic impedance CPW com-pared to microstrip. A numerically optimized transitionfrom CPW to microstrip is then made for matching to therest of the superconducting circuitry.

Details of the OMT design and its optimization, aswell as the CPW-to-microstrip transition, are availablein McMahon et al.[3]

Microstrip filters

The existing prototype polarimeters are designed foroperation at a nominal band-center of 145 GHz and afractional bandwidth of 25%. The spectral band is de-fined by 5-pole λ/4 shorted stub filters (see Figure 2).These resonant filters transmit at odd harmonics, andthe leakages above the main band must be attenuated.This is accomplished by using two back-to-back stepped-impedance low-pass filters with staggered cut-off char-acteristics. The out-of-band transmission has been simu-lated to be ∼−30 dB or lower.

Lossy absorber and TES

After filtering, the microstrip lines terminate onto free-standing bolometer islands (see Figure 3). The lossless

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Nb lines abruptly transition into long meanders of lossyAu microstrip so that almost all of the incident poweris absorbed. Simulations have shown the reflection to be< −20 dB across the entire spectral band. Because Aumicrostrip is well-matched to Nb, the reflection at theabrupt transition is minimal.

The absorbed power is then detected by a MoCu bi-layer TES with nominal Tc = 530 mK and Rn = 5 mΩ,appropriate for the time-domain SQUID multiplexedreadout used in prototype testing thus far. A ∼ 2 Ω goldheater is available on the island for calibration and test-ing purposes. The thermal conductance to the 250 mKbath is determined by the silicon nitride legs that sup-port the two Nb microstrip lines as well as the lines forTES and heater biases. By varying the aspect ratio of thenitride legs we have been able to target thermal conduc-tance G(T = Tc) of 50 to 600 pW/K, which cover therange of interest for both low-background CMB obser-vations and ambient temperature load laboratory testing.

Si platelet feedhorn

As a first step towards building monolithic arrays ofSi platelet feedhorns, we have fabricated single stacks ofvarious geometries to characterize the loss of metalizedSi waveguides on a vector network analyzer (VNA). TheVNA measurements show an upper limit of 0.15 dB/cmloss over 80−110 GHz at room temperature. We expectthis to become negligible at the operational temperaturesbelow 1 K. Detailed report on the Si waveguide measure-ments is given in Britton et al.[4]

FILTER DESIGN

Traditionally, observation bands have been defined us-ing elements in the optical path of the instrument, suchas wire mesh filters. Our pixel design allows another op-tion: the filters are constructed in microstrip for on-chipprocessing.

We chose a 5-pole, 1/4! shorted stub filter as ourbandpass filter. The simple rectangular geometry makesthese filters easy to fabricate. Our substrate is very thincompared to distance between stubs, so that couplingbetween stubs is expected to be extremely low (this waschecked by simulations). This allows for easy simulationof the filters using linear circuit analysis tools such asMicrowave Office. Design formulas for these filters areavailable in standard references (e.g. [4], [5]).

Our filter was based on a 5-pole Chebyshev prototypefilter with .1 dB ripple. Five poles were chosen to giveswift roll-off at the band edges. 1/4! stub filters haveharmonic passbands at frequencies of 3, 5, 7, etc timesthe design frequency. They are also susceptible to narrowspikes in transmission near 2, 4, 6 times design frequencyif not tuned correctly [5]. To suppress these resonanceswe designed two simple stepped impedance low-passfilters with roll-offs at 190 and 310 GHz.

To validate our design process, we first designed filtersassuming perfect conductors and simulated these usingMicrowave Offices linear circuit analysis tools. We alsosimulated the filters using MWOs 2.5D EM tools [6] andSonnet EM [7]. We compare these simulations with themeasured spectra in the next section.

MEASURED SPECTRUM

Spectra were obtained using a symmetric polarizingMichelson Fourier Transform Spectrometer (FTS). Inour FTS design the common input/output ports are off-set from each other by a small offset angle [8]. At oneinput port we have either a 1000C thermal blackbody ora 77K piece of eccosorb that we use as our source. Acurved piece of light pipe placed at the correspondingoutput port runs up to the window of the dewar wherethe device is mounted. We have covered the two addi-tional ports with 300K eccosorb to provide an isothermalreference and to help damp spurious reflections throughthe FTS. The FTS is calibrated in a frequency range be-tween 45 and 3000 GHz and has a resolution of 2GHz.Scans were taken at a variety of speeds between .25 and2 optical cm/s with sampling rates of either 128 or 512Hz.

