A Polarization Sensitive Bolometric Receiver for Observations of the Cosmic Microwave Background

APolarizationSensitiveBolometricReceiverforObservationsoftheCosmicMicrowaveBackgroundW.C.Jonesa,R.S.BhatiaJ.J. Bo cka,bA.E.LangeaCalifornia Instituteof Technology,Pasadena,CA,USAbJetPropulsion Lab oratory,Pasadena,CA,USAABSTRACTWehavedevelop ed a b olometric receiver that is intrinsically sensitive to linear p olarization for the purp ose ofmaking measurements of the p olarization of the cosmic microwave background radiation.The receiver consistsofapairco-lo catedsiliconnitridemicromeshabsorb ers whichcoupleanisotropically tolinearlyp olarizedradiationthroughacorrugatedwveguidestructure.Thissystemallowsbackgroundlimited,simultaneousmeasurementoftheStokesIandQparametersover30%bandwidthsatfrequenciesfrom60to600GHz.Since b oth linear p olarizations traverse identical optical paths from the sky to the p oint of detection, thesusceptibility to systematic e ects is minimized.The amount of uncorrelated noise b etween the to p olarizationsenses is limited to the quantum limit of thermal and photon shot noise, while drifts in the relative resp onsivityto orthogonal p olarizations are limited to the e ect of non-uniformity in the thin lm dep osition of the leads andthe intrinsic thermistor prop erties.Devices using NTD Ge thermistors haveachieved NEPs of 21017W=pHzwith a 1=fknee b elow 100 mHz at a base temp erature of 270 mK. Numerical mo delling of the structures hasb een used to optimize the b olometer geometry and coupling to optics.Comparisons of numerical results andexp erimental data are made.A description of how the quantities measured by the device can b e interpreted interms of the Stokes parameters is presented.The receiver develop ed for theBoomerangandPlanckHFI fo calplanes is presented in detail.Keywords:Sub-mm Detectors, Bolometers, Cosmic Microwave Background Polarization, Cosmology1. INTRODUCTIONObservationalcosmologistshaveyettodetectp olarizationinthecosmicbackgroundradiation(CBR),andupp er limitsare still well ab ovethe level exp ected due to Thompson scattering of quadrup ole anisotropies inthe background radiation during the ep o ch of recombination.1, 2The small amplitude of this p olarized signal,p eaking at p erhaps 5Kat100scales, demands not only extremely high raw sensitivity,but also exquisitecontrol of systematics.Over the last twentyyears, all published e orts to detect p olarization in the CBR have used coherent receivers.Hetero dyne,quasitotal-p ower,andcorrelationreceiverswithfront-endRFlonoiseampli erblo cksbasedon HEMTs are mature technologies at millimeter wavelengths.The fundamental design principles of these ofreceivers are well established and have b een used to construct p olarized receivers at radio to mm-wae frequenciesfor manyyears.4Although cryogenic b olometric receivers achieve far higher instantaneous sensitivities over widerbandwidths than their coherent analogs, the intrinsic p olarization sensitivity of coherent systems has made themthe choice of the rst generation of CBR p olarization exp eriments.Inthispap erwedescrib eanewb olometricsystemwhichcombinesthesensitivity,bandwidth,andstabilitofacryogenic b olometer withtheintrinsicp olarization capabilitytraditionally asso ciated withcoherent sys-tems.In addition, the design obviates the need for orthogonal mo de transducers (OMTs), hybrid tee networks,Corresp onding Author:William Jones, 1200 E. California Blvd., MC 59-33, Pasadena CA 91125.email: wcj@astro.caltech.edu, tel: 626.395.2020To the author's knowledge, the pioneering e orts of Caderni, et al., are the only published CBR p olarization limitsset by a b olometric system.3