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Ekström H, Karasik B, Kollberg E, Gol'tsman G, Gershenzon E. 350 GHz NbN hot electron bolometer mixer. In: Proc. 6th Int. Symp. Space Terahertz Technol.; 1995. p. 269–83.
Abstract: Superconducting NbN hot-electron bolometer (HEB) mixer devices have been fabricated and measured at 350 GHz. The HEB is integrated with a double dipole antenna on an extended crystalline quartz hyper hemispherical substrate lens. Heterodyne measurement gave a -3 dB bandwidth, mainly determined by the electron- phonon interaction time, of about 680 and 1000 MHz for two different films with Tc = 8.5 and 11 K respectively. The measured DSB receiver noise temperature is around 3000 K at 800 MHz IF frequency. The main contribution to the output noise from the device is due to electron temperature fluctuations with the equivalent output noise temperature TFL-100 K. TH, has the same frequency dependence as the IF response. The contribution from Johnson noise is of the order of T. The RF coupling loss is estimated to be = 6 dB. The film with lower Tc, had an estimated intrinsic low-frequency conversion loss = 7 dB, while the other film had a conversion loss as high as 14 dB. The difference in intrinsic conversion loss is explained by less uniform absorption of radiation. Measurements of the small signal impedance shows a transition of the output impedance from the DC differential resistance Rd=dV/dI in the low frequency limit to the DC resistance R 0 =Uoff 0 in the bias point for frequencies above 3 GHz. We judge that the optimum shape of the IV-characteristic is more easily obtained at THz frequencies where the main restriction in performance should come from problems with the RF coupling.
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Karasik BS, Elantiev AI. Analysis of the noise performance of a hot-electron superconducting bolometer mixer. In: Proc. 6th Int. Symp. Space Terahertz Technol. ; 1995. p. 229–46.
Abstract: A theoretical analysis for the noise temperature of hot–electron superconducting mixer has been presented. Thecontributions of both Johnson noise and electron temperature fluctuations have been evaluated. A set of criteriaensuring low noise performance of the mixer has been stated and a simple analytic expression for the noisetemperature of the mixer device has been suggested. It has been shown that an improvement of the mixer sensitivitydoes not necessarily follow by a decrease of the bandwidth. An SSB noise temperature limit due to the intrinsic noisemechanisms has been estimated to be as low as 40–90 K for a mixer device made from Nb or NbN thin film.Furthermore, the conversion gain bandwidth can be as wide as is allowed by the intrinsic electron temperaturerelaxation time if an appropriate choice of the mixer resistance has been made. The intrinsic mixer noise bandwidthis of 3 GHz for Nb device and of 5 GHz for NbN device. An additional improvement of the theory has been madewhen a distinction between the impedance measured at high intermediate frequency (larger than the mixerbandwidth) and the mixer ohmic resistance has been taken into account.Recently obtained experimental data on Nb and NbNbolometer mixer devices are viewed in connection with thetheoretical predictions.The noise temperature limit has also been specified for the mixer device where an outdiffusion coolingmechanism rather than the electron–phonon energy relaxation determines the mixer bandwidth. A consideration ofthe noise performance of a bolometer mixer made from YBaCuO film utilizing a hot–electron effect has been done.
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Semenov AD, Nebosis RS, Gousev YP, Heusinger MA, Renk KF. Analysis of the nonequilibrium photoresponse of superconducting films to pulsed radiation by use of a two-temperature model. Phys Rev B. 1995;52(1):581–90.
Abstract: Photoresponse of a superconducting film in the resistive state to pulsed radiation has been studied in the framework of a model assuming that two different effective temperatures can be assigned to the quasiparticle and phonon nonequilibrium distributions. The coupled electron-phonon-substrate system is described by a system of time-dependent energy-balance differential equations for effective temperatures. An analytical solution of the system is given and calculated voltage transients are compared with experimental photoresponse signals taking into account the radiation pulse shape and the time resolution of the readout electronics. It is supposed that a resistive state (vortices, fluxons, network of intergrain junctions, hot spots, phase slip centers) provides an ultrafast connection between electron temperature changes and changes of the film resistance and thus plays a minor role in the temporal evolution of the response. In accordance with experimental observations a two-component response was revealed from simulations. The slower component corresponds to a bolometric mechanism while the fast component is connected with the relaxation of the electron temperature. Calculated photoresponse transients are presented for different ratios of the electron and phonon specific heat, radiation pulse durations and fluences, and frequency band passes of registration electronics. From the amplitude of the bolometric component we determine the radiation energy absorbed in a film. This enables us to reveal an intrinsic electron-phonon scattering time even if it is much shorter than the time resolution of readout electronics. We analyze experimental voltage transients for NbN, YBa2Cu3O7, and TlBa2Ca2Cu3O9 superconducting films and find the electron-phonon interaction times at the transition temperatures of 17, 2.5, and 1.8 ps, respectively. The values are in reasonable agreement with data of other experiments.
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Ekstrom H., Karasik B. S., Kollberg E.L., Yngvesson K.S. Conversion Gain and Noise of Niobium Superconducting Hot-Electron-Mixers. IEEE Trans. Microw. Theory Techn.. 1995;43:938–47.
Abstract: A study has been done of microwave mixing at 20 GHz using the nonlinear (power dependent) resistance of thin niobium strips in the resistive state. Our experiments give evidence that electron-heating is the main cause of the nonlinear phenomenon. Also a detailed phenomenological theory for the determination of conversion properties is presented. This theory is capable of predicting the frequency-conversion loss rather accurately for arbitrary bias by examining the I-V-characteristic. Knowing the electron temperature relaxation time, and using parameters derived from the I-V-characteristic also allows us to predict the -3-dB IF bandwidth. Experimental results are in excellent agreement with the theoretical predictions. The require ments on the mode of operation and on the film parameters for minimizing the conversion loss (and even achieving conversion gain) are discussed in some detail. Our measurements demon-strate an intrinsic conversion loss as low as 1 dB. The maximum IF frequency defined for -3-dB drop in conversion gain, is about 80 MHz. Noise measurements indicate a device output noise temperature of about 50 K and SSB mixer noise temperature below 250 K. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies.
