Terahertz Superconducting Hot Electron Bolometer ... - IEEE Xplore

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 17, NO. 2, JUNE 2007

Terahertz Superconducting Hot Electron Bolometer Heterodyne Receivers J. R. Gao, M. Hajenius, Z. Q. Yang, J. J. A. Baselmans, P. Khosropanah, R. Barends, and T. M. Klapwijk

Abstract—We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and—structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.

Fig. 1. Schematic diagram of the heterodyne detection technique.

Index Terms—Heterodyne receiver, superconducting hot electron bolometer mixer, terahertz, THz quantum cascade laser.

I. INTRODUCTION

H

ETERODYNE receivers are the only detection systems that can offer high spectral resolution ( where is the frequency) combined with high sensitivity. A heterodyne receiver (see Fig. 1) mixes an astronomical (or atmospheric) signal with a local-oscillator (LO) signal in a non-linear or a power-law detector (mixer) to produce a signal at an intermediate frequency (IF), which is the beat between the LO frequency and the signal frequency. The IF signal at several GHz, conserving incredible spectral detail is suitable for further amplification and spectral analysis. Nowadays, heterodyne receivers combine an electronically tunable solid-state LO source with either a SIS mixer or a HEB mixer. In the past decade a great deal of progress has been made to achieve low noise hot electron bolometer (HEB) mixers [1]–[4], which essentially are a superconducting thin-film strip contacted with two normal metal antenna pads. HEBs become the detector of choice for frequencies above 1.5 THz since SIS Manuscript received August 28, 2006. This work was supported in part by EU RadioNet and in part by INTAS. J. R. Gao and M. Hajenius are with SRON Netherlands Institute for Space Research, Utrecht/Groningen, the Netherlands and are also with Kavli Institute of NanoScience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ, Delft, the Netherlands (e-mail: [email protected]. nl). Z. Q. Yang, J. J. A. Baselmans, and P. Khosropanah, are with SRON Netherlands Institute for Space Research, Utrecht/Groningen, the Netherlands. R. Barends and T. M. Klapwijk are with Kavli Institute of NanoScience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ, Delft, the Netherlands. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2007.898066

mixers work only up to this frequency due to their mixing principle and due to the superconduting gap of available materials. The physics of HEBs is based on “hot electrons”, meaning electrons having an elevated temperature with respect to the phonons in the film. Since in a superconductor the resistance is a steep function of temperature (near ), the HEB is a sensitive transducer from temperature to resistance. It was demonstrated by Gershenson et al. [1] that the response and decay time of electrons in thin NbN films could be very fast, suggesting the possibility of fast bolometric process. The mixing signal at IF is the result of the fact that the bolometer is a power-law detector and the electron temperature can follow the beat-frequency. The sensitivity has always been the first priority in the development of HEB’s simply because it sets the ultimate sensitivity of a heterodyne instrument and determines the observation time in practice. The noise performance of phonon cooled HEB mixer has been reported from 0.6 to 5.3 THz. The most sensiline, in which is Planck’s tivity data lie above the constant, and Boltzman’s constant. A crucial point in the HEB development is the IF bandwidth (actually roll-off), which needs to be sufficiently large to allow sampling a given spectrum with only one LO tuning step. Phonon-cooled NbN HEBs on Si substrate have a gain bandwidth of about 3 GHz [3], [4]. It is known that the bandwidth depends strongly on the electron-phonon interaction, the film thickness, and the substrate. The improvement can only be achieved by optimizing the film/substrate. On the other hand, the growth of such thin film with a high critical temperature and good reproducibility is extremely challenging. At the moment, the best films are produced by the group at Moscow State Pedagogical University, Russia. The development of short niobium diffusion-cooled HEBs, considered as another route to improve the bandwidth, has

