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QUASIOPTICAL TERAHERTZ

    SPECTROMETER BASED ON A JOSEPHSON

    OSCILLATOR AND A COLD ELECTRON

    NANOBOLOMETER

     1 2,32M. Tarasov, L. Kuzmin, E.Stepantsov, A.Kidiyarova-Shevchenko 1Institute of Radio Engineering and Electronics RAS, Moscow 125009, Russia 2Chalmers University of Technology, Göteborg SE41296, Sweden 3Institute of Crystallograph RASy, Moscow 117333, Russia

    Abstract: We have developed a low temperature transmission spectrometer operating in

    a wide range of frequencies from 100 GHz to 1.7 THz. The spectrometer has

    utilized unique properties of high-Tc superconducting Josephson junctions and

    wideband response of sensitive Cold-Electron Bolometers (CEB). The voltage

    response of the CEB integrated with log-periodic and double-dipole antennas,

    has been measured using an oscillator consisting of high-Tc Josephson

    junction integrated on separate substrate with a log-periodic antenna.

    Superconducting Josephson junctions with high characteristic voltages (IR cnlarger than 4 mV at 4.2 K) are fabricated by depositing YBaCuO on miscut 237-xsapphire bi-crystal substrates, where the tilting axis is along the grain boundary.

    A cold electron bolometer of superconductor-insulator-normal metal-insulator-

    superconductor (SINIS) structure was 200 nm wide, 10 µm long, and

    terminating tunnel junctions were 200x300 nm area. The response of the

    bolometer with a double dipole antenna has resonance shape with maximum

    corresponding to the designed central frequency of 300 GHz. A voltage .8response of the bolometer up to 410 V/W corresponds to noise equivalent .-171/2power of the bolometer of 1.210 W/Hz. Our measurements demonstrate

    that Josephson junction is overheated by transport current up to 3 K at 1 mV

    bias when it is placed on millikelvin stage. A high-Tc Josephson junction

    operated at temperatures below 2 K shows advantages of high IR product that cnenhances the oscillation frequency to over 2 THz. The resolution of the

    spectrometer is determined by the linewidth of Josephson oscillations and for

    this temperature is below 1 GHz. Further development of such a device can be

    using a carbon nanotube.

    Key words: terahertz spectrometers, Josephson oscillators, cold electron bolometers

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    and a cold electron nanobolometer

1. CRYOGENIC DETECTORS

    It has become increasingly evident in the last few years that superconducting devices will play a major role in radiation detection and characterization of electromagnetic spectrum in terahertz frequency range. The critical difference between detection at terahertz frequencies and detection at shorter wavelengths lies in the low photon energies (1-10 meV). The ambient background thermal noise almost always dominates narrow-band signals requiring cryogenic cooling. The use of superconducting sensors has been further promoted through the recent development of ultra-low temperature coolers which no longer need liquid cryogens. Superconducting devices offer the prospect of fundamental thermodynamic or quantum limited sensitivity, spectral sensitive detection with ability to measure photons individually, ability to manufacture large arrays with modern thin-film technology. Ultimate NEP of a bolometer (see [1]) is determined by fundamental thermodynamics and is essentially the 22same for all types NEP=4kTG in which k is Boltsman constant, T bb

    is electron temperature, G is thermal conductance. The main limitation for practical detector NEP is determined by a background 2power load P that gives NEP=2PkT [2]. At present the state-of-00be-171/2the-art practical bolometers demonstrate NEP below 10 W/Hz at

    0.1 K. Main generic types of cold direct detectors are following: Transition Edge Sensor (TES) is the most acknowledged type of superconducting bolometer. It consists of a thin superconducting film that can change its resistance under the incoming radiation power. Depending on the frequency range, such a bolometer can be integrated with THz band planar antenna, or attached to absorber film that interacts with the optical or X-ray radiation. For X-ray detectors at .-181/2100 mK one of the best NEP=310 W/Hz was demonstrated in [3].

    The dynamic range of TES detectors is strictly limited by dc heating power applied before real operation. Any attempt to increase the dynamic range lead to additional heating with unavoidable degradation of the sensitivity. Practical TES sensors sacrifice sensitivity to avoid saturation.

