Koshelets, V. P.

Lapytskaya, I. L.

Mygind, Jesper

Shchukin, A. V.

Shitov, S. V.

Flux flow oscillators (FFO) have been experimentally investigated at frequencies up to 850 GHz. At 440 GHz the received power in an on-chip integrated SIS mixer is 5 μW. The first experimental measurements of the FFO linewidth as a function of applied dc bias current and magnetic field have been performed both for two autonomous FFOs and with one of the FFOs injection-locked to a narrow-band external microwave source. By beating the two autonomous FFOs an integral spectral linewidth as low as 750 kHz is measured at 280 GHz. Mechanisms leading to a broadening of the FFO linewidth are discussed. Experimentally it is shown that the FFO can be used as harmonic generato

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V.; Lapytskaya, I. L.Published in:I E E E Transactions on Applied SuperconductivityLink to article, DOI:10.1109/77.403211Publication date:1995Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mygind, J., Koshelets, V. P., Shchukin, A. V., Shitov, S. V., & Lapytskaya, I. L. (1995). Properties of autonomousand injection locked flux flow oscillators. I E E E Transactions on Applied Superconductivity, 5(2), 2951-2954.DOI: 10.1109/77.403211IEEE TRANSACTIONS ON APPLIED SUPERCONDUCI’IVITY, VOL. 5, NO. 2, J U N E  1995 ’ 295 1 .-. _ _ _  m67 ,r: Properties of Autonomous and Injection Locked Flux Flow Oscillators J .  Mygind Physics Department, Technical University of Denmark, B309, DK-2800 Lyngby, Denmark (4 _-. ’2 V. P. Koshelets, A. V. Shchukin, S. V. Shitov, and I. L. Lapytskaya Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Moscow, Russia .-. Abstract-Flux flow oscillators (FFO) have been ex- perimentally investigated at frequencies up to 850 GHz. At 440 GHz the received power in an on-chip integrated SIS mixer is 5 p W .  The Arst experimental measurements of the FFO linewidth as a function of applied dc bias current and magnetic Aeld have been performed both for two autonomous FFOs and with one of the FFOs injection-locked to a narrow-band external microwave source. By beating the two au- tonomous FFOs an integral spectral linewidth as low as 750 kHz is measured at 280 GHz. Mechanisms leading to a broadening of the FFO linewidth are dis- cussed. Experimentally it is shown that the FFO can be used as harmonic generator. H N b  W N O ,  Si substrate .- (a) I. INTRODUCTION The uni-directional and viscous flow of magnetic flux quanta in a long (length, L >> A J ,  the Josephson penetra- tion depth) Josephson tunnel junction with high damping [l] has recently been successfully used in the development of local oscillators (LO) for fully superconducting inte- grated submillimeter wave SIS receivers [2]-[6]. The fre- quency of this so called Flux Flow Oscillator (FFO) can be tuned over a wide frequency range, limited only by the superconductor gap frequency. Moreover, the FFO pro- vides sufficient output power to pump a SIS mixer array detector. Preliminary FFO spectral linewidth measure- ments have demonstrated encouraging values: 130 kHz at 70 GHz [2], about 1 MHz at 140 GHz [5] and 2.1 MHz at 320 GHz [SI. 11. EXPERIMENTAL SYSTEM For comprehensive studies of the FFO properties a series of different integrated circuits comprising long Josephson junctions and microwave SIS detectors have Manuscript received October 18, 1994. J. Mygind, e-mail myg@mips.fys.dtu.dk; fax (+45) 4593 1669; V. P. Koshelets, e-mail iree235@glas.apc.org; fax (+7) 095 203 8414. The work was supported in part by the Russian Program of Fun- damental Research (G 92-02-3484), the Russian State Scientific Pro- @am ”Superconductivity” (G 91009), NATO (LG 921040), and the Danish Research Academy. been made. A cross-section (a), a layout (b) and a sim- plified equivalent diagram (c) of one of the tested on-chip integrated circuits are shown in Fig. 1. The circuit comprises two identical FFOs (FFO1 - ”lo” and FF02 - ”11”; length, L = 200pm, width, W = 1.5pm), a SIS mixer array detector with a capacitance- tune-out circuit (”,,”, junction area, S = 1.3 * 1.3pm2), three impedance matching transformers (”V, ” 12” and ”13”), and a fin-line antenna (”8”). The tuning-out in- ductances for the two SIS junction (parallel dc biased [7]) array (”14”) and the transformers (” 12”, ”13”) were de- signed for optimum performance at 350 GHz.  