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GAO et al.: TERAHERTZ SUPERCONDUCTING HOT ELECTRON BOLOMETER HETERODYNE RECEIVERS

not led to useful devices because of poor reliability and sensitivity [5]. Another practical issue is the requirement of local oscillator power to operate a HEB mixer at THz frequencies. At such high frequencies, one can rely on optically pumped FIR gas lasers in the lab condition, which usually can deliver enough power at several strong emission lines to pump HEB mixers. Such lasers are in general massive, bulky, and power-hungry and thus are inappropriate for instruments in space. Solid-state LOs are more attractive. Thanks to the progress on solid-state LOs based on multiplier chains for the ESA Herschel Space Telescope, it is output power up to about now possible to reach roughly 10 1.9 THz [6]. Because of limited power at THz frequencies, HEB mixers requiring lower LO power will continue to be preferable. Success of THz QCLs [7] shines new light on this issue because of the mW output at several THz. However, due to their high power consumption and low operating temperature, significant cooling capacity is needed. This, in combination with the QCL’s poor beam pattern [8] makes that the application of QCL as LO in a practical instrument still needs much improvement. NbN HEB mixers have been operated at ground-based spectra and telescopes, obtaining 1037 GHz CO demonstrating true heterodyne detection [9] and are forecasted to be employed in the High Elevation Antarctic Terahertz Telescope (HEAT) on Dome A, Antarctica [10]. Future astronomical and atmospheric space missions demand better performance of THz heterodyne detectors. Those are NASA’s astronomical mission SAFIR [11], and a potential European mission ESPRIT (the Exploratory Submm Space Radio-Interferometric Telescope, formed by a free-flying, six-element, far-infrared imaging interferometer using heterodyne detection covering a frequency range between 0.5 to 6 THz [12]. In this paper we will review the progress on NbN HEB mixers achieved in our laboratories. II. HEB DEVICES A. NbN Thin Films Our HEBs are based on thin NbN films on high resistive Si substrates, produced by the Moscow group. These films, optimized for phonon cooling, have a thickness around 5.5 nm, a (at 300 K) and a of 9.5 K. sheet resistance of A recent study of such films using high-resolution transmission electron microscopy (HRTEM) shows this thickness and also indicates that the film has a polycrystalline structure [13]. Until now it has always been thought to be 3.5 nm, which was based on the sputtering rate calibrated with thick films. Although reducing the film thickness further seems to favor a large IF bandwidth because of the shorter phonon escape to the substrate, the practical thickness is limited by the quality and reliability of the film, which degrade when it gets thinner. B. Contact Structures The bolometer is an NbN strip with a finite size. The choice of its size is made based on the DC resistance, which should match to the RF impedance of the antenna, and LO power requirement, for which a small area (volume) means lower LO power needed. To allow coupling an RF signal into the

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Fig. 2. A cross-sectional view of the bolometer and contact pads region in our NbN HEB mixers. The Si substrate used is not shown here.

Fig. 3. Normal state resistance, measured at 16 K, of a number of NbN HEBlike devices of various bridge lengths, but with a fixed width of 2 m (data points), together with a linear fit to the data. An Ar sputter cleaning of 16 sec was applied before depositing the contact. Evidently, the contact resistances are negligible.

bolometer and biasing with a DC current, additional metallic contacts are needed. Fig. 2 shows a cross-sectional view of the bolometer with contact structures [14]. We improve the existing NbN HEB mixers by cleaning the NbN surface before deposition of the contact pads and by adding an additional superconductor interlayer of 10 nm thick NbTiN between the NbN film and the Au pads. The cleaning was done using an etching to remove possible contact resistance. in-situ To confirm absence of the contact resistance, we studied the resistance (just above the ) as a function of bolometer length. As shown in Fig. 3, the contact resistances were negligible. The around 8 K, is additional superconductor, NbTiN with a to keep the contact structure superconducting under operating conditions. Without this layer, due to the proximity effect of normal metal and superconductor structure, the NbN film will lose its superconductivity. In contrast, for the earlier conventional NbN HEB mixers, the Au pads are directly deposited on the top of an NbN film. Due to the possible thin oxide layer or surface contamination after exposing a film to normal air, the interface between the film and the Au pads is poor, resulting in an additional contact resistance. Furthermore, it is difficult to realize reproducible contact structures. Applying the new contact structure leads to an improved performance in spiral antenna coupled mixers [4]. Intuitively one can understand the improvement in the following way, in which the contact pads are superconducting for DC current, and also almost lossless for RF current due to the presence of the Au.

