SUPERCONDUCTING HOT-ELECTRON BOLOMETER MIXER FOR TERAHERTZ HETERODYNE RECEIVERS

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1 SUPERCONDUCTING HOT-ELECTRON BOLOMETER MIXER FOR TERAHERTZ HETERODYNE RECEIVERS Alexei D. Semenov, Heinz-Wilhelm Hiibers, Heiko Richter, Konstntin Smirnovc, Gregory N. Gortsmn c, Ntli Kurov c, Boris M. Voronove DLR Institute of Spce Sensor Technology nd Plnetry Explortion, Berlin, Germny Moscow Stte Pedgogicl University, Moscow, Russi A number of on-going stronomicl nd tmospheric reserch progrms re imed to the Terhertz (THz) spectrl region. At frequencies bove bout 1.4 THz heterodyne receivers plnned for these missions will use superconducting hot-electron bolometers s mixers. We present recent results of the terhertz ntenn development of superconducting NbN hot-electron bolometer mixer for GREAT (Germn Receiver for Astronomy t Terhertz Frequencies, to be used bord of SOFIA) nd TELIS (Terhertz Limb Sounder). The mixer is incorported into hybrid ntenn consisting of plnr feed ntenn, which hs either logrithmic spirl or double-slot configurtion, nd hyper hemisphericl silicon lens. The hybrid ntenn showed lmost frequency independent nd symmetric rdition pttern with the bem-width slightly broder thn expected for diffrction limited ntenn. The noise temperture s well s its spectrl dependence chnges with the bolometer sizes tht provides dditionl tool for mixer optimiztion. FTS spectr mesured in the direct detection regime greed with the noise temperture spectr. Mixer nd Antenn Design Hot-electron bolometers (HEBs) were mnufctured from superconducting NbN film with nominl thickness of bout 3 nm. The film ws deposited by dc rective mgnetron sputtering on 35-gm thick Si substrte. The HEB ws incorported in plnr feed ntenn ptterned from 2 nm thick gold film. The bolometer hd n re of one tenth of squre, i.e. the width mounted ten times the length. Given the norml sheet resistnce 6 Ohm t the trnsition temperture of the NbN film, the resistnce of the bolometer just bove the superconducting trnsition should be 6±6 Ohm s determined by the ccurcy of the mnufcturing process. However, the contct resistnce between the HEB nd the inner terminls of the ntenn ffected the ctul device resistnce. The contct resistnce vried depending on the contct re nd contct qulity. Two types of feed ntenn hve been studied: self-complementry 33

2 logrithmic-spirl nd twin-slot. We tested three logrithmic-spirl ntenns with different sizes of the inner terminls nd double-slot ntenns optimized for severl frequencies in the rnge from 1.5 THz to 3 THz. Antenn prmeters re depicted in Fig.1 nd specified in the Tble. The substrte crrying the HEB nd the feed ws glued with its bckside onto the flt opticlly polished side of n extended hemisphericl 12 mm dimeter silicon lens. The 3.5 mm extension of the lens together with the substrte thickness positioned the feed in the second ellipticl focus [1]. The lens hd Prylene ntireflection coting optimized for 2.5 THz [2]. The lens with the HEB ws mounted in n Infrred Lbs helium dewr with wedged TPX vcuum window nd cold (77 K) qurtz filter. Fig. 1 Plnr log-spirl (left) nd twin-slot (right) ntenns. Gold is shown gry. Blck rectngle in the geometric center of the ntenn depicts the bolometer. For log-spirl feeds: D is the dimeter of the circle tht demrctes the spirl structure of the feed from the contct pd re; B is the lrgest bolometer width tht the feed my hve embedded. Tble. Sizes in micrometers of studied log-spirl nd twin-slot plnr feeds. Antenn type Log-spirl feed B D Sp_ 4 14 Sp_b Sp_c Twin-slot feed Antenn type L S W b TW TW TW _ 2 4 TW TW

