An Antenna Coupled Cold-Electron Bolometer for High Performance Cosmology Instruments

Transcription

1 An Antnna Coupld Cold-Elctron Bolomtr for High Prformanc Cosmology Instrumnts Lonid Kuzmin 1), Ghassan Yassin ), tafford Withington 3) and Paul Grims ) 1) Chalmrs Univrsity of Tchnology, Gothnburg, wdn, ) Oxford Univrsity, Unitd Kingdom 3) Cambridg Univrsity, Unitd Kingdom Abstract Th nwly mrging CMB polarization xprimnts (g CLOER, EBEX) mploy dtctors comprising transitiondg snsors (TE). Th dtctors will oprat at tmpraturs of approximatly 1 mk and will b rad out by tim division or frquncy multiplxd QUID amplifirs. Although dtctors ar xpctd to dlivr imprssiv snsitivity, futur spac B-mod xprimnts (g B-Pol) can bnfit gratly from an incras in snsitivity, much highr saturation powr, and flxibility in thir ralization on planar substrats. In this papr w dscrib a Cold-Elctron Bolomtr (CEB), which is a srious candidat for th nxt spac cosmology missions. W analyz th suitability of various dvics for th 7 GHz channl of th proposd B-Pol polarimtr. Th dtctor may also b of intrst to ground-basd xprimnt as a rsult of th simplicity of its intgration to planar circuit tchnology. Th Capacitivly Coupld CEB is a planar antnna-coupld suprconducting dtctor with high snsitivity and high dynamic rang. Th CEB can mt nois rquirmnts with both QUID and JFET radouts. Th QUID radout can b usd th sam as for TE bolomtrs with typical QUID snsitivity of.5 pa/hz 1/. An attractiv ralisation of th dtctor at millimtr wavlngths is to fabricat th CEB dirctly connctd to th antnna on a planar substrat. Th propr matching can b achivd by fabrication of an absorbr strip of rsistanc qual to th wav impdanc of th antnna. Thr variants of th CEB concpt hav bn considrd. Th optimum ralization of a CEB with IN and N tunnl junctions givs nois lss than photon nois with QUID radout. Estimations of th CEB nois with a JFET radout (at 3 K and 4. K) has shown an opportunity to raliz background-limitd prformanc for ralistic powr loading. Matching to a JFET is bst obtaind by using th CEB (with wak suprconducting absorbr), and choosing a voltag bias in th flat rgion of th I curv with vry high dynamic rsistanc. This configuration can givs photon nois limitd prformanc with JFET radout ovr a wid rang of optical loading lvls. Anothr possibility for matching to a JFET is a currnt-biasd sris array of CEBs with normal mtal absorbrs, connctd in paralll for HF signal. Th antnna-coupld CEB bolomtr is asy to coupl to a wid rang of planar antnna systms, ithr on bulk or mmbran substrats. Applicability of th CEB to B-Pol and similar spac missions looks vry promising for all of th frquncy bands and with both JFET and QUID radout schms. Indx Trms Cold-Elctron Bolomtr, IN tunnl junction, Josphson junction, Andrv contact, QUID radout Manuscript rcivd Octobr 3, 7. L. Kuzmin is with th Chalmrs Univrsity of Tchnology, Gothnburg, wdn (phon: ; fax: ; -mail: Lonid.Kuzmin@ mc.chalmrs.s). I. INTRODUCTION Rcnt Cosmology xprimnts hav discovrd that th Univrs consists mainly of mystrious Dark Enrgy and Dark Mattr [1]. Indd, in 6, a Nobl Priz was awardd for th xprimntal obsrvation of anisotropis in th Cosmic Microwav Background (CMB) radiation, and th subsqunt ralization that th xpansion of th Univrs is controlld by unknown forcs []. Thr ar svral cosmology instrumnts (B-Pol [3], BOOMERanG, [4], CLOER [5], EBEX, BICEP, QUIET,) that ar bing dsignd to masur th polarization stat of th Cosmic Microwav Background (CMB), in particular th B-mod polarization, which is gnratd by primordial gravitational wavs. It is wll known, howvr, that ground-basd xprimnts ar svrly limitd by atmosphric nois vn