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In the conventional HEBs, energy dissipation also takes place at the contacts. In our case, HEB mixers are operated in the hotspot mixing mode [16], [17] or a more realistic mixing mode based on the local resistivity [15]. This approach brings also several other advantages: good reproducibility and predictable HEB resistance, which is important for the impedance matching to the antenna. Among them, the greatest advantage of this new contact approach is the manufacture of small size HEB mixers (1 m wide HEB) without suffering from the contact resistance, which becomes a serious challenge for the conventional fabrication process. III. PLANAR ANTENNAS We use two types of antenna, spiral and twin slot, to couple a RF signal from free space to the superconducting bridge. An alternative coupling scheme is the waveguide which generally has a better beam pattern than that of a coupling scheme based on planar antennas. However, due to fabrication difficulties, a waveguide is usually applied at frequencies around 1 THz or below. Spiral antennas are extremely useful for evaluating HEB mixers for the laboratory tests because of the broad RF bandwidth as a result of non-resonating frequency response. However, such antennas have a circular polarization, so they are less favorable for actual applications in a telescope. In contrast, twin-slot antennas are resonant ones with a linear polarization and an acceptable beam pattern. Therefore they are more desirable for real applications. Because of the resonant type of antenna, to reach the maximum RF coupling at a designed frequency the impedance matching between antenna and HEB is challenging. This is partly due to the fact that the theoretical model has not been fully developed for THz frequencies and partly due to the fact that the bridge impedance should be under good control during the fabrication. Antenna structure is made of highly conductive normal metal layer such as Au, but not superconducting layers. This is because at THz frequencies of interest, a normal metal layer shows much lower surface impedance than superconductors, like Nb or NbTiN, when their gap frequency is exceeded. A. Twin Slot Antenna Mixer The twin-slot mixer used is illustrated by the SEM micrograph in Fig. 4. The bridge has a width of 1.5 m and a length of 0.2 m, which results in a normal state DC resistance of 130 at low temperature above . In our case it is assumed to be same as the RF impedance. The antenna is designed for the center frequency of 1.6 THz and has the following dimensions: with m the free space the slot length L is 0.30 wavelength. The slot separation S is 0.17 , the slot width W is 0.07 L. Note that all these design values are only valid for a Si substrate. The CPW transmission line used to connect the two slots to the HEB has a central line width of 2.8 m and a gap of 1.4 m, yielding a characteristic impedance of 51 [18]. The RF filter structure consists of three sections each consisting of one high-(70 ) and one low-impedance (26 ) segment, all of which are quarter long. Here is the effective wavelength. Applying the same approach in [19], we calculate

Fig. 4. SEM micrograph of a twin slot antenna coupled NbN HEB mixer. The bright area is covered with metal Au layer, while the dark area is the Si substrate. Between the two slots, there is a CPW transmission line that connects the slots to the superconducting bridge. The HEB is located in the middle of the CPW line. Note that the bridge is covered by the resist (dark area). The RF filter structure is shown in the right side of the micrograph.

Fig. 5. SEM micrograph of a self-complementary spiral antenna coupled NbN HEB mixer on Si substrate. The dark part in the center is a remaining e-beam resist, which is used to define the width of the HEB.

a real impedance of 44 for the twin-slot antenna, while a reactance of only . The CPW transmission line transforms the antenna impedance to the feed impedance of 116 as the real part and 9 as the imaginary. We find a power coupling efficiency of 90% for this mixer if we take the main beam efficiency into account [19], but nearly 100% if we ignore the effect of main beam efficiency. B. Spiral Antenna Mixer Here we apply a self-complementary spiral antenna, which is similar to what used in our previous work [4]. The detailed antenna structure is illustrated by the SEM micrograph in Fig. 5. The bolometer is in the center of the antenna and has a width of 2 m and a length of 0.2 m. The normal state resistance is 96 . The antenna feed impedance has been simulated using HFSS for the real part of the impedance and [20]. We find 88.6 for the reactance around 1.6 THz, which is consisonly tent with what calculated using a textbook analytical expression.