3 Experimentl rrngement The intermedite frequency (IF) signl ws guided out of the mixer vi the 5- coplnr line. A circultor ws used to feed the bis to the mixer nd to trnsmit the IF signl to low noise HEMT mplifier, which hd either 1 2 GHz or 4 8 GHz bndwidth. The IF signl ws filtered by n electriclly tunble filter with bndwidth of 75 MHz, further mplified nd rectified with microwve crystl detector. An opticlly pumped FIR lser providing lines t frequencies.69, 1.63, 2.53, 3,1 nd 4,2 THz ws used s locl oscilltor (LO). Signl rdition nd LO rdition were superimposed by 6 gm thick Mylr bem splitter. The double sidebnd (DSB) noise temperture of the receiver ws mesured by the Y-fctor method. Hot nd cold lods (Eccosorb) t 293 K nd 77 K lterntively covered the receiver bem. Output signl, tht is the dc voltge t the crystl detector, ws continuously redout by computer, which performed sttisticl nlysis of the signl nd computed the noise temperture. The bem pttern of the hybrid ntenn ws mesured t two LO frequencies, 1.6 THz nd 2.5 THz, by moving hot, point-like source in the fr field of the receiver. The output heterodyne signl ws registered s function of the position of the source. FTS mesurements were performed in the direct detection regime with the mixer kept t the middle of the superconducting trnsition. We used simple cube interferometer with 12-mm thick Mylr bem splitter nd mercury dischrge lmp s rdition source. The mirror ws moved with step motor. Typiclly, ten or more computed spectr were ccumulted nd verged in order to increse the sensitivity. o -2 H-plne U. Ai E-plne -6-1 on U Angie (Degree) Angle (Degree) Fi g. 2. Bem profile of the twin-slot ntenn TW2 t 1.6 THz in the H- (left pnel) nd E- (right pnel) plnes. Solid lines show the Gussin fit with the 3-dB width of 1.1. Antenn bem pttern At 1.6 THz we found for the twin-slot feed ntenn 3-dB bem width of 1.1 in both E- nd H-plnes. The pttern ws firly symmetric (Fig. 2) with the side lobes ppering slightly below 1 db. The log-spirl feed hd t this frequency the sme side 35

4 14th Interntionl Symposium on Spce Terhertz Technologv lobe level nd lso showed symmetric bem pttern (Fig. 4). The polriztion of the ntenn ws ellipticl with the middle vlue 1.3 of the 3-dB bem width. There ws 15% difference between the bem width in two min ellipticl plnes, indicting tht t this frequency the twin-slot feed is probbly the better choice thn logrithmic-spirl. However, this chnges t higher frequencies where the required dimensions of the twinslot ntenn become comprble to the size of the IF embedding circuitry. This brings prsitic impednces nd destroys coupling [3] between the feed nd the embedding circuitry. -. i, IP -1 R3 D dm i t Angle (Degree) Fig. 3. Bem profile of the twin-slot ntenn TW3 t 2.5 THz in the H-plne (filled symbols) nd in the E-plne (open symbols). Solid line presents the Gussin fit to the min lobe with the 1.5 bem width t 3 db " ṙii I, WAIN ir MIIII SIM i,,,,....., Wiill E11 : II, : IRE i1 11 ', $ IBM ' III %I Angle (Degree) Angle (Degree) Fig. 4. Bem profile of the log-spirl ntenn Sp_b t 1.6 THz (solid line) nd 2.5 THz (dotted line) in min ellipticl plnes. 36

5 At 2.5 THz, we found (Fig. 3) higher side lobes (-3 db) nd wider bem pttern (1.5 ) for the twin-slot feed. The log-spirl feed hd t this frequency (Fig. 4) n lmost symmetric circulr polrized bem with 3-dB width of.8 nd side-lobes t 5 db. Spectrl properties: Noise temperture Generlly, the noise temperture of HEB mixer imbedded in the plnr feed increses with frequency. Tht might be mnifesttion of the detection mechnism [1] s well s the electrodynmics of the feed or opticl coupling. We mde n ttempt to seprte the contribution of the feed nd the HEB geometry in tht we mesured the noise temperture s function of frequency for severl identicl feeds hving bolometers with different plnr dimensions. The length nd the width of the bolometer were both chnged proportionlly in order to keep the number of squres nd the impednce of the bolometer unchnged. Dt shown in Fig. 5 suggest tht the noise temperture grows fster for bolometers with lrger width. This result seems to be nturl consequence of the skin effect in the bolometer. Indeed, the skin depth in our NbN films [1] iscz.,'.57 gm t 2.5 THz tht is noticebly smller thn the bolometer width pm d. 2 Hl.2um fe) 1.5 pm 1 3.pm 1 2 Frequency (THz) 3 Fig. 5 Noise temperture for severl bolometers with different width (shown in the plot) embedded in the log-spirl feed of the type Sp. Although the noise temperture t prticulr frequency tends to increse when the bolometer width decreses, this trend cn be hrdly treted numericlly. Becuse of very lrge spred of the noise tempertures between btches of identicl mixers, the error br would be lrger thn the expected vritions. The reson for tht is most likely poorly 37

