Cite this Article
Zhou Kang-Min, Miao Wei, Geng Yue, Delorme Yan, Zhang Wen, Ren Yuan, Zhang Kun, Shi Sheng-Cai. Noise temperature distribution of superconducting hot electron bolometer mixers. Chinese Physics B, 2020, 29(5): 058505  
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Noise temperature distribution of superconducting hot electron bolometer mixers
Zhou Kang-Min1, 2, Miao Wei1, 2, Geng Yue1, 2, 4, Delorme Yan3, Zhang Wen1, 2, Ren Yuan1, 2, Zhang Kun1, 2, Shi Sheng-Cai1, 2, †
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210023, China
Observatoire de Paris, 75014 Paris, France
University of Science and Technology of China, Hefei 230026, China

 

† Corresponding author. E-mail: scshi@pmo.ac.cn

Project supported by the Chinese Academy of Sciences (Grant Nos. GJJSTD20180003 and QYZDJ-SSW-SLH043), the National Key Basic Research and Development Program of China (Grant Nos. 2017YFA0304003 and 2018YFA0404701), the National Natural Science Foundation of China (Grant Nos. 11603081, 11673073, U1831202, and 11873099), and PICS projects between the CAS and the CNRS.

Abstract

We report on the investigation of optimal bias region of a wide-band superconducting hot electron bolometer (HEB) mixer in terms of noise temperature performance for multi-pixel heterodyne receiver application in the 5-meter Dome A Terahertz Explorer (DATE5) telescope. By evaluating the double sideband (DSB) receiver noise temperature (Trec) across a wide frequency range from 0.2 THz to 1.34 THz and with a large number of bias points, a broad optimal bias region has been observed, illustrating a good bias applicability for multipixel application since the performance of the HEB mixer is uniquely determined by each bias point. The noise temperature of the HEB mixer has been analyzed by calibrating the noise contribution of all RF components, whose transmissions have been measured by a time-domain spectroscopy. The corrected noise temperature distribution shows a frequency independence relation. The dependence of the optimal bias region on the bath temperature of the HEB mixer has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature until 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.

PACS: ;85.25.Pb;;85.25.Am;
Keyword:;superconducting hot electron bolometer (HEB) mixer;;noise temperature distribution;;bath temperature dependence;;frequency dependence;
1. Introduction

Superconducting hot electron bolometer (HEB) mixers[1] have been widely used in ground-based[2] and space-based observatories[3] due to the advantages of high sensitivity and low local oscillator (LO) power requirement comparing to other THz detectors.[4–6] The use of multipixel receivers can improve the mapping speed of the telescope significantly compared with that with single pixel beam receivers. Superconducting HEB mixer array has and will be used in several telescopes such as GLT,[7] upGREAT,[8] GUSTO,[9] and FIRSPEX.[10] Since Dome A holds the best terahertz window on the earth,[11] 5-meter Dome A Terahertz Explorer (DATE5) is proposed by Purple Mountain Observatory and a 1 × 4 superconducting HEB mixer array has been chosen as the mixer for the 200 μm wavelength band.

Due to the extreme environment at Dome A, the DATE5 telescope will be unmanned. For the 1 × 4 beams heterodyne receiver array, a DC bias reuse technology will also be applied to reduce the complexity of the bias system. What is more, it is difficult to get a uniformed LO power distribution for each pixel,[12,13] and the operation point of each superconducting HEB mixer is uniquely determined by the absorbed LO power, DC bias voltage, and bath temperature. Therefore, it becomes important to study the noise temperature distribution of superconducting HEB mixers for the multipixel application in DATE5 telescope.

Previous research[11] has proved that Dome A has good transmittance at frequency beyond 7 THz. Considering the possible application of HEB mixer in far-infrared frequency range, we investigated the frequency dependence of the optimal bias region of a wideband superconducting HEB mixer.

2. Experiment system
2.1. HEB mixer design

The 0.2–2 THz superconducting HEB device consists of a NbN micro-bridge and a planar spiral antenna. The NbN micro-bridge, located at the center of the planar spiral antenna, has a width of 2 μm and a length of 0.2 μm. The thickness of the NbN film is 5.5 nm and is connected to the spiral antenna by two Ti/Au pads. Both micro-bridge and spiral antenna are fabricated on the high-resistivity silicon substrate.

Figure 1 presents the SEM picture (inset image) of the superconducting HEB device. The spiral antenna arms are defined by r1 = keaφ and r2 = kea(φ – δ), where r1 = 120 μm is the outer radius, r2 = 3.6 μm is the inner radius, k = 4 μm is the initial inner radius, a = 0.35 is the growth rate, and φ is the angular position. The simulated input resistance and reactance of the spiral antenna are shown in Fig. 2, the resistance is nearly constant (∼ 80 Ω) in the whole operating frequency range.