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Having known the impedances of the antenna and the HEB, we calculate the power coupling efficiency from the antenna to the HEB and find it to be nearly 100% around 1.6 THz. In the impedance simulation, we assume a zero thickness of the metal layer and neglect any resistive loss. Furthermore, we also neglect the effect of the main beam efficiency [19] in estimating the coupling efficiency. IV. HETERODYNE CHARACTERIZATION METHOD AND SETUP We measure the receiver noise temperature using a standard hot/cold load method. A blackbody source (Eccosorb) is used as the signal, which defines a hot load at 295 K and a cold load at 77 K. A directly measured parameter is the Y-factor, which is the ratio of the IF output powers of the receiver responding to the hot and cold loads. Consequently, one can derive the receiver noise temperature through a standard way provided one knows the effective temperatures of hot/cold loads. At THz frequencies, the temperatures should be calculated using the Callen and Welton definition [21]. Radiation is coupled to the antenna using a quasi-optical technique: the Si chip with the HEB is glued to the back of an elliptical, anti-reflection coated Si lens. The lens is placed in a metal mixer block, which is fixed to the 4.2 K cold plate mechanically as well as thermally. A standard LO source in our case is an optically pumped gas laser, which generates relatively strong lines at 1.63, 1.89, and 2.5 THz. In QCL-HEB experiments, we replace the gas laser by a QCL as LO. The calibration signal is combined with the LO signal via a 3.5 m thick Mylar beam splitter, which acts as a directional coupler. Both of the signals pass further into the lens through a 1.1 mm thick HDPE window and a Zitex G104 heat filter at 77 K for 1.6 THz or a metal mesh heat filter at 77 K for 2.8 THz. The IF signal, resulting from the mixing of the LO and the hot/cold load signal, is amplified using a low noise amplifier operated at 4.2 K, and is further fed to a room temperature amplifier and filtered at 1.4 GHz in a band of 80 MHz. The entire IF chain has a gain of 80 dB and a noise temperature of 4 K. V. PERFORMANCES OF NbN HEB MIXERS A. Receiver Noise Temperature at 1.6 THz Fig. 6 shows the measured receiver noise temperature of a twin slot antenna HEB mixer at 1.6 THz, together with the current-voltage (IV) curves. The bolometer is relatively small (see Section III-A for details). We measured a minimum noise temperature of 700 K at a bath temperature of 2.4 K, which is 10% lower than what was found at 4.3 K. This represents the lowest value reported for twin slot antenna mixers at this frequency. Note that this sensitivity is similar to our earlier result using a twin-slot mixer, but after annealing the device in vacuum [22]. It might be interesting also to mention that a very similar noise temperature at this frequency was measured from a spiral mixer fabricated in the same batch, suggesting that there are no fundamental differences between two types of antenna regarding to RF coupling.

(symbols, right axis) Fig. 6. Measured receiver noise temperature T versus the bias voltage of a twin slot antenna coupled NbN hot electron bolometer mixer for different 1.63 THz LO power levels at the HEB, together with the current-voltage characteristics (lines, left axis) without and with radiation.