6 controllble contct resistnce between the bolometer nd the inner terminls of the feed. The best noise tempertures t 2.53 THz (1.5 GHz intermedite frequency) were 16 K for Sp_ with 3 mm bolometer width, 22 K for Sp_b with 1.3 mm bolometer width. nd 26 K for Sp_c with 1.5 mm bolometer width. We shll note here tht the power of the locl oscilltor required for optiml (lowest noise temperture) opertion increses in proportion to the bolometer volume. We hve directly mesured the W LO power in front of the cryostt window t the optiml opertion of 1.6-pm wide bolometer imbedded in the Sp_b feed. Expected power for 3-rim wide bolometer is four times lrger tht might be crucil for pplictions relying on solid stte LO sources. The frequency dependence of the noise temperture for log-spirl feeds Sp_ nd Sp_c is shown in Fig. 6. Typiclly, the feed Sp_, which is cpble to crry lrger bolometer, shows smller noise temperture t ny prticulr frequency. In order to compre electrodynmics of feeds nd possibly void the effect of contct resistnce, we used two different feeds with identicl bolometers hving the dimensions 1.5 x.15 pm2. The feed Sp_c with the spirl structure going down to smller rdius (see Fig. 1 nd Tble) better performs t higher frequencies. According to [4], the upper cut-off frequency of log-spirl feed should be (c is the speed of light nd e is the dielectric constnt of the substrte) 1/ = (1) 5DV(1+ e) I 2 For our Sp_ feed ntenn vo 1.6 THz tht grees within the ccurcy of the cut-off definition with the dt presented in Fig. 6. Following the sme criteri, one should expect for Sp_c the cut-of t 6 THz Sp 12 rj. 1 et. ' Sp_c Frequency, THz Fig. 6 Noise temperture of the log-spirl feeds Sp (tringles) nd Sp_c (squres) with identicl bolometers. 38

7 14th Interntionl S y mposium on Spce Terhertz Technology. 1.2 Frequency (THz) ;.6 - C Wvenumber (cm-1) _ Fig. 7. Normlized to mximum FTS spectr of three twin-slot feeds. Dimensions re presented in the Tble. Blck squres show normlized noise temperture mesured with the sme TW3 feed. -- O8 _6.4 _2 Wvelength (p.m) Fig. 8. Normlized to mximum FTS spectr of two log-spirl feeds (solid line Sp_b, dotted line Sp_) nd mesured noise tempertures (closed symbols Sp_b, open symbols Sp). Spectrl properties: Fourier trnsform spectroscopy FTS spectr of few twin-slot feeds re shown in Fig. 7. For ll three, noise tempertures mesured t vilble LO frequencies correspond well to the obtined FTS spectr. Blck squres illustrte tht for the TW3 feed. Our dt shows, in greement with [3]. tht the common design rule [5] L = 1.95-S nd L =.33 X does not properly 39

8 work t frequencies bove 2 THz. In order to relize the required resonnce frequency. the slot length nd seprtion should decrese in excess to the wvelength. Fig. 8 shows FTS spectr of log-spirl feeds. For this ntenn type we lso found resonbly good greement between spectr nd mesured noise temperture. There is lthough contrdiction with common understnding of how the cut-off frequency should vry with the feed size. According to (1), the rtio of the smllest spirl dimeters D (see Fig. 1 nd the Tble) dicttes the cut-off frequency of the Sp_b lmost twice s lrge s the cut-off frequency of the Sp_. In fct we hve found much smller increse of the cut-off frequency. In conclusion, we hve studied spectrl properties nd bem pttern of log-spirl nd twin-slot feeds t terhertz frequencies nd found feed sizes resulting in resonbly good performnce. Comprison of two types of fed ntenn suggests tht t frequencies bove 2 THz the spirl feed, lthough it hs not very prcticl ellipticl polriztion, provides better overll performnce. Acknowledgement The work ws prtly supported by INTAS, K.S. cknowledges support through personl grnt YSF by INTAS. References 1. A.D. Semenov, H.-W. Htibers, J. Schubert, G.N. Gol'tsmn, A.I. Elneev, B.M. Voronov, nd E.M. Gershenzon, Design nd performnce of the lttice-cooled hot-electron terhertz mixer, J. Appl. Phys 88, 6758 (2). 2. H.-W. Hiibers, J. Schubert, A. Krbbe, M. Birk, G. Wgner, A. Semenov, G. Gtsmn, B. Voronov, nd E. Gershenzon, Prylene ntireflection coting of qusi-opticl hot-electron-bolometric mixer t terhertz frequencies, Infrred Physics & Technology 42, 41(21). 3. R.A. Wyss, A. Neto, W.R. McGrth, B. Bumble, H. LeDuc, Submillimeter-wve spectrl response of twin-slot ntenns coupled to hot electron bolometers, Proceedings of the 11 th Int. Symposium on Spce Terhertz Technology, Uni. Of Michign, Ann Arbor, MI, My 2, pp J.D. Dyson, The equingulr spirl ntenn, IRE Trnsctions on Antenns nd Propgtion AP-7, 181(1959). 5. W. Gnzevles, A qusi-opticl THz mixer bsed on Nb diffusion-cooled hotelectron bolometer, Ph. D. thesis, Delft University oftecimology, 21. 4