Fig. 1 Measured R–T curve of the superconducting HEB mixer. The inset presents the SEM picture of the log-spiral antenna coupled HEB device. The light gray area is the planar antenna, and the dark gray area is the Si substrate.
Fig. 2 Simulated input impedance of the log-spiral antenna.

The measured R–T and I–V curves of the HEB device are presented in Figs. 1 and 3, respectively. The critical temperature Tc is around 9 K, and the critical current Ic is around 100 μA at 4 K. The normal state resistance is about 160 Ω, as shown in Fig. 1.

Fig. 3 The I–V curves at different pumping levels at (a) 0.2 THz and (b) 0.85 THz, negative resistance can still be found even at well-pumped I–V curves at 0.2 THz.
2.2. Measurement setup

The double sideband (DSB) receiver noise temperatures of the 0.2–2 THz HEB mixer are measured at 0.2 THz, 0.5 THz, 0.85 THz, and 1.34 THz by the Y-factor method. The measurement system is illustrated in Fig. 4. The HEB device is stuck to the flat surface of a Si elliptical lens which has a long axis length of 5.228 mm and a minor axis length of 5 mm, the extension length is 1.149 mm. The lens, without anti-reflection coating, is mounted in a cupreous mixer block which is mounted in a close-cycled cryostat. A Zitex G104 film is used to filter the infrared radiation and a HDPE sheet with a thickness of 1.5 mm is employed as the vacuum window of the cryostat. The LO and RF signals are coupled to the HEB mixer by a 25 μm Mylar film. The LO signals are provided by a signal generator together with different frequency multiplier chains.

Fig. 4 Diagram of the noise temperature measurement setup.

The intermediate frequency (IF) output signal from the HEB mixer is amplified firstly by a low noise amplifier (LNA) and then a room temperature amplifier. The amplified IF power is then filtered by a bandpass filter (80 MHz) at 1.5 GHz and recorded by a square-law detector.

Figure 3 shows the measured I–V curves of the HEB mixer at 0.2 THz and 0.85 THz with different local oscillator pumping levels. It is obvious that the well-pumped I–V curves at 0.2 THz still show negative resistances, which can be attributed to the non-uniformly absorption of LO power in the NbN micro-bridge.[14] The DSB noise temperatures Trec have been measured at these pumping levels (shown in Fig. 3), and are then plotted in a two-dimensional graph of the bias voltage and current in order to get a better view of its dependence on bias points. At each fixed bias voltage, the noise temperature distribution corresponding to different bias currents (or LO pumping levels) is obtained by linear interpolation of the measured data.

3. Noise temperature distribution of superconducting HEB mixer
3.1. Optimal bias region

The DSB receiver noise temperatures have been measured at a large number of bias points at four frequencies, and are illustrated in Fig. 5. The effective radiation temperatures of 300 K and 77 K (blackbody) are calculated based on the Callen–Welton definition[15] at each LO frequency in the Y-factor method. What is more, the compensation of the direct detection effect is fulfilled by keeping the bias current unchanged between the 300 K and 77 K blackbody measurements.[16] A broad optimal bias range can be observed at each frequency, indicating a good bias applicability of the HEB mixer for multipixel application. Note that, the bias current of the HEB mixer (Y axis in Fig. 5) has been normalized to the critical current Ic for universal comparison. A raised region (marked by circle) at low bias voltage can be observed at 0.2 THz and 0.5 THz, which can be attributed to the negative resistance region as shown in the measured I–V curves. The lowest uncorrected DSB noise temperatures at these frequencies are also listed in Table 1, indicating that this HEB mixer has high sensitivity in the whole operating frequency range. The noise temperature at 1.34 THz is slightly higher than that at other frequencies, which is because the RF loss of 25 μm Mylar at 1.34 THz is higher than that at other frequencies (as shown in Table 2).

Fig. 5 Measured DSB receiver noise temperature Trec in a two-dimensional graph of the bias voltage and current at (a) 0.2 THz, (b) 0.5 THz, (c) 0.85 THz, and (d) 1.34 THz. The isoline indicates the absolute value of Trec.
Table 1.

Lowest uncorrected and corrected DSB receiver noise temperatures

.
Table 2.

Loss of RF components.