    Superconducting Tunnel Junctions (STJ) of the Superconductor-Insulator-Superconductor (SIS) type, as well as Superconductor-Insulator-Normal metal (SIN) type can be used from microwave to X-

quasioptical terahertz spectrometer based on a josephson oscillator 3

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    ray wavebands. They are currently being developed as photon counting detectors. STJ converts the incident radiation energy into a population of excited charges whose number is proportional to the deposited energy and to the inverse of the superconducting gap. The superconducting electrode in which this conversion takes place serves as absorber. By measuring the tunnel current it is possible to estimate the incoming energy and frequency [4]. Due to the leakage current -16and quantum efficiency about unity, the best NEP is about 10 1/2W/Hz.

    Superconducting Hot Electron Bolometer (SHEB) is mainly used in HEB mixers as a relatively fast (up to 10 GHz) power meter of interfered signal and LO waves, and in general it is the same type of power detector, because at signal and LO frequencies it is too slow and can’t multiply these components. It has a very low thermal capacitance and a large thermal conductance, and in this way it is optimized for speed, but not for sensitivity. This type of sensor can be optimized for direct detection, a so-called Hot Electron Direct -20Detector (HEDD). The theoretical estimations of NEP below 10 1/2W/Hz [5] seems to be very optimistic. Taking into account the background power load, HEDD NEP should be limited at the same -181/2thermodynamic level of above 10 W/Hz.

    Normal metal Hot Electron Bolometer with Andreev mirrors (ANHEB) was proposed by M.Nahum and P.L.Richards [6] and consists of a thin normal-metal strip between superconducting electrodes. Low electron-phonon interaction at low temperatures together with Andreev reflection at the boundary of a normal metal and a superconductor prevent heat leakage form hot electrons to phonons and to the electrodes. A superconductor-insulator-normal metal (SIN) junction attached to the normal metal strip is used for temperature .-181/2sensing. The best NEP=510 W/Hz was achieved at 100 mK [7].

    SINIS normal metal cold electron bolometer (CCNHEB or CEB) was proposed in [8] and experimentally demonstrated in [9]. As in all previous cases, the responsivity and noise equivalent power (NEP) of the bolometer are mainly determined by its electron temperature. To improve CCNHEB performance it was suggested using direct electron cooling of the absorber by a superconductor-insulator-normal metal (SIN) tunnel junction [10]. The effect of electron cooling was demonstrated in [11]. The CEB is essentially a nanorefrigerator that

quasioptical terahertz spectrometer based on a josephson oscillator 4

    and a cold electron nanobolometer

    cools the electrons within a thin metal film by extracting the hottest electrons through SIN junctions. This effect is similar to that used in a thermoelectric cooler (Peltier effect). In contrast to TES, an unavoidable dc heating for electrothermal feedback is replaced by a deep electron cooling, removing all incoming power from the absorber to the next stage. Thereby the electron temperature is maintained at the minimum level below the phonon temperature independently on the relatively high power load. The refrigeration effect allows this detector to operate with high sensitivity under high power load. The response time is determined by the tunneling time of electrons which can be very fast ((10ns).

    NISIN hot electron bolometer [12] is completely complementary to SINIS bolometer and clarify difference between hot electron and cold electron bolometers. In NISIN case a photon assisted tunneling through the barrier heats the middle S electrode. Hot electrons are injected into a superconductor, they reduce the energy gap, which in turn increase the current through the junction.

    Kinetic Inductance Bolometer (KID) is based on the fast change of the kinetic inductance in a superconducting strip when a pair breaking process reduces the superfluid density [13]. The basic principle of operation of KID is to measure the resonant frequency of a thin-film superconducting resonator, operating at about 5 GHz. The frequency shift is detected by monitoring the transmission or reflection phase, and this shift is proportional to the energy of the absorbed photon. According to [14] the NEP is determined by the quasiparticle -19generation-recombination noise and at T~1K it can be as small as 10 1/2W/Hz, again without accounting for the background power load. Among these six generic superconducting bolometers the most are operated at electron temperature that is equal or above the bath or phonon temperature, and only CEB works at reduced electron temperature. Moreover, the electron cooling allows extracting the heating power, which leads to increase of the saturation level. As a result the effect of the background power load is not so severe as for the rest of bolometer types.