at bst sits. Consquntly, spac-born CMB polarization instrumnts ar now bing plannd both in th UA and Europ. A Europan consortium has alrady bn assmbld to dsign th nxt EA CMB cosmology instrumnt. An xprssion of intrst has rcntly submittd to EA, as part of th Cosmic ision Call, to support a mdium-scal spac mission calld B-Pol [3]. A nw gnration of dtctors is ndd for ths advancd tlscops, and ths dtctors must achiv snsitivitis bttr than ~1-18 W/Hz 1/. On of ths tchnologis is th Capacitivly Coupld Cold-Elctron Bolomtr (CEB) [6]-[8]. Th CEB can b usd with both JFET [9] and with QUID radout [6],[8]. Th JFET radout has bn usd for th latst astronomy missions, and th QUID radout and multiplxing has bn dvlopd for TE (Transition-Edg nsor) bolomtrs [1,11]. Ovrall, th goal is to achiv, with a CEB rad out by a JFET or QUID, a nois-quivalnt powr that is lss than th photon nois of th CMB radiation. Th CEB is a planar antnna-coupld suprconducting dtctor with high snsitivity and high dynamic rang. It oprats through lctron ing by IN tunnl junctions, and with strong lctrothrmal fdback [6]. To achiv nois matching to th stimatd in-flight optical powr load, diffrnt concpts of th CEB, with QUID and JFET radouts, must b analyzd. In this papr, w analyz an optimal configuration having on IN junction and an Andrv N contact, and QUID radout. W also analys a CEB with a suprconducting absorbr, and a paralll/sris array of CEBs with JFET radout for th 7 GHz channl of B-Pol. 93

2 To incras th CEB fficincy in voltag-biasd mod, for currnt radout, an optimal configuration with a capacitivly coupld IN junction and an Andrv N contact [1] has bn slctd (Fig. 1). This concpt has bn invntd to improv th nois proprtis by incrasing th rsponsivity of th CEB with QUID radout. An important fatur of th dsign is that th volum of th normal mtal is partly squzd du to th proximity ffct of th suprconducting lctrod of th Andrv contact. This squzing furthr incrass th fficincy of th lctron ing without dgrading th HF coupling. Fig 1. chmatic of th optimal Cold-Elctron Bolomtr (CEB) with capacitiv coupling to th antnna and a QUID radout. Th CEB compriss a planar suprconducting antnna and an absorbr coupld through th capacitanc of an IN tunnl junction, and an N Andrv contact. Th IN tunnl junction is usd also for lctron ing, and for rading out th signal with a QUID. Dtction using this dvic is obtaind by allowing th incoming signal to pass from th antnna to th absorbr through th capacitanc of a tunnl junction and an Andrv contact. RF matching is ralizd by th rsistanc of a normal absorbr, which is indpndnt of th tunnl junction paramtrs. Th concpt is basd on dirct lctron ing of th absorbr, which provids strong ngativ lctrothrmal fdback for th signal. This fdback is analogous to th TE [1,11], but artificial dc hating is rplacd by dirct lctron ing to a minimum tmpratur. This innovation can lad to a major brakthrough in ralizing suprsnsitiv dtctors. Th nois proprtis of this dvic ar improvd considrably by dcrasing th lctron tmpratur. Th loop gain of th lctrothrmal fdback can xcd 1. Th rspons tim is rducd, by lctrothrmal fdback, to 1 ns compard to th intrinsic -ph tim constant of 1 μs. Th CEB in voltag-biasd mod allows a substantial incras in th dynamic rang, by rmoving incoming powr from th absorbr. Th currnt flowing through th tunnl junction is radout by a QUID, which intrinsically has a high dynamic rang. Th CEB with on IN junction and on Andrv contact has almost twic th rsponsivity of th traditional CEB with two IN junctions in sris. II. MODEL In what follows w shall us th basic concpt of th CEB with strong lctrothrmal fdback du to lctron ing. This structur has