.
3.2. Frequency dependence of HEB mixer noise temperature

The measured DSB receiver noise temperature Trec includes three parts: RF noise TRF, HEB mixer intrinsic noise Tmixer, and IF noise TIF, and can be expressed as

where GRF is the RF loss, and Gmixer is the conversion gain of the HEB mixer. In order to investigate the distribution of HEB mixer noise temperature Tmixer, we need to calibrate other noises.

The IF noise TIF is mainly produced by the LNA and is normally very small (about 3–4 K). The conversion gain Gmixer of the HEB mixer is about –5 dB and is proved to be frequency independent.[14] Therefore we ignore the term TIF/Gmixer in Eq. (1) and focus on the calibration of RF loss GRF and RF noise TRF.

The RF noise is produced by the RF optics in the receiver system which consists of a 25 μm Mylar beam splitter, a 1.5 mm HDPE window, a Zitex G104 infrared filter, an air path, a Si lens, and a planar spiral antenna. The equivalent input noise temperature Teq of each RF component with a gain G is given by

According to Eq. (2), the gain G of each RF component is the most crucial parameter. In order to obtain a precise value of the RF loss, we measure and simulate the transmittance of all RF components, as shown in Figs. 6 and 7. The transmittances of the Mylar and Zitex films are measured by a time-domain spectrometer (TDS), and the complex permittivity is derived from the measured result. The simulation of transmittance is performed by FEKO software[17] with measured complex permittivity, and as shown in Fig. 6 good agreement has been found between simulation and measurement. The transmittance of HDPE has been measured in Ref. [11]. The transmittance of the air path is calculated by an atmospheric model (AM).[18] The coupling efficiency between the planar spiral antenna and superconducting NbN film has been measured by a FTS with a calibration method described in Ref. [19]. The measured coupling efficiency of the spiral antenna shows the same trend with the simulation one, where the difference is largely due to the water absorption in the air path, as shown in Fig. 7. The reflection loss of the air and Si lens interface on both sides is about 1.54 dB according to the measurements in Ref. [20], where the absorption loss inside the high resistivity silicon material is ignored. All measured and simulated RF losses are listed in Table 2.

Fig. 6 Measured and simulated transmission of (a) 25 μm Mylar and (b) Zitex. Both amplitude (red solid line) and phase (green square line) are measured by a TDS, the simulated amplitude (black dash line) and phase (blue star line) are obtained by FEKO software with the measured permittivity.
Fig. 7 (a) Simulated transmission of the air path in our lab by AM model. (b) Normalized coupling efficiency of spiral antenna by simulation and FTS measurement.

After calibrating the RF noise and RF loss by Eqs. (1) and (2), we plot the HEB mixer noise temperature in a two-dimensional graph of the bias voltage and current at 0.2 THz, 0.5 THz, 0.85 THz, and 1.34 THz in Fig. 8. The corrected mixer noise temperature shows the same distribution on bias point at all frequencies, in other words a frequency independent optimal bias region has been observed. The corrected lowest noise temperature has also been listed in Table 1, a frequency independent mixer noise temperature has been obtained without considering the quantum noise contribution.

Fig. 8 Corrected HEB mixer noise temperature Tmixer in a two-dimensional graph of the bias voltage and current at (a) 0.2 THz, (b) 0.5 THz, (c) 0.85 THz, and (d) 1.34 THz. The isoline indicates the absolute value of Tmixer.
3.3. Bath temperature dependence

The receiver noise temperature of the superconducting HEB mixer has also been measured at 4 K, 5 K, 6 K, and 7 K to investigate the effect of the bath temperature on the optimal bias region. Figure 9 shows the DSB receiver noise temperatures distribution at different bath temperatures at 0.85 THz. Note that similar phenomenon has been observed at other frequencies, we only plot the result at this frequency for illustration. Figure 10 illustrates the bath temperature dependence of the measured results. The lowest receiver noise temperature Trec only increases from 750 K to 810 K when the bath temperature increases to 6 K, however it rapidly raises to 1000 K when the bath temperature increases to 7 K due to the rapid decrease of the critical current of the HEB mixer. The optimal bias area, defined as the bias region where the noise temperature is lower than 900 K, has been calculated by counting the bias points with a voltage interval of 0.1 mV and a current interval of 0.01 × I/Ic. Obviously, the optimal bias points decrease rapidly with increasing bath temperature. In conclusion, we believe that the bath temperature has limited effect on the lowest receiver noise temperature until it raises to 7 K (about 0.8 times critical temperature Tc[21]), however the optimal bias region deteriorates obviously with increasing bath temperature.