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2. SAMPLES LAYOUT AND FABRICATION

    A general view on cold electron bolometer with capacitive coupling (CCNHEB) chip is presented in Fig. 1. One can see in the center a broadband log-periodic antenna for frequency range 0.1-2 THz, and double-dipole antennas for 300 and 600 GHz to the left and to the right from the center. Besides above and below the central antenna there are two structures with additional SIN junctions for studies of electron cooling in SINIS structures. The first step of sample fabrication was thermal evaporation of 60 nm Au for fabrication of the normal metal traps and contact pads. The pattern for the traps and the pads were formed using photolithography. The next step was the fabrication of the tunnel junctions and the absorber. The structures were patterned by e-beam lithography and the metals were thermally evaporated using the shadow evaporation technique. The Al (superconductor) was evaporated at an angle of about 60? up to a -1thickness of 65 nm and oxidized at a pressure of 10 mbar for 2

    minutes. A Cr/Cu (1:1) absorber of a total thickness of 75 nm was then evaporated directly perpendicular to the substrate. The cooling junctions have a normal state resistance R equal to 0.86 k, while N

    the two inner junctions have R equal to 5.3 k. The inner junctions N

    have a simple cross geometry, where a section of the normal metal absorber overlaps the thin Al electrodes. The area of overlap, which makes to the area of each of the tunnel junction, is equal to 0.2 x 0.3 2m. The structure of the outer junctions is such that the ends of the normal metal absorber overlap with a corner of each of the Al electrodes, which have a much larger area compared to the middle Al 2electrode. The area of each of these junctions is 0.55 x 0.82 m. The

    purpose of the larger area Al electrode is to give more space for quasiparticle diffusion compared to the middle Al electrode with simple cross geometry. In the described structure, the two outer and inner junctions have the R equal to 0.85 k and 5.4 k, respectively. N3The volume of the absorber was 0.18 m.

    A bias cooling current is applied through the outer junctions and the absorber. These tunnel junctions act as the cooling junctions, and therefore serve to decrease the electron temperature of the absorber. To determine the electron temperature, the voltage across the inner junctions is measured. A small current bias is applied to these junctions. The bias has to be optimal to obtain the maximum linear

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    voltage response on temperature, and yet not too large so as to disturb the cooling process in the absorber.

    High critical temperature Josephson junctions on tilted bicrystal sapphire substrates were fabricated in YBaCuO epitaxial films with c-ooaxis inclined in <100> direction by angle 14+14. Films 250 nm

    thick were deposited by pulsed laser ablation on tilted sapphire bicrystal substrates covered by a CeO buffer layer. The critical 2

    temperature of the film was T=89 K and T=1.5 K. Bicrystal cc

    Josephson junctions of width from 1.5 to 6 m demonstrated a

    characteristic voltage IR of over 4 mV at a temperature of 4.2 K. cn

    This makes them promising candidates as oscillators for Terahertz frequency band applications.

    Figure 1. View of the central part of CEB chip

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3. POWER AND TEMPERATURE RESPONSES OF THE

    BOLOMETER

    We measured the temperature response of the bolometers at temperature down to 260 mK. The dc response was measured at upper and lower structures with four SIN junctions. Two external junctions were used as thermometers and two internal as heaters. The highest measured value of voltage response to temperature variations is over 1.6 mV/K and the largest current response about 37 nA/K for a 10 k junction and 55 nA/K for a 6 k junction.

    It was possible to apply a dc power to the central pair of junctions and measure the response of the outer pair of SIN junctions for these samples with four SIN junctions. We observed the largest voltage response of 400 V/W for a 70 k junction and 550 A/W for a 10 k

    junction. The obtained values of current and voltage responses can be converted to the natural figure of merit for the sensitivity of the bolometer in terms of a Noise Equivalent Power (NEP).

    NEP=I/S or NEP=V/S ninv

    in which I is the current noise, V is the voltage noise, S=dI/dP is the nni

    current response, S=dV/dP is the voltage response of the bolometer. v

    Taking the voltage noise of a room-temperature preamplifier about 3 1/2nV/Hz one can obtain the technical TNEP value

    .-171/2TNEP=1.2510 W/Hz

    Using measured values of the temperature response and the power response one can also obtain the thermal conductivity of the bolometer.