Inside the dewar, the device being tested is mountedin a brass holder that is screwed into a 350mK coldstage. The holder is connected to a corrugated feed hornwith standard WR6 waveguide flange via a square to

FIGURE 2. Measured Spectrum of a Prototype Pixel. Plottedare the simulated filter response function as well as the mea-sured spectral response for polarization channels A and B. Notethat the response for channels A and B were scaled so that thelower edge of the band roughly matched the simulated results.

rectangular(?) waveguide transformer. The waveguidecutoff of the holder is 115GHz , and does not define thepixel’s lower band edge. There is a small gap between thehorn and a piece of lightpipe at 4K. Stycasted into oneend of the lightpipe is 1/8" of fluorigold which is usedto roll off high frequency radiation. At the other end ofthe 4K pipe is either a 1/8" piece of black polyethylene(which acts as a neutral density filter in band and as anadditional strong attenuator at high frequencies) or a 1/8"piece of expanded teflon to low pass radiation below3THz. Next there is a small gap between the 4K lightpipe and the piece of light pipe at 77K. A pair of 1/8"pieces of expanded teflon cap the light pipe on the 300Kside. Following the plastic there is a small gap and then a15 mill polypropylene window to 300K. There are smallmisalignments/tilts at each light pipe juncture.

In Figure 2 a spectrum for a prototype pixel is shown.Plotted are spectra from both polarization channels aswell as the simulated bandpass of the µ-strip filters. Themeasured spectra are plotted assuming a flat spectralsource, (ie assuming that the source is single-moded andbeam filling). However, as the light pipe setup limits thebeam, some spectral dependence up to "2 is expected.We expect 25% ripples due to channel spectra from ourplastic filters. While the band edges are well defined fromscan to scan, we find that modifying the coupling opticsbetween the dewar and the FTS changes the amplitudeof the ripples in the band. Because of the difficulty ofcorrectly normalizing the band in the presence of theseripples, the plotted spectra have been scaled so that thelower band edge roughly matches the simulated filterresponse.

One unexpected problem was the presence of largeamounts of coupling outside of our desired band. Thisout of band power was confirmed with chopped thickgrill measurements. As seen in Figure 3 this excesspower does not display narrow resonances at the ex-pected filter harmonics and extends well above the Nbbandgap energy of 700GHz leading us to believe the

Optical properties of Feedhorn-coupled TES polarimeters for CMB polarimetry July 8, 2009 2

FIGURE 4. Measured vs. simulated spectral band.

TEST RESULTS

A full set of dark characterization and a limited numberof optical tests have been carried out on the prototype 145GHz pixels. In summary, the OMT-coupled polarimetershave band-averaged optical efficiencies of ∼ 54% andcrosspolar coupling of <∼ 4%. The optical efficiencymay be improved by using CPW stub filters instead ofmicrostrip if the loss is due to the dielectric. We havedesigned an OMT-coupled ring resonator to measure thedielectric loss and verify if it is dominating the end-to-end optical efficiency.

The spectral band definition of the stub filters is in ex-cellent agreement with simulations (see Figure 4). Thereis significant out-of-band broadband coupling at higherfrequencies, however, accounting for ∼ 50% of the totalintegrated power. We have been able to eliminate this en-tirely by introducing lossy material into the 50 µm air-gap above the OMT membrane outside the diameter ofthe coupling circular waveguide.

Full detail of the optical tests is available in Bleemet al.[5] In addition, Austermann et al.[6] describes theresults of the dark characterization of the prototype pix-els, including measured uniformities of TES Tc, G(T ),time constants and noise properties. The modeling of themeasured noise and complex impedance is presented inAppel et al.[7].

FUTURE PLANS

Further optical and dark characterizations of the proto-type pixels are currently being carried out. We are plan-ning to measure the polarized beam response functionsusing metal corrugated feedhorns coupled to the single-pixel polarimeters to acertain that the OMT couples to

the fundamental waveguide mode and that the beams arewell-behaved. In addition, work is currently under wayto extend the existing design to 90 and 220 GHz bands.

Future single-pixel polarimeters are planned for de-ployment in the ABS telescope in 2010, and monolithic6" arrays of Si feedhorns/polarimeters are planned for theSPTpol and ACTpol telescopes starting in 2011.

ACKNOWLEDGMENTS

Work at the University of Chicago is supported bythe National Science Foundation through grant ANT-0638937 and the NSF Physics Frontier Center grantPHY-0114422 to the Kavli Institute of CosmologicalPhysics at the University of Chicago. It also receives gen-erous support from the Kavli Foundation and the Gordonand Betty Moore Foundation. Work at NIST is supportedby the NIST Innovations in Measurement Science pro-gram. Work at the University of Colorado is supportedby the National Science Foundation through grant AST-0705302. Work at Princeton University is supported byPrinceton University and the National Science Founda-tion through grants PHY-0355328 and PHY-085587.

REFERENCES

1. Chiang, H. C. et al., 2009, arXiv:0906.1181v22. Brown, M. L. et al., 2009, arXiv:0906.1003v23. McMahon, J. et al., 2009, this volume4. Britton, J. et al., 2009, this volume5. Bleem, L. E. et al., 2009, this volume6. Austermann, J. E. et al., 2009, this volume7. Appel, J. W. et al., 2009, this volume