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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Gol'tsman, G. N., Karasik, B. S., Okunev, O. V., Dzardanov, A. L., Gershenzon, E. M., Ekstrom, H., et al. (1995). NbN hot electron superconducting mixers for 100 GHz operation. IEEE Trans. Appl. Supercond., 5(2), 3065–3068.
Abstract: NbN is a promising superconducting material for hot-electron superconducting mixers with an IF bandwidth larger than 1 GHz. In the 1OO GHz frequency range, the following parameters were obtained for 50 /spl Aring/ thick NbN films at 4.2 K: receiver noise temperature (DSB) /spl sim/1000 K; conversion loss /spl sim/10 dB; IF bandwidth /spl sim/1 GHz; and local oscillator power /spl sim/1 /spl mu/W. An increase of the critical current of the NbN film, increased working temperature, and a better mixer matching may allow a broader IF bandwidth up to 2 GHz, reduced conversion losses down to 3-5 dB and a receiver noise temperature (DSB) down to 200-300 K.
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Karasik, B. S., Gol'tsman, G. N., Voronov, B. M., Svechnikov, S. I., Gershenzon, E. M., Ekstrom, H., et al. (1995). Hot electron quasioptical NbN superconducting mixer. IEEE Trans. Appl. Supercond., 5(2), 2232–2235.
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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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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Gol'tsman, G., Jacobsson, S., Ekstrom, H., Karasik, B., Kollberg, E., & Gershenzon, E. (1994). Slot-line tapered antenna with NbN hot electron mixer for 300-360 GHz operation. In Proc. 5th Int. Symp. Space Terahertz Technol. (pp. 209–213a).
Abstract: NbN hot-electron mixers combined with slot-line tapered antennas on Si wdnitride membranes had been fabricated. Several strips of 1 gm wide and 5 tan long made from 100 A NbN film are inserted into the slot antenna. IV-curves under local oscillator power in 300-350 GHz frequency range and conversion gain dependencies on intermediate fre- quency in the 0.1-1 GHz range are measured and compared with that for 100 GHz frequency band. Our results show that pumped IV-curves and intermediate frequency bands are different for 100 GHz and 300 GHz frequency ranges. The interpretation exploits the fact that for the lowest radiation frequency the superconducting energy gap is larger than the radiation quantum energy while they are comparable at the higher frequency. Tha results show that such mixers have good perspectives for terahertz receiving technology.
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