The receiver noise temperature depends on the operating conditions, i.e. the LO power and DC bias voltage. To characterize a mixer, usually only the minimum noise temperature is mentioned together with the bias conditions. In this case, it was taken at a DC bias voltage of 0.7 mV and at a LO power of 330 nW. This optimal operating condition reflects interplay of DC and LO power inside the bridge, which bring the HEB to a resistive state where the ratio between the output noise and mixer gain of the HEB reaches its minimum. The LO power is estimated from the optimally pumped IV curve using the isothermal technique by assuming the equivalent heating effect due to LO and DC power in a HEB. B. Performance of Small Twin Slot HEB Mixers We have studied small twin slot antenna coupled HEB mixers extensively [23], [24], which have an area of m m . These devices have a similar resistance versus temperature (RT) behavior as large devices, suggesting a good contact structure. The motivation for such small HEBs is the use of solid-state LOs based on multiplier chains, which are only able to deliver a limited amount of power. Reducing the volume is an important means to decrease LO power. But the required LO power depends also on the critical temperature and the thermal cooling process in the film. Strictly speaking, one has to predict the LO requirement by means of a heat balance equation. In practice, it is known that the LO is determined by the area and critical current of the device. The and the thickness of the film. latter reflects We found that small area mixers of m m with a (at 4.2 K) can be operated with LO critical current of 50–70 power as low as 30 nW [23]. A measured receiver noise temperature is 950 K at 1.63 THz. An increase of 20% to the previous device is caused by the direct detection effect due to broadband radiation of the hot/cold loads [25], [26]. This effect becomes prominent when the RF power from the calibration load absorbed in the bridge is no longer much smaller than the “biasing” power of LO and DC. In this case, additional change in bias current introduces an error in the heterodyne sensitivity. This

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the best sensitivity around 2.8 THz ever reported in the litera, suggesting that ture. This sensitivity corresponds to the receiver noise temperature of an NbN HEB mixer may reeven when the frequency moves beyond main below 2.8 THz. D. Other Progress

Fig. 7. Measured receiver noise temperature (symbols, right axis) of a spiral antenna HEB mixer versus the bias voltage at 2.8 THz using a QCL as LO, together with the current-voltage characteristics (lines, left axis) without and with radiation. The inset shows the measured IF power versus the bias voltage corresponding to the hot /cold loads.

effect has been well understood qualitatively. However, there is no simple expression that can be applied generally. This is because the influence of this effect is in practice determined by a combination of the operating power of LO and DC, the voltage responsivity of the HEB, and the transmission and bandwidth of the radiation from hot/cold loads to the bridge. To get rid of this, as demonstrated, one can introduce a narrow bandpass filter in the signal path to reduce the hot/cold load power [22], [25], [26]. C. Receiver Noise Temperature at 2.8 THz We have reported a record sensitivity of 950 K at 2.5 THz using a spiral antenna coupled large HEB mixer. In this case, an optically pumped FIR gas laser was used as LO. Motivated by the recent success of THz quantum cascade lasers, we have performed the first successful heterodyne experiment using this novel device as LO. Fig. 7 shows IV curves of a HEB with and without radiation (300 nW power absorbed at the HEB), together with the receiver noise temperature as a function of voltage. The lowest value is 1400 K [27]. It should be said that this mixer has a large HEB size m and has a different contact structure as shown in Fig. 2, in which an additional Nb layer was used instead of a NbTiN layer. Furthermore, a slightly thicker beam splitter was used in this measurement because of difficulties to couple enough LO power to the HEB, the achieved sensitivity was excellent, but lower than what we could obtain. Nevertheless, this is a breakthrough in the THz technology because this is the first successful demonstration of an all solid-state heterodyne receiver above 2 THz, which is suitable for space-based observatories. We have also shown that the output power of the free-running QCL has excellent long-term stability, which allows recording the IF response to a hot and cold load as a function of voltage (see the inset of Fig. 7). It is usually difficult to perform the same measurement with the FIR gas laser because of its poor stability. This noise temperature data was recently improved using a different type of QCL with an improved output beam pattern in combination with a new HEB mixer with an improved design on the spiral structure [28]. We obtained a receiver noise temperature of 1050 K (at 2 K bath temperature), which represents