All mi- crocircuits had three superconducting Nb layers and two double insulating S i0  layers. The integrated circuits have been fabricated with a technique developed for produc- ing SIS mixer elements and Rapid Single Flux Quantum (RSFQ) digital devices [8]. Details of the circuit design 1051-8223/95$04.00 0 1995 IEEE Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 09:05 from IEEE Xplore.  Restrictions apply. 2952 - 6 E W 10 5 I I I I I I 1 I I I I I I I I I I 0 1 2 3 Voltage (mV) 1 I I I I I 530.GHz I 0.61 0 2 4 Voltage (mV) 6 .Fig. 3. IVCs of the SIS detector (circuit ”WB”) pumped by the FFO at Fig. 2. IV-curves of the FFO at differenk magnetic fields produced by the control line current, circuit ”HF”. frequencies. and experimental set-up will be described elsewhere [9]. The high quality Nb-Al0,-Nb tunnel junctions have a critical current density, j ,  =’ 8 kA/cm2 and a Josephson penetration depth, A, = 4pm. The product of the normal state resistance and the junction area, l3,, *S, is 25 Rpm’. For the investigation of the properties of the au- tonomous FFO we used two different test circuits made on the same chip with identical FFOs (length, L = 500pm, width, W = 4.5pm) and SIS mixer array detectors (area, S = 1.1 * l .lpm2). The tuning-out inductances for the two-junction SIS array (connected in parallel both for dc and rf currents) and the two-step impedance transform- ers were different for the two test circuits. One (named WB) had a fairly low center frequency of 400 GHz and wide band frequency band (f250 GHz) while the other (named HF) had a higher center frequency but smaller bandwidth (600 GHz f l O O  GHz). 111. PROPERTIES OF THE AUTONOMOUS FFOS A typical FFO IV-curve (IVC) recorded at different values of the magnetic field, produced by a dc current, Im, in a control line, are shown in Fig. 2. The control line was integrated in the base electrode of the FFO. We used a low-value resistor (R 5 l a )  incorporated in the base electrode to ”disconnect” the superconducting films and allow for the magnetic field to enter mainly perpen- dicular to the junction. The test circuits had very similar IVCs. The FFOs were designed with one end specially taylored (length about 50 pm) in order to steepen the flux flow steps [l], [6], and both FFOs showed the asym- metric l c ( I m )  interference pattern typical for long overlap junctions with a uniform dc bias current distribution. The voltage separation between the tiny Fiske steps barely ob- servable at very low voltages in Fig. 2 is 22 pV. The peculiarities of the IVCs at voltages below 900 p V .  which are seen in all our FFOs, are fully reproducible and might be due to internal resonant phenomena. They are also seen in FFOs not equipped with transformers and detectors. Due to self-inductance the integrated thin-film resistor, which shunts the full length of the FFO, provides a shunt loss damping parameter, a x 0.2, only up to around 200 GHz. Above this frequency a x 0.01 so even with a normalized length, 1 = L/Aj x 100 we were not in the ”pure” flux flow regime until V > 9OOpV where the surface loss, /3, of the superconducting films starts to play a dominant role. The junction quality factor Qj is strongly frequency dependent: Qj  F l /{(fj /fp~)2 /3 + a}, where fj  = V~,/@O is the Josephson frequency in the bias point and fPo is the maximum plasma frequency. = h/(2e) is the magnetic flux quantum. Numerous experiments in which FFO power was de- tected by different SIS elements coupled via other trans- formers have demonstrated that the FFO itself can oscil- late in a very wide frequency range. The IVCs of the SIS detector (circuit WB) pumped by the FFO at different frequencies are shown in Fig. 3. One can see that there is enough power to pump the SIS mixer two-junction paral- lel array in a wide frequency range from 260 GHz up to 665 GHz. The highest detectable frequency was 850 GHz. The dependence of the current of the first Shapiro step on the FFO frequency is shown in Fig. 4 for circuit HF. The SIS detector in this case was shunted by a 10R resistor in order to ease the detection of the Shapiro steps. The FFO power was kept constant by adjusting the bias so that the IV-product remained the same. Notice the Riedel peak at 635 GHz corresponding to a gap voltage of 2.65 pV. Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 09:05 from IEEE Xplore.  Restrictions apply. 2953 500 600 700 800 FFO frequency (GHz) Fig. 4. Amplitude of the first Shapiro step of the SIS detector as a function of the FFO frequency (constant FFO power, test circuit HF) . The microwave power at a given frequency may be ad- justed by simultaneously changing the dc bias current and the control current keeping the FFO voltage constant. We have measured the microwave power received by the SIS detector by recording its critical current and the varia- tion of the Shapiro steps. At 440 GHz it was possible to suppress the critical current ( I c  = 1.3 mA, R, = 2.2Q) to zero and measure the maximum amplitude of the first Shapiro step 11 = 0.55 mA. It corresponds to an ampli- tude of the microwave current in the detector of I,,, x 1.8 mA and a delivered power of about 5 pW. Iv.. MIXING TWO AUTONOMOUS FFOS In the first experiments the signals from two free- running autonomous FFOs were mixed in the small SIS array detector. The frequencies of the FFOs were individ- ually tuned by adjusting the dc bias current and the ap- plied dc magnetic field so that their difference frequency was within the band of the intermediate frequency (IF) amplifier connected to the SIS mixer array. The room temperature IF amplifier (center frequency,  IF = 1.5 GHz, bandwidth, ABIF = 600 MHz) had an effective noise temperature of 100 K referred to the SIS junction array. With this set-up we have detected the mixed signal up to a FFO frequency of 450 GHz. The integral (half power, full width) linewidth of the two FFOs was as low as 750 kHz at 280 GHz, see Fig. 5. The results confirm the rather similar experiments done by Zhang et al. [6]. The inherent problem with this kind of measurements is that one may find an unrealistic small integral linewidth because most external disturbances are common-mode to the two FFOs placed on the same chip. FL k S P A N  2 0 M H z z  VAVG 1 0 0  -----I-- -- i \i I I 1 I I I I Frequency (2 MHzldiv) Fig. 5. Integral linewidth of two autonomous FFOs at 280 GHz (see text) v. FFO AS HARMONIC MULTIPLIER In order to measure the linewidth of a single FFO at dif- ferent frequencies we used a scheme in which a signal from an external frequency-locked room temperature Gunn os- cillator (frequency: fc = 56 - 74 GHz, linewidth: A f G  < 80 kHz) was applied to FFOl via the fin-line antenna (”8” in Fig. 1) and the threestep Chebyshev stepped stripline transformer (”9” in Fig. 1) both designed for 70 GHz cen- ter frequency. As a result rf-induced Shapiro steps with ”usual” voltage spacing, AV = f @ o ,  could be generated in the IVC of FFOl. . The applied frequency, however, is much lower than the maximum plasma frequency of FFO1, fPo > 200 GHz and the fundamental microwave signal cannot propagate inside the FFOl tunnel junction. Nevertheless, from the rf-induced steps in the IVC of the SIS array at voltages corresponding to higher harmonics of the applied signal it is inferred that higher harmonic signals (generated by the non-linearity of the FFO1) reach the SIS detector. For small levels of applied power. the third harmonic signal (frequency 3* fG x 220 GHz, which roughly coincides with half of the design frequency for the transformer (” 12”) and the SIS mixer tuning-out inductance) dominantly affects the detector. At higher power levels (where the critical current of the FFOl and thus the plasma frequency is suppressed) zero-crossing Shapiro steps appear and also a signal at fG reaches the detector. The IF frequency  IF resulting from the mixing of the n’th harmonic of the Gunn oscillator with the FF02 sig- nal (frequency ~ F F O ~ )  obeys f ~ ~ 0 2  = n * f~ f  IF so it is concluded that the FFOl has successfully been used as a harmonic multiplier [2]. The experimental results Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 09:05 from IEEE Xplore.  