bn analyzd in dtail in Rf. [6],[13]. Th opration of CEB can b dscribd using th hat balanc quation: 5 5 ) dt (,, T ) ( T T I R C P P( t) T (1) P +ΣΛ ph = + δ ph R abs Λ dt j 5 5 Hr, ΣΛ( T is th hat flow from th lctron to th T ph ) phonon subsystms in th absorbr, Σ is a matrial constant, Λ is th volum of th absorbr, T and T ph ar, rspctivly, th lctron and phonon tmpraturs of th absorbr; P (, T, Tph) th ing powr of th IN tunnl junction; C Λ = ΛγT is th spcific hat capacity of th absorbr; Rj th subgap rsistanc of th tunnl junction; Rabs th rsistanc of th absorbr; P(t) th incoming rf powr. W can sparat Eq. (1) into th tim indpndnt trm, 5 5 ΣΛ (T Tph ) + P (,T,T ph ) = P, and th tim dpndnt trm, ( P T + 5ΣΛT 4 + iω C δt = δp Λ ). () Th first trm, G = P T, is th ing thrmal conductanc of th IN junction that givs th ngativ lctrothrmal fdback (ETF); whn it is larg, it rducs th tmpratur rspons δτ bcaus ing powr, P, compnsats th chang of signal powr in th bolomtr. Th scond trm, G ph = 5ΣΛ T 4, is lctron-phonon thrmal conductanc of th absorbr. From Eq. () w dfin an ffctiv complx thrmal conductanc which controls th tmpratur rspons of CEB to th incidnt signal powr G ff = G + G ph + iω C Λ (3) In analogy with TE [11], th ffctiv thrmal conductanc of th CEB is incrasd by th ffct of lctron ing (ngativ ETF). Hr w assum that th IN tunnl junction is voltag-biasd, and th currnt is masurd by a QUID [6],[1]. Th snsitivity of th dvic is thn charactrizd by th currnt rsponsivity I, which is th ratio of th currnt chang and th chang in th powr load of th bolomtr, ω / T / T L I = = = (4) P G + G + iωc G L+ [ + iωτ] ω ph Λ ( 1)1 whr L = G G ph >> 1 is ETF gain and τ = C Λ G ph = τ (L + 1) (5) is an ffctiv tim constant, τ = C Λ G ph ( 1μs at 1 mk). Th strngth of th lctrothrmal fdback is stimatd as: 94

3 L(ω ) = G = G ph (1 + iωτ ) / T G + G ph + i ωc Λ (6) Nois proprtis ar charactrizd by th, which is th sum of thr diffrnt contributions: = + + δi (7) total ph ph IN = 1k B ΣΛ(T 6 + T ph 6 ) (8) is th nois associatd with lctron-phonon intraction; IN is th nois of th IN tunnl junctions, and th last trm δ I / I is th nois of an amplifir (QUID): δ I, is xprssd in pa/hz 1/. Th nois of th IN tunnl junctions, IN, has thr componnts: shot nois I/ I, th fluctuations of th hat flow through th tunnl junctions, and th anticorrlation trm btwn ths two procsss [13],[14]. IN = δp ω δp ωδi ω + δi ω I. (9) I This anticorrlation is a form of th lctrothrmal fdback discussd arlir by Mathr [15]. III. THE CEB WITH IN TUNNEL JUNCTION AND N CONTACT AND QUID READOUT Th analysis of th Cold-Elctron Bolomtr (CEB) shows that th optimal configuration of th bolomtr in voltag-biasd mod is a CEB with a singl IN Junction and an Andrv N contact [1]. Any us of a doubl junction in Fig.. A Cold-Elctron Bolomtr (CEB) coupld to a finlin antnna with QUID radout. voltag-biasd mod [6,8] would lad to th splitting powr btwn two junctions and a dgradation of rsponsivity. Th optimal radout is a QUID, and voltag bias. W hav analyzd th concpt of an optimal cold-lctron bolomtr for 7 GHz channl of B-Pol polaromtr in th prsnc of th typical powr load (P =. pw I phot I N =.1pA/Hz 1/ oltag/δ.5.8 Fig. 3 Total of th CEB with IN tunnl junction for th 7 GHz channl, with a QUID nois currnt from.1 pa/hz 1/ and.8 pa/hz 1/. R=. kohm, =μm, ol=.3um 3, powr load P =. pw, T=1 mk. Th phot= 4.3*1-18 W/Hz 1/ is shown by dashd lin. pr polarization componnt) [3]. Photon nois: = P * hf. (1) phot For th 7GHz channl, phot= 4.3*1-18 W/Hz 1/. Fig. 3 shows th rsults of a simulation of a CEB with a singl IN junction, with ralistic paramtrs for th tunnl junction and absorbr, and valus of QUID nois from.1 