Apart from the extensive studies on the receiver noise temperatures, we have also investigated the IF bandwidth of small HEBs. We have observed an IF gain bandwidth of 3 GHz [24]. The frequency dependent IF impedance of a small HEB has been measured and modeled [29]. It was found that the measured impedance data could be well described by a model presented by Nebosis et al. [30]. The frequency dependences can be modeled using three time constants, governed by the electron-phonon scattering time, phonon escape time, and the electron temperature. of the normalWe have measured the Allan Variance , where ized IF output power, given by is the average squared standard deviation of each number from its mean and is the sampling period. The noise of any receiver is a combination of white (uncorrelated) noise, 1/f electronic noise, and low frequency drift. Since, to first order, only white noise can be integrated out, there is an optimum integration time, , beyond which the signal/noise known as the “Allan” time ratio no longer improves. Thus this time scale has a great impact on the integration time during an observation. We obtained of about 0.5 sec within the 80 MHz bandwidth of the IF a increases chain at the optimal bias voltage [23], [24]. This with increasing the bias voltage. Interestingly, it is independent of HEB size [31]. Our recent success in modeling the IV characteristics of a HEB by including the current-dependent intrinsic resistance in the solution of the heat-balance equations [32] suggests the importance of the vortex-antivortex dissociation in thin superconducting films, as proposed by Berezinskii–Kosterlitz–Thouless [33], which sheds new light in the understanding of HEBs. A detailed study of the resistivity of ultra thin NbN films has been made to deepen the understanding [34]. So far no HEB theory, comparable to the Tucker theory for SIS mixers, exists. This is partly due to the fact that, though the device has a simple structure and geometry, it reflects in practice a very complicated physical situation with several unknowns. Today’s HEB analysis relies still heavily on an extremely simplified model [35], which is only applicable for ideal HEBs and incapable of describing practical devices with contact structures. New models, such as the electronic hotspot [16], [17], have advanced the understanding but are unable to describe the measurements quantitatively. Good theoretical models are needed for the understanding and optimization of future HEBs and are regarded as one of the important development areas. VI. WORLDWIDE HEB RESULTS During the last 10 years, enormous improvements of HEB mixers have been achieved, that is a progress with contribution from more than 10 different research groups studying either diffusion-cooled or phonon-cooled HEB mixers. We summarize


Fig. 8. Summary of the receiver noise temperatures of phonon-cooled HEB mixers, reported by different research groups in the world.

the results from the literature in Fig. 8 and also make a comparison between our results and others. The data show a general trend of increasing receiver noise temperature with increasing RF frequency. Most noise data are above the sensitivity line of . Our spiral mixers demonstrate the best sensitivity at 1.9, 2.5, and 2.8 THz, while the twin slot antenna mixers show the lowest noise temperature at 1.6 and 1.9 THz. VII. CONCLUSION We have made a significant improvement of the performance of NbN HEB mixers as a result of a combination of new device physics insight, new fabrication process, optimal RF circuitry design, and improved measurement techniques. We have made twin slot antenna coupled HEB mixers at 1.6 THz ready for space and ground based instruments, with the highest sensitivity and with extremely low LO power requirement. By combining a spiral antenna NbN HEB mixer with a QCL as local oscillator, we demonstrated an unprecedented low receiver noise temperature at 2.8 THz. ACKNOWLEDGMENT The authors would like to thank J. N. Hovenier, A. Baryshev, J. W. Kooi, A. J. L. Adam, T. O. Klaassen, and W. Jellema for their contribution to the results described in this paper, H. Hoevers for his support, G. N. Gol’tsman and B. Voronov for providing their NbN films, Q. Hu and his group at MIT, and C. Sirtori and his group in Paris for providing their THz QCLs. REFERENCES [1] E. M. Gershenzon, G. N. Gol’tsman, I. G. Gogidze, Y. P. Gusev, A. I. Eliantev, B. S. Karasik, and A. D. Semenov, “Millimeter and submillimeter range mixer based on electron heating of superconducting films in the resistive state,” Sov. Phys. Superconductivity, vol. 3, p. 1582, 1990. [2] A. D. Semenov, H.-W. Hübers, J. Schubert, G. N. Gol’tsman, A. I. Elantiev, B. M. Voronov, and E. M. Gershenzon, “Design and performance of the lattice-cooled hot-electron terahertz mixer,” J. Appl. Phys., vol. 88, pp. 6758–6767, 2000. [3] S. Cherednichenko, P. Khosropanah, E. Kollberg, M. Kroug, and H. Merkel, “Terahertz superconducting hot-electron bolometer mixers,” Physica C, vol. 372–376, p. 407, 2002.

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