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19 th International Symposium on Space Terahertz Technology Sensitivity of a heterodyne receiver at 4.3 THz based on a NbN hot electron bolometer mixer 2-3 P. Khosropanah 1 , W. M. Laauwen 1 , M. Hajenius 1,2 , J.N. Hovenier 2 , T. Bansal 1,2 , J. R. Gao 1,2 , and T.M. Klapwijk 2 1 SRON Netherlands Institute for Space Research, Utrecht/Groningen, the Netherlands 2 Kavli Institute of NanoScience, Delft University of Technology, Delft, the Netherlands Correspondence: P.Khosropanah@sron.nl Today hot electron bolometer (HEB) mixer is considered a mature technology below 2 THz as it is used in band 6 and 7 (1.4-1.9 THz) of HIFI on the Herschel space observatory. Future space missions will move to higher frequencies, e.g. 2-6 THz and thus call for sensitive mixers beyond 2 THz. Additionally, the successful demonstration of the new technology will play a crucial role in defining ESA/NASA future mission plans. We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna. Using hot/cold blackbody loads and a beam splitter all in vacuum and applying an optically pumped gas laser at 4.3 THz as a local oscillator (LO), we measured a double sideband (DSB) receiver noise temperature of 1300 K at the optimum LO power of 330 nW and a bias voltage of 0.8 mV [1], which is an unprecedented sensitivity at such a high frequency and is about 12 times the quantum noise (hν/2k B ). By comparing to the measurement in a usual setup, we find that the use of vacuum setup not only reduces the loss in the air and the window, but also reduces the fluctuations considerably, which makes the measurement much more reliable (see the figure, which shows the receiver output power and receiver noise temperature versus bias voltage in the vacuum and in the usual setup). The LO power fluctuations caused by either the power fluctuations of the laser itself or by air and beam splitter vibrations can have a significant impact on the stability of a receiver. Although this is usually not the case in a real astronomical instrument, such fluctuations can inevitably occur in the laboratory environment where a gas laser is applied as LO. Here we also introduce a new measurement method that can accurately determine the receiver noise temperature despite of LO power fluctuations or drift. [1] P. Khosropanah, J. R. Gao, W. M. Laauwen, M. Hajenius, and T. M. Klapwijk, “ Low noise NbN hot electron bolometer mixer at 4.3 THz”, Appl. Phys. Lett. 91, 221111 (2007). Receiver output power (dBm) -20 -22 -24 -26 -28 -30 -32 Air Vacuum Air Vacuum Hot Load Cold Load -4 -3 -2 -1 0 1 2 3 4 Voltage (mV) 12000 10000 8000 6000 4000 2000 0 DSB receiver noise temperature (K) 24
19 th International Symposium on Space Terahertz Technology 2-4 Demonstration of a heterodyne receiver for detection of OH line at 3.5 THz based on a superconducting HEB mixer and a distributed feedback quantum cascade laser W. Zhang 1,a , P. Khosropanah 1 , J.N. Hovenier 2 , J.R. Gao 1,2 , T. Bansal 1,2 , M. Hajenius 1,2 , T.M. Klapwijk 2 , M.I. Amanti 3 , G. Scalari 3 , and J. Faist 3 1 SRON Netherlands Institute for Space Research, Utrecht/Groningen, The Netherlands 2 Kavli Institute of Nanoscience, Delft University of Technology, Delft, The Netherlands 3 Institute of Physics, University of Neuchâtel, 1 A.-L. Breguet, Neuchâtel CH–2000, Switzerland Problems related to the Earth's atmosphere such as global warming and ozone destruction can be monitored and better understood by observations in the far-infrared regime. This regime holds the most important spectral signatures of the relevant molecules. Hydroxyl (OH) molecules at 1.8, 2.5 and 3.5 THz have been identified as being crucial. Among them, the OH line at 3.5 THz should have the highest intensity and is well isolated from other molecular lines. Therefore, it is ideal for monitoring and retrieval. THz quantum cascade lasers (QCLs) are currently the only choice of solid-state local oscillators (LO) for this specific frequency since solid state LOs based on multipliers are unable to deliver enough power and gas lasers have no strong line. Although THz QCLs based on Fabry-Pérot cavity have been demonstrated as LOs for the Y-factor measurement, for this purpose a QCL with a stable single-mode emission at a precisely designed wavelength is desirable. Distributed feedback (DFB) QCLs, which are based on first-order Bragg gratings incorporated into the waveguide, are able to offer a reliable method to achieve required single-mode operation. Here we report heterodyne measurements using a surface plasmon DFB QCL emitted at 3.42 THz as a LO and an NbN HEB mixer integrated with a tight spiral antenna [1]. The QCLs were developed by University of Neuchâtel. First, we demonstrate that the DFB QCL, after improving the beam [2], can pump the HEB mixer with a 3 μm-thick Mylar beam splitter. Second, we succeeded in measuring receiver noise temperatures (see the figure). Although the best receiver noise temperature is 2700 K at 3.42 THz, this value can be improved considerably by optimizing the setup, such as using an antireflection coating Si lens. Third, we illustrate a few aspects of this DFB QCL which needs to be improved significantly. For example, the high input power imposes a challenge to operate this laser at L-He temperature. This is an ongoing experiment. So we will report the latest results during the symposium. DSB Trec(K) 10000 8000 6000 4000 2000 0.8mV1.2mV 1.6mV 0.6mV 1.0mV 1.4mV 1.8mV 0.01 0.02 0.03 0.04 0.05 0.06 Current(mA) a On leave from Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008, China [1] P. Khosropanah, J. R. Gao, W. M. Laauwen, M. Hajenius and T. M. Klapwijk, Appl. Phys. Lett. 91, 221111 (2007). [2] J.N. Hovenier, S. Paprotskiy, J.R. Gao, P. Khosropanah, T.M. Klapwijk, L. Ajili, M. A. Ines, and J. Faist, Abstract, ISSTT 2007. 25
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