pa/hz 1/ to.8 pa/hz 1/. Th lvl of phot has bn achivd for QUID nois lowr than.5 pa/hz 1/ I. THE CEB WITH I AND JOEPHON JUNCTION IN OLTAGE-BIAED MODE WITH JFET READOUT W shall now discuss a scond schm, which matchs th modrat dynamic rsistanc of th CEB (~1 1 kohm) to th high nois quivalnt rsistanc of a JFET (~1 MΩ). To achiv nois matching to a JFET, a Cold Elctron Bolomtr with a wak uprconducting absorbr (CEB) has bn proposd [9]. In voltag-biasd mod, with a voltag highr than th diffrnc gap, an I junction has a considrably incrasd dynamic rsistanc that is usd to supprss voltag nois of JFET. Howvr, th us of two sris tunnl junctions is not optimal for voltag-biasd mod [1]. A CEB with an I junction and a Andrv-typ contact (similar to optimal bolomtr with IN and N contacts [1]) could solv this problm, but would bring complicatd 3-layr tchnology. 95

4 Fig 4. chmatic of a uprconducting Cold-Elctron Bolomtr (CEB) with I and Josphson Tunnl Junctions and a JFET radout [16]. Th I junction is usd for capacitiv coupling to th antnna, thrmal isolation, lctron ing and dc radout by a JFET. Th Josphson junction is usd for dc and RF contacts, and for thrmal isolation. I junction is mad in a loop gomtry for asy supprssion of a critical currnt by a wak magntic fild. A novl concpt of a uprconducting Cold-Elctron Bolomtr (CEB) with I and Josphson tunnl junctions (Fig. 4) has bn proposd rcntly [16]. Th main innovation in comparison with prvious concpts of th CEB in voltagbiasd mod is th ffctiv us of a Josphson junction for dc and RF contacts, and for thrmal isolation. Th I junction (for RF coupling, thrmal isolation, lctron ing and dc radout) is proposd in a loop gomtry for supprssing th critical currnt by a wak magntic fild. A rmarkabl fatur of this concpt is that th critical currnt of th Josphson junction is not compltly supprssd by a wak magntic fild. As a rsult, a robust two layr tchnology can b usd in fabrication of both th I and Josphson tunnl junctions simultanously. In this papr w analyzd a ralization of th CEB for th 7 GHz channl of B-POL. For RF coupling w hav chosn a 4-prob antnna in circl wavguid with dirct connction of CEBs to th antnna (Fig. 5a) [16]. In contrast to a prvious concpt of th CEB with coplanar lins [9], th RF rgion is strictly limitd by th circular wavguid ara. Th optimal point for th CEB is shown in th diagram, whr th RF currnt is gratst. Th problm of DC bias of an I junction could b solvd by introducing on mor Josphson junction at th right nd of th absorbr (Fig. 5b). Two opposit CEBs ar connctd in paralll, for ach polarization, by dc lads, and masurd by JFET in voltag-biasd mod. Th optimal bias point of I junction is btwn th diffrnc and sum gaps whr th I- curv has incrasd dynamic rsistanc (Fig. 6). For th JFET nois, 3 n/hz 1/ & 5 fa/hz 1/, th ffctiv nois impdanc is around 6 KOhm. Th supprssion of th JFET voltag nois is important for this ralization. Currnt nois in a JFET ar rathr low, at th lvl of 5fA/Hz 1/. Th high nois impdanc of a JFET amplifir is on of th rasons why a low-ohmic TEs [9,1] cannot b matchd with JFETs. Fig 5. a) Dirct connction of CEBs to a 4-prob antnna in a circular wavguid [16]. CEBs in opposit probs ar connctd in paralll for ach polarization. b) A dtail of th CEB connctd to a prob antnna with an additional Josphson junction for dc bias supply. For th analysis w us a prvious concpt of th CEB with strong lctrothrmal fdback, du to lctron ing [6,8,13], and with a suprconducting absorbr [9]. For an optical powr load of P =. pw pr polarization for th 7 GHz channl of B-Pol, th photon nois is phot = 4.3*1-18 [3]. Figur 3 shows simulations of th diffrnt contributions to JFET -ph phot tot Currnt oltag, μ I' Fig. 6. componnts of th CEB with JFET radout for I JFET =5 fa/hz 1/, JFET =3 n/hz 1/, R=1 kohm, Λ=.μm 3, powr load P =. pw pr polarization. I curv is shown for stimation of a high dynamic rsistanc of th junctions. Th phot= 4.3*1-18 W/Hz 1/ is shown by dashd lin

