Low noise NbN hot electron bolometer mixer at 4.3THz

Abstract

We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3 THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300 K at the optimum local oscillator ͑LO͒ power of 330 nW, which is about 12 times the quantum noise ͑h / 2k B ͒. Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2819534͔Superconducting mixers 1 play a key role in astrophysics at terahertz frequencies, where the early universe radiates strongly. The availability of low noise superconductorinsulator-superconductor ͑SIS͒ mixers and hot electron bolometer ͑HEB͒ mixers has made the realization of highly sensitive spectrometers on ground, airborne, and space telescopes possible. An example of this is the heterodyne instrument for far infrared on the Herschel space telescope, 2 to be launched in 2008, where the heterodyne spectrometers are operated up to 1.3 THz using SIS mixers and further up to 1.9 THz using HEB mixers. For the next generation of space telescopes, it becomes highly desirable to demonstrate sensitive mixers in the frequency range between 2 and 6 THz. HEB mixers, which are currently the only devices suitable for this frequency range, have been reported up to 5.3 THz. [3][4][5] However, only few experiments have so far been done at the frequencies above 3 THz, namely, superterahertz frequencies, and the performance is relatively poor. 3,4 The noise temperature of a receiver is a crucial parameter that defines the ultimate sensitivity of the heterodyne spectrometer and the observation time. To achieve the low noise at superterahertz, several challenges are expected either in the mixer itself or in the testing technique. First, it is unclear whether the performance of HEBs will degrade. The relaxation of highly excited electrons due to increased photon energy can be complicated by cascade processes of emission and absorption of phonons. This can compete with the electron-electron interaction and thus may decrease the mixing efficiency. 6 Also, there is a concern of the quantum noise. 7 Second, it becomes more difficult to couple terahertz radiation to the HEB. Third, there is lack of local oscillators ͑LOs͒. Optically pumped far infrared ͑FIR͒ gas lasers are commonly used, but achieving stable output power is cumbersome. Terahertz quantum cascade lasers 8 ͑QCLs͒ are promising, stable solid-state LOs, 9 but still in a development stage. Finally, there is an increase in the air loss due to the absorption of terahertz radiation by water vapor, which can increase the receiver noise temperature and may also cause instability.In this letter, we report the measurement of a quasioptical NbN HEB mixer at 4.3 THz using a hot/cold load built in vacuum an...

Phase-locking of a 2.7-THz quantum cascade laser to a mode-locked erbium-doped fibre laser

“…To obtain the beat signal between the QCL and the THz reference, we use a lens-antenna coupled superconducting NbN HEB mixer [17]. It requires low LO power (< 300 nW).…”

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Quantum noise in a terahertz hot electron bolometer mixer

“…First, we used a spiral antenna coupled NbN HEB mixer with a 0.2ϫ 2 m 2 NbN bridge, which is similar to the one described in Ref. 11. It has shown unprecedentedly high sensitivity at the upper terahertz frequencies, which is vital to exclude the contribution of any unknown effect other than T CL,MIX out .…”