Restrictions apply. 2954 Differential resistance (a) Fig. 6. Linewidth of FF02 versus the dynamic resistance for four runs recorded with the same sample on different days. Solid line and dot-and-dashline - Eqn. (1) with effective temperature, Tee = 32 K and 4.2 K, respectively; dashed line - Eqn. (2) with the amplitude of the low frequency noise current, I f  = 0.17pA. showing that the FFO can be phase-locked by injection will be given elsewhere [9]. It was possible to realize har- monic mixing at n = 4, 5, 6 and 7 with appropriate Gunn oscillator and FF02 frequencies. VI. LINEWIDTH OF A SINGLE FFO The observed dependence of the linewidth of the mixed signal, A f ,  on the differential resistance, R d ,  of the FF02 is shown in Fig. 6 .  The corresponding IVC of the FF02 is depicted in the inset. The frequency of the FF02 was tuned around 350 GHz and we used the 6th harmonic of the frequency-locked Gunn oscillator. The contribu- tion from the Gunn oscillator linewidth (80 kHz * 6 M 0.5 MHz) can be neglected here and, accordingly, the mea- sured linewidth of the mixed signal can be attributed to FF02 alone. The R d  values used in Fig. 6 were derived from the slope of the dc IVC in the bias point. One can see from Fig. 6 that the FF02 linewidth changes proportional to R d 2  above 5 MHz. Below this value A f versus approaches a linear dependence on R d .  Presently, no theory exists for the linewidth of the ra- diation emitted from a FFO. Both theoretical models for the single Josephson junction [lo] and the low-damping soliton oscillator [ll] give the same dependence of the linewidth on the differential resistance: A f oc R d 2  assum- ing a wideband (fnoise 5 Af) Nyquist noise spectrum. The linewidth of the high frequency oscillations in a short Josephson tunnel junction in the limit of small junction current fluctuations is given by [lo] spectrum with effective temperature Teff. Vdc and Idc is the dc voltage and current in the bias point. The best fit (solid line in Fig. 6) to Eqn. (1) using the experimental values for R d ,  Vdc and Idc is obtained for Teff = 32 K, which is approx. eight times the physical temperature, T= 4.2 K (dot-and-dash line in Fig. 6. It should be noted that in some experiments with the same sample (for apparently the same experimental conditions) the Af 0; R d 2  dependence is still observed but a Teff as Iarge as 90 K (0 in Fig. 5 )  was needed to obtain the best fit. Low frequency noise with a cut-off frequency much smaller than the width of the spectral line (0 % fnoise << Af) e. g. from external disturbances (bias supplies. tem- perature fluctuations, hum, l/f noise, etc. ) may be di- rectly converted and give rise to excess noise. A low fre- quency current noise with amplitude, 11, will increase the linewidth proportional to R d :  The corresponding dependence for I j  = 0.17 p A  is shown in Fig. 6 by the dashed line fitting the experimental results well for Af below 5 MHz. REFERENCES T. Nagatsuma, K. Enpuku, F. Irie, and K. Yoshida, J. Appl. Phys. 54 3302 (1983); see also Part 11-IV in: J. Appl. Phys. 56 3284 (1984); J. Appl. Phys. 58 441 (1985); J. Appl. Phys. 63 1130 (1988). A. V. Ustinov, J. Mygind, and V. A. Oboznov, J. Appl. Phys. 72 1203 (1992). A. V. Ustinov, T. Doderer, R. P. Huebener, J. Mygind, V. A. Oboznov, and N. F. Pedersen, IEEE Trans. on Appl. Su- percond. 3 2287 (1993). V. P. Koshelets, A. V. Shchukin, S. V. Shitov, and L. V. Fil- ippenko, IEEE Trans. on Appl. Supercond. 3 2524 (1993). V. P. Koshelets, A. V. Shchukin, S. V. Shitov, L. V. Filip- penko, G. M. Fisher, and J. Mygind, Physica B 194-196 81 (1994). Y. M. Zhang, D. W i d e r ,  and T. Claeson, Appl. Phys. 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Properties of autonomous and injection locked flux flow oscillators

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