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Hans Ekstrom, Karasik BS, Kollberg EL, Sigfrid Yngvesson. Conversion gain and noise of niobium superconducting hot–electron–mixers. IEEE Trans. Appl. Supercond.. 1995;43(4):938–47.
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Krasnosvobodtsev SI, Shabanova N P., Ekimov EV, Nozdrin VS, Pechen E V. Critical magnetic field of NbC: new data on clean superconductor films. Zh Eks TeorFiz. 1995:534–7.
Abstract: The temperature dependence of the upper critical magnetic fields of exceptionally low-defect-density films of the superconducting compound NbC has been investigated, and previously unknown parameters of this clean superconductor and its electronic characteristics have been evaluated. An electron density of states at the Fermi level equal to 1.3 states/ eV. Nb atom, a Fermi velocity equal to 2.2X lo7 cmls, a plasma frequency equal to 3.6 eV, and a coherence length to 24 nm have been obtained with an electron mean free path exceeding 40 nm. A vortex-free state existing over the entire temperature range below T, which causes a many-fold increase in the critical magnetic field of the films when the field is aligned parallel to their surface, has been discovered in very thin films of superconducting niobium carbide.
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Verevkin AI, Ptitsina NG, Chulkova GM, Gol'tsman GN, Gershenzon EM, Yngvesson KS. Electron energy relaxation in a 2D channel in AlGaAs-GaAs heterostructures under quasiequilibrium conditions at low temperatures. JETP Lett. 1995;61(7):591–5.
Abstract: The energy relaxation time of 2D electrons, Te, has been measured under quasiequilibrium conditions in AlGaAs—GaAs heterojunctions over the temperature range T= 1.5—20 K. At T> 4 K, Te depends only weakly on the temperature, while at T< 4 K 7;'(T) there is a dependence fr; lNT. A linear dependence 7: 1 (T) in the Bloch—-Grfineisen temperature region (T< 5 K) is unambiguous evidence that a piezoacoustic mechanism of an electron—phonon interaction is predominant in the inelastic scattering of electrons. The values of T6 in this temperature range agree very accurately with theoretical results reported by Karpus [Sov. Phys. Semicond. 22 (1988)]. At higher temperatures, where scat—tering by deformation acoustic phonons becomes substantial, there is a significant discrepancy between the experimental and theoretical re-sults.
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Karasik BS, Milostnaya II, Zorin MA, Elantev AI, Gol'tsman GN, Gershenzon EM. High speed current switching of homogeneous YBaCuO film between superconducting and resistive states. IEEE Trans Appl Supercond. 1995;5(2):3042–5.
Abstract: Transitions of thin structured YBaCuO films from superconducting (S) to normal (N) state and back induced by a supercritical current pulse has been studied. A subnanosecond stage in the film resistance dynamic has been observed. A more gradual (nanosecond) ramp in the time dependence of the resistance follows the fast stage. The fraction of the film resistance which is attained during the fast S-N stage rises with the current amplitude. Subnanosecond N-S switching is more pronounced for smaller amplitudes of driving current and for shorter pulses. The phenomena observed are viewed within the framework of an electron heating model. The expected switching time and repetition rate of an optimized current controlling device are estimated to be 1-2 ps and 80 GHz respectively.
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Gerecht E, Musante CF, Schuch R, Lutz CR, Jr., Yngvesson KS, et al. Hot electron detection and mixing experiments in NbN at 119 micrometer wavelength. In: Proc. 6th Int. Symp. Space Terahertz Technol.; 1995. p. 284–93.
Abstract: We have performed preliminary experiments with the goal of demonstrating a Hot Electron Bolometric (HEB) mixer for a 119 micrometer wavelength (2.5 THz). We have chosen a NbN device of size 700 x 350 micrometers. This device can easily be coupled to a laser LO source, which is advantageous for performing a prototype experiment. The relatively large size of the device means that the LO power required is in the mW range; this power can be easily obtained from a THz laser source. We have measured the amount of laser power actually absorbed in the device, and from this have estimated the best optical coupling loss to be about 10 di . We are developing methods for improving the optical coupling further. Preliminary measurements of the response of the device to a chopped black-body have not yet resulted in a measured receiver noise temperature. We expect to be able to complete this measurement in the near future.
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Karasik BS, Gol'tsman GN, Voronov BM, Svechnikov SI, Gershenzon EM, Ekstrom H, et al. Hot electron quasioptical NbN superconducting mixer. IEEE Trans Appl Supercond. 1995;5(2):2232–5.
Abstract: Hot electron superconductor mixer devices made of thin NbN films on SiO/sub 2/-Si/sub 3/N/sub 4/-Si membrane have been fabricated for 300-350 GHz operation. The device consists of 5-10 parallel strips each 5 /spl mu/m long by 1 /spl mu/m wide which are coupled to a tapered slot-line antenna. The I-V characteristics and position of optimum bias point were studied in the temperature range 4.5-8 K. The performance of the mixer at higher temperatures is closer to that predicted by theory for uniform electron heating. The intermediate frequency bandwidth versus bias has also been investigated. At the operating temperature 4.2 K a bandwidth as wide as 0.8 GHz has been measured for a mixer made of 6 nm thick film. The bandwidth tends to increase with operating temperature. The performance of the NbN mixer is expected to be better for higher frequencies where the absorption of radiation should be more uniform.
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