Fig. 9 Measured DSB noise temperature Trec in a two-dimensional graph of the bias voltage and current at 0.85 THz at different bath temperatures.
Fig. 10 Lowest receiver noise temperature (square) and optimal bias region (star) at different bath temperatures.
4. Conclusion

The noise temperature distribution of a wide-band superconducting HEB mixer has been investigated for the multi-pixel application. A broad optimal bias region has been observed at 0.2 THz, 0.5 THz, 0.85 THz, and 1.34 THz. The HEB mixer noise temperature has been calculated at all frequencies by calibrating the noise contribution of the RF components. The corrected mixer noise temperature has the same distribution on bias point at all frequencies, a frequency independent optimal region can be observed. The bath temperature dependence has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature Trec until it raises to 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.

Reference
[1] Gershenzon E M Gol’tsman G N Gogidze I G Elant’ev A I Karasik B S Semenov A D 1990 Sov. Phys. Supercond. 3 1582
[2] Meledin D Pavolotsky A Desmaris V Lapkin I Risacher C Perez V Henke D Nystrom O Sundin E Dochev D Pantaleev M Fredrixon M Strandberg M Voronov B Gol’tsman G N Belitsky V 2009 IEEE Trans. Microw. Theory Tech. 57 89
[3] Cherednichenko S Drakinskiy V Berg T Khosropanah P Kollberg E 2008 Rev. Sci. Instrum. 79 034501
[4] Gu M Kang L Zhang L B Zhao Q Y Jia T Wan C Xu R Y Yang X Z Wu P H 2015 Chin. Phys. Lett. 32 038501
[5] Li J Wang M J Shi S C Matsuo H 2007 Chin. Phys. Lett. 24 570
[6] Zhang Q Y Dong W H He G F Li T F Liu J S Chen W 2014 Acta Phys. Sin. 63 200303 in Chinese
[7] Huang Y R Chiu C P Lu W C Chang H H Chiang Y Y Wang M J 2016 Proceeding of URSI Asia-Pacific Radio Science Conference (URSI AP-RASC) August 21–25 2016 Seoul, South Korea 569 571
[8] Pütz P Büchel D Jacobs K Schultz M Honingh C E 2015 Proceeding of 26th Int. Symp. Space THz Technology March 16–18 2015 Cambridge, MA, USA
[9] Silva J R G D 2016 Study on the viability of a 4×2 HEB mixer array at super-THz based on a Fourier phase grating LO for space applications Master Dissertation
[10] Rigopoulou D Caldwell M Ellison B Pearson C Caux E Cooray A Gallego J D Gerin M Goicoechea J R Goldsmith P Kramer C Lis D C Molinari S Ossenkopf-Okada V Savini G Tan B K Tielens X Viti S Wiedner M Yassin G 2016 Proceeding of SPIE 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave July 29, 2016 Edinburgh, United Kingdom 99042K
[11] Shi S C Paine S Yao Q J Lin Z H Li X X Duan W Y Matsuo H Zhang Q Z Yang J Ashley M C B Shang Z H Hu Z W 2016 Nature Astronomy 10001 10.4161/auto.19496
[12] Zhu H T Valentin J Vacelet T Herth E Delorme Y Wiedner M 2019 Proceeding of 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) September 1–6, 2019 Paris, France 1 2
[13] Ding J Q Hu J Shi S C Zhao Y 2018 IEEE Access 6 42507 42515
[14] Miao W Zhang W Zhong J Q Shi S C Delorme Y Lefevre R Feret A Vacelet T 2014 Appl. Phys. Lett. 104 052605
[15] Kerr A R 1999 IEEE Trans. Microw. Theory Tech. 47 325
[16] Baselmans J J A Baryshev A Reker S F 2005 Appl. Phys. Lett. 86 163503
[17] FEKO: A computer code for the analysis of electromagnetic problems EM Software & Systems-S.A. (Pty) Ltd. Stellenbosch, South Africa
[18] Paine S, Am Atmospheric Model, Available: https://www.cfa.harvard.edu/∼spaine/am/
[19] Zhou K M Miao W Lou Z Hu J Li S L Zhang W Shi S C Lefevre R Delorme Y Vacelet T 2015 IEEE Trans. Appl. Supercond. 25 3 1 5
[20] Zhou K M Miao W Lefevre R Delorme Y Shi S C 2016 Proceeding of URSI Asia-Pacific Radio Science Conference (URSI AP-RASC) August 21–25, 2016 Seoul, South Korea 2010 2012
[21] Zhang W Li N Jiang L Ren Y Yao Q J Lin Z H Shi S C Voronov B M Gol’tsman G N 2007 Proceeding of SPIE 6840, Terahertz Photonics 684007 February 8, 2008 Beijing, China 684007