5 th total of th bolomtr. W s that for a rang of bias voltag from 155 μ to 195 μ, th total of th CEB is wll blow th photon nois: tot < phot. Th rang of voltags from 155 μ to 17 μ is not rcommndd for us bcaus, du to ngativ slop th Icurv, th oprating point would b unstabl. In addition, th tot of th CEB is dominatd at th optimum point by th shot/hat nois of th dtctor, I, (9) corrsponding to th background limitd mod of opration. Equation (9) includs th ffct of th nois rduction of I tunnl junction du to th anticorrlation trm. Th final nois, I, is lss than nois componnts. Th ffct is strongr than for IN junction nois [1] du to th wll-dfind lvl of th quasiparticl nrgy just nar th suprconducting gap.. THE CEB ARRAY WITH IN TUNNEL JUNCTION IN CURRENT-BIAED MODE WITH JFET READOUT An altrnat mod of CEB opration is a novl concpt mploying a paralll/sris array of CEBs with IN Tunnl Junctions, for ffctiv matching to a JFET amplifir [17] (Fig. 7). Prvious analysis of a singl currnt-biasd CEB with JFET radout showd that th JFET input voltag nois limits th snsitivity [9]. Th main rason is th dgradation of voltag rsponsivity undr high optical powr load. Th main innovation of th CEB array is th distribution of powr btwn N sris CEBs, and summarizing th incrasd rspons from th array. Effctiv distribution of powr is achivd by a paralll connction of CEBs, which coupl to th RF signal through additional capacitancs (Fig. 7b). Th rspons is incrasd bcaus th CEB is snsitiv to th lvl of powr, and th powr is dcrasd N tims for th individual CEBs, with a proportional dcras of absorbr ovrhating. Th high snsitivity of th CEB for small powr loads has bn analyzd thortically [6,8,13], and dmonstratd xprimntally [18]. In this papr w analyz a ralization of th CEB array for th 7 GHz channl of B-Pol. For RF coupling w analyz a systm with th dirct insrtion of th CEB arrays into a 4-prob antnna insid a circular wavguid (Fig. 7). Th systm is similar to th prvious 4-prob systm with CEBs (Fig. 5), with th rplacmnt of th CEBs by th CEB arrays, and with th rplacmnt of th paralll connction to a sris connction of opposit CEBs. Th problm of DC biasing th CEB arrays can b solvd by intrconncting opposit probs by a narrow strip with vry high inductiv impdanc (Fig. 7a). A small isolation layr should b placd btwn strips in th cntr of th wavguid. Two opposit CEB arrays ar connctd in sris to gt twotims highr rspons for ach polarization. Th voltag rspons is masurd by a JFET amplifir in a currnt-biasd mod. Th main purpos of this concpt is to match th total dynamic rsistanc of th array to th nois impdanc of a JFET (~.6 MΩ). Th powr should b dividd btwn th CEBs in th array to incras th rsponsivity du to lowr ovrhating and modrat lctron ing. Fig 7 a) Dirct connction of CEBs to a 4-prob antnna in circlar wavguid [17]. CEBs in opposit probs ar connctd in sris by a narrow strip for ach polarization. b) Each prob is rally connctd to an array of CEBs with sris connction for DC and paralll for RF (schmatically shown as a singl CEB in th top figur). For RF th CEBs ar connctd in paralll by additional capacitancs btwn suprconducting islands and antnna. Th opration of a CEB array can b analyzd using th hat balanc quation for a singl CEB [13] taking into account powr distribution btwn th N bolomtrs. Th rsponsivity is dscribd by th voltag rspons to an incoming powr δ = ω = δ P ω Th scond trm G ph / T + G IN + iω C Λ (1) PIN PIN G IN = / (11) T T is th ing thrmal conductanc of th IN junction, G IN, which givs som lctron ing and hlp to avoid ovrhating of th absorbr. Nois proprtis ar charactrizd by th nois quivalnt powr (), which is th sum of thr contributions: tot = N * ph + N * IN + JFET. (1) 97

