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Quantum Cascade lasers in combination with Hot electron bolometers by hcj


									Sensitivity of a heterodyne receiver at 4.3 THz based on a NbN hot electron bolometer mixer
P. Khosropanah1, W. M. Laauwen1, M. Hajenius1,2, J.N. Hovenier2, T. Bansal1,2, J. R. Gao1,2, and T.M. Klapwijk2
1 2

SRON Netherlands Institute for Space Research, Utrecht/Groningen, the Netherlands Kavli Institute of NanoScience, Delft University of Technology, Delft, the Netherlands

Correspondence: 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 (h/2kB). 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)

Air -22 -24 -26 -28 Air -30 Vacuum -32 -4 -3 -2 -1 0 1 2 Vacuum

Hot Load 12000 Cold Load 10000 8000 6000 4000 2000 0 3 4 Voltage (mV)

DSB receiver noise temperature (K)

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