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Gol'tsman, G. N., Karasik, B. S., Svechnikov, S. I., Gershenzon, E. M., Ekström, H., & Kollberg E. (1995). Noise temperature of NbN hot—electron quasioptical superconducting mixer in 200-700 GHz range. In Proc. 6th Int. Symp. Space Terahertz Technol. (268).
Abstract: The electron heating effect in superconducting films is becoming very attractive for the development of THz range mixers because of the absence of frequency limitations inherent in the bolometric mechanism. However, the evidence for the spectral dependence of the position of optimal operating point has been found recently for NbN thin film devices 1.2 • The effect is presumably attributed to the variation in the absorption of radiation depending on the frequency. Since the resistive state is not spatially uniform the coupling efficiency of the mixer device with radiation can be different for frequencies larger than Zeilh and those smaller than 2Alh (d is the effective superconducting gap in the resistive state). To study the effect more thoroughly we have investigated the noise temperature of quasioptical NbN mixer device with broken hue tapered slot antenna in the frequency range 200-700 GHz. The device consists of several (5-10) parallel strips 1 jim wide and 6-7 tun thick made from NbN film on Si0 2 -Si 3 N 4 -Si membrane. The strips are connected with the gold contacts of the slot-line antenna which serves both as bias and IF leads. We used backward wave oscillators as LO sources and a standard hot/cold load technique for noise temperature measurements. The frequency dependence of noise temperature is mainly determined by two factors: frequency properties of the antenna and frequency dependence of the NbN film impedance. To separate both factors we monitored the frequency dependence of the device responsivity in the detector mode at a higher temperature within the superconducting transition where the impedance of NbN film is close to its normal resistance. In this case the impedance of the device itself is frequency independent. The experimental results will be reported at the Symposium. 1. G. Gollsman, S. Jacobsson, H. EkstrOm, B. Karasik, E. Kollberg, and E. Gershenzon, “Slot-line tapered antenna with NbN hot electron mixer for 300-360 GHz operation,” Proc of the 5th Int. Symp. on Space Terahertz Technology, pp. 209-213a, May 10-12,1994. 2. B.S. Karasik, G.N. Gol i tsman, B.M. Voronov, S.I. Svechnikov, E.M. Gershenzon, H. Ekstrom, S. Jacobsson, E. Kollberg, and K.S. Yngvesson, “Hot electron quasioptical NbN superconducting mixer,” presented at the ASC94, submitted to IEEE Trans. on Appl. Superconductivity.
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Ekström, H., Karasik, B., Kollberg, E., Gol'tsman, G., & Gershenzon, E. (1995). 350 GHz NbN hot electron bolometer mixer. In Proc. 6th Int. Symp. Space Terahertz Technol. (pp. 269–283).
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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Gerecht, E., Musante, C. F., Schuch, R., Lutz, C. R., Jr., Yngvesson, K. S., et al. (1995). Hot electron detection and mixing experiments in NbN at 119 micrometer wavelength. In Proc. 6th Int. Symp. Space Terahertz Technol. (pp. 284–293).
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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Гольцман, Г. Н., Веревкин, А. А., Гершензон, Е. М., Птицина, Н. Г., Смирнов, К. В., & Чулкова, Г. М. (1995). Исследования процессов неупругой релаксации и примесная спектроскопия-релаксометрия в двумерном электронном газе в полупроводниковых структурах с квантовыми ямами.
Abstract: В гетероструктурах GaAs/AlGaAs впервые прямым методом измерена температурная зависимость вpемени энеpгетической pелаксации двумерного электронного газа te(T) в широком диапазоне температур Т=1,5 – 50 К в квазиравновесных условиях. Для измерений использовался высокочувствительный спектрометр миллиметрового диапазона волн с высоким временным разрешением, который позволял измерять релаксационные времена до 150 пс с погрешностью не более 20%. Верхний предел температуры определялся временным разрешением спектрометра. Исследования проводились на высококачественных образцах с поверхностной концентрацией носителей ns = 4,2 1011 см-2 и подвижностью m = 7 105 см2В-1с-1 (при Т = 4,2К). В квазиравновесных условиях из температурной зависимости tе определен предел подвижности при низких температурах (T<4.2 K), связанный с рассеянием на пьезоакустическом потенциале, получено время неупругой релаксации, связанное с рассеянием на деформационном потенциале (15 K25 K), получено характерное время испускания оптического фонона (tLO>4,5пс), которое существенно превышает время сронтанного излучения оптического фонона (примерно в 30 раз), что связано с большой ролью процессов перепоглащения фононов электронами.При низких температурах проведены измерения tе в условиях сильного разогрева. Полученные значения tе и зависимость tе от температуры электронов Те совпадают с tе(Т) в квазиравновесных условиях при Т=Те. Из полученных значений tе(Те) построена зависимость мощности энергетических потерь от Те, которая хорошо согласуется с литературными данными.Начаты измерения в магнитном поле, которые показывают переспективность использованного нами метода измерений как в области слабых магнитных полей при факторе заполнения >10, так и в области сильных магнитных полей при факторе заполнения >1-2.
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