6 Hr -ph is th sam lctron-phonon nois as in Eq. 8. IN is th nois of th IN tunnl junctions. Th IN nois has thr componnts: th shot nois I/I, th fluctuations of th hat flow through th tunnl junctions and th corrlation btwn ths two procsss [13-15]: δiω < δp ω δ Iω > IN = + + δp ω ( ). (13) Du to this corrlation th shot nois is incrasd at 3-5% in contrast to th CEB in voltag-biasd mod (9) whr strong anti-corrlation dcrass th shot nois. Th last trm is du to th voltag δ and currnt δ I nois of a JFET, which ar xprssd in n/hz 1/ and pa/hz 1/ : JFET = ( δ + ( δi * (Rd + Ra) * N ) ) (14) Th strong dpndnc on N, dcrasing this nois is includd in th rsponsivity, which is proportional to N. Th stimations wr mad for th 7 GHz channl of BPol phot tot JFET -ph IN,5,6,7,8,9 1 oltag/dlta Fig. 8. componnts of th array of 1 CEBs with JFET radout at 7 GHz with powr load of. pw for I JFET =5 fa/hz 1/, JFET =3 n/hz 1/, R=4 kohm, Λ=.3μm 3, T=1 mk. W hav simulatd arrays of CEBs with diffrnt numbrs of CEBs, from 1 to 14, to achiv a low with JFET radout. Fig. 8 shows typical rsults of an simulation for th optimal array of 1 CEBs. W s that for a rang of normalizd voltag from.6 to.93, th total of th CEB array is lss than th photon nois. At th optimum point, background limitd prformanc is ralizd (th total nois is dtrmind by th nois of IN junctions, IN, (13) du to background powr load). Th dpndnc of th nois componnts on th numbr of bolomtrs is shown in Figur 9. Th total dcrass to a lvl lss than photon nois for a numbr of CEBs largr than 6 (3 for ach prob). It is achivd mainly through th supprssion of th JFET nois componnt du to th incrasd rsponsivity (1). Figur 9 dmonstrats a strong linar incras of th rsponsivity proportional to N whn th numbr of bolomtrs is incrasd. Th nois of th JFET (14) is proportionally dcrasd, which is th main goal of this ralization. Around th optimum point (N=1) th JFET is lss than IN, which is a manifstation of backgroundlimitd opration. Th IN incrass proportionally to N (according to q. 6), but dcrass du to a dcras of th hat flow (and currnt) and an incras of th rsponsivity. Ths two ffcts approximatly compnsat ach othr, and IN is not vry snsitiv to th numbr of th bolomtrs. Th most surprising rsult is that th ph (8) is not incrasd proportionally to th numbr of bolomtrs whn th total volum of absorbr is incrasd proportionally to N. Th rason is du to a compnsation of this dpndnc by som dcras in T that is in th 6 th powr for ph (8) JFET I' phot -ph 1 1 Numbr of CEBs, N tot Fig. 9. componnts and photon in dpndnc on th numbr of CEBs in a sris array. Th paramtrs of CEBs ar th sam as in Fig. 8. Th rsponsivity is shown for illustration of th ffct of th CEB numbr. Optimal numbr of CEBs in sris array. Th optimal numbr is dtrmind mainly by th powr load Po and th volum of absorbr Λ. Th gnral rul of array dsign is th following: th numbr of bolomtrs, N, should b incrasd to split Po btwn bolomtrs up to th point whn P /N= P ph, whr P ph =T 5 ph ΣΛ. Th phonon powr is dtrmind by only on paramtr, th volum of th absorbr, Λ. Thr is no nd to incras th numbr of bolomtrs mor than this figur bcaus th optical powr loading in ach bolomtr bcoms lss than th powr from phonons. Rsponsivity is saturatd aftr this lvl. I. CONCLUION W hav analyzd svral variations of th concpt of a Cold-Elctron Bolomtr (CEB) with an IN (uprconductor- Insulator-Normal Mtal) or I (uprconductor-insulator- Wak uprconductor) tunnl junctions for ralization of th rquirmnts of futur spac projcts. Ths concpts giv uniqu opportunitis to achiv s lss than photon nois for any optical powr loading with standard JFET or QUID radouts. Thr variants of th CEB concpt hav bn considrd for th 7 GHz channl of B-Pol. Th first optimal solution is a CEB with IN and N tunnl junctions which givs nois lss than photon nois with QUID radout. For matching th dvic to a JFET radout w hav analyzd th CEB (with wak suprconducting absorbr) choosing a voltag bias in th flat rgion of th I curv with vry high dynamic rsistanc. This configuration can giv background 98

7 limitd prformanc with total nois considrably lss than photon nois. Anothr concpt for matching with JFET radout is a currnt-biasd sris array of CEBs with normal mtal absorbrs, connctd in paralll for th RF signal. This concpt has also dmonstratd total nois lss than th background photon nois. vral variants of RF coupling of CEBs to finlin and a 4- prob antnna hav bn proposd. W hav shown that th CEB is an idal antnna-coupld bolomtr that can b asily intgratd to any antnna systm on a bulk or mmbran substrat. Applicability of th CEB to BPol and similar spac missions looks vry promising for th whol rang of frquncy bands, and with both JFET and QUID radout. Th authors would lik to thank Paolo d Brnardis, Philip Mauskopf and Dmitri Golubv for intrsting discussions. Th work was supportd by NB and TINT wdish agncis. REFERENCE [1] C. if, BREAKTHROUGH OF THE YEAR 3: Illuminating th Dark Univrs, cinc, vol. 3, pp , Dc. 3. [] 6 Nobl Priz in Physics for discovry of th blackbody form and anisotropy of th cosmic microwav background radiation. [3] B-Polarization atllit Proposal for Dtcting Primordial Gravitational wavs from Inflation. EA, Jun 7. [4] BOOMERANG - balloon tlscop: Masurmnts of CMB Polarization, [5] G. Yassin t al., CLOER- A Novl Instrumnt for Masuring th CMB Polarization, Proc. of th 15th Int. ymposium on pac Trahrtz Tchnology, pp 15-16, UMA, UA (4). [6] L. Kuzmin Ultimat Cold-Elctron Bolomtr with trong Elctrothrmal Fdback, Proc. of PIE confrnc Millimtrs and ubmillimtr Dtctors, 5498, p 349, Glasgow, Jun 4. [7] L. Kuzmin On th Concpt of a Hot-Elctron Microbolomtr with Capacitiv Coupling to th Antnna, Physica B: Condnsd Mattr, 84-88, 19 (). [8] L. Kuzmin D. Golubv On th concpt of an optimal hot-lctron bolomtr with NI tunnl junctions. Physica C , 378 () [9] L. Kuzmin, P. Mauskopf, and D Golubv, uprconducting Cold-Elctron Bolomtr with JFET Radout for OLIMPO Balloon Tlscop. Journal of Physics: Confrnc ris (JPC), olum 43, pp (6). [1] K. Irwin. Applid Physics Lttrs, 66, (1995) 1998 [11] A. L, P. Richards,. Nam, B, Cabrra, K. Irwin, Applid Physics Lttrs, 69, (1996) 181. [1] L. Kuzmin, Optimal Cold-Elctron Bolomtr with an IN Tunnl Junction and an Andrv Contact. Proc. of th 17 th Int. ymp. On pac Trahrtz Tchnol., ITT-6 (Paris, May 6). [13] D. Golubv and L. Kuzmin. Nonquilibrium thory of th hot-lctron bolomtr with NI tunnl junction. Journal of Applid Physics. 89, (1). [14]. Golwala, J. Johum, and B. adoult, Proc. of th 7 Int. Workshop on Low Tmpratur Dtctors, July 1997, Munich, pp [15] J. C. Mathr, Appl. Opt. 1,115 (198). [16] Lonid Kuzmin, A uprconducting Cold-Elctron Bolomtr with I and Josphson Tunnl Junctions, Journal of Low Tmpratur Physics, 151, pp (8). [17] Lonid Kuzmin, Array of Cold-Elctron Bolomtrs with IN Tunnl Junctions for Cosmology Exprimnts, Journal of Physics: Confrnc ris (JPC), 97, 131 (8). [18] I. Agulo, L. Kuzmin and M. Tarasov, Nois Charactrization of th Cold- Elctron Bolomtr, subm. to Applid Physics Lttr (7). 99