    PV/T;11G0.810W/KVTV/P

    Now we can calculate the thermodynamic NEP arising from the 22electron-phonon interaction NEP=4kTG in which thermal ep4-11conductivity G=5?,T=10 W/K, is the absorber volume. This .-18brings a thermodynamical noise equivalent power NEP=1.410 TD1/2W/Hz , and if we compare with the thermal conductivity in the .-181/2voltage bias mode it corresponds to a NEP=1.310 W/Hz. V

quasioptical terahertz spectrometer based on a josephson oscillator 8

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3. IRRADIATION OF BOLOMETER BY A JOSEPHSON JUNCTION

    To increase the output microwave power from the Josephson junction and increase the oscillation frequency it is necessary to increase the critical current of the Josephson junction. Placing the Josephson junction on the He4 stage prevents the sample from overheating by the relatively high power absorbed by the Josephson junction. As the example if we take a junction with 10 k normal resistance and

    oscillation frequency 300 GHz, it brings the absorbed power over 0.2 W. At 1 THz it is already 2.5 W.

    The layout of the Josephson sample was with similar log-periodic antennas, and the critical current was over 500 A at 2 K. As a result

    the IR product exceeds 5 mV for non-hysteretic junctions and such cn

    oscillators can in principle operate at frequencies over 2.5 THz. Experimental curves in Fig.2 measured by bolometers integrated with double-dipole and log-periodic antennas, and reveals that there is clear maximum at the design frequency 300 GHz for DDA and smooth spectrum for LPA. The response at higher bias voltages for Josephson oscillator is presented in Fig. 3. The highest maximum corresponds to an oscillation frequency of 1.75 THz.

    8

    6

    V

    4

    Response,

    2

    00.00.20.40.60.81.01.21.41.6

    Frequency, THz

     Figure 2. Response measured by double-dipole and log-periodic antennas for the

    same radiation source.

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    1.2

    1.0

    V0.8

    0.6

    Response,

    0.4

    0.2

    0.00.51.01.52.0

    Frequency, THz

    Figure 3. Response measured for high bias voltages of the Josephson junction. Last

    maximum corresponds to oscillation frequency 1.75 THz.

    For measurements at frequencies below 600 GHz we use also a low Tc Josephson oscillator with resistively shunted Nb SIS tunnel junction. It is integrated with the log-periodic antenna designed for 0.2-2 THz. Samples were fabricated by HYPRES 3 µm Nb process (for details see www.hypres.com). The linewidth of Josephson oscillations was measured irradiating such junction by a backward wave oscillator and monitoring the detector response by a lock-in amplifier. The selective detector response in Fig. 4 shows the voltage distance between bipolar maxima below 1 μV that corresponds to the Josephson oscillations linewidth below 0.5 GHz.

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    300

    200

     RESP89

    100

    0

    -100HYPRES A21 J2Response, nVmeasured 30.08.2004T=1.8 K-200BWOV=1V-300

    425426427428429430431432433434435

    Voltage, V

     Figure 4. Selective detector response of Nb shunted tunnel junction at 215 GHz with

    voltage distance between maxima about 1 μV that corresponds to the linewidth 0.5

    GHz.

5 CONCLUSION

    We demonstrated the first experimental response of a normal metal cold electron bolometer at frequencies up to 1.7 THz. Noise .-171/2equivalent power of the bolometer is 1.310 W/Hz. We use

    electrically tunable high critical temperature Josephson quasioptical oscillator as a source of radiation in the range 0.2-2 THz and shunted Nb SIS Josephson junction for frequencies below 600 GHz. Combination of a Terahertz-band Josephson junction and a high-sensitive hot electron bolometer brings a possibility to develop a quasioptical cryogenic transmission spectrometer with a resolution below 1 GHz. Such cryogenic spectrometer can be used for low-temperature spectral evaluation of biological and chemical samples. Cold electron bolometers can be used for remote atmosphere monitoring for pollution detection, etc.

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