We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.
We present an analytic method to extract Schottky diode parasitic model parameters. All the ten unknown model parameters are extracted via a straightforward step-by-step procedure. The challenges for a proper finger inductance and series resistance extraction are discussed and solutions are recommended. The proposed method is evaluated using three sets of -parameter data for GaAs-based planar Schottky diodes, i.e., data from measurement up to 110 GHz and 3-D electromagnetic full-wave simulations up to 600 GHz. The extracted models agree well with the measured and simulated data.
The authors report on a terahertz (600GHz) mixing experiment with MgB2 microbolometers in the resistive state. The authors observed that for a 20nm film a mixer gain bandwidth of 2.3GHz can be achieved, corresponding to an energy relaxation time of 70ps. The experimental results were analyzed using a two-temperature model. As a result, the phonon escape time of ∼20ps was deduced. At 1.6THz the MgB2 mixer uncorrected noise temperature was 11000K. The obtained results show that MgB2 bolometers are good prospects for the terahertz range as both broadband mixers and fast direct detectors.
We report on low noise terahertz bolometric mixers made of MgB 2 superconducting thin films. For a 10-nm-thick MgB 2 film, the lowest mixer noise temperature was 600 K at 600 GHz. For 30 to 10-nm-thick films, the mixer gain bandwidth is an inverse function of the film thickness, reaching 3.4 GHz for the 10-nm film. As the critical temperature of the film decreases, the gain bandwidth also decreases, indicating the importance of high quality thin films for large gain bandwidth mixers. The results indicate the prospect of achieving a mixer gain bandwidth as large as 10-8 GHz for 3 to 5-nm-thick MgB 2 films. Superconducting NbN hot-electron bolometer (HEB) 1 mixers are widely used for high resolution terahertz radio astronomy. 2 Such mixers have superior performance over other types of mixers (e.g., SIS, Schottky diodes) 2 at frequencies higher than 1.2 THz. 3-5 A large RF bandwidth, a low noise temperature, and low LO power requirements determined the choice of NbN HEB mixers for the Herschel space observatory. 6,7 In contrast to SIS mixers, the useful IF bandwidth of NbN HEB mixers is practically limited to 3-5 GHz, 8 as the noise temperature rises drastically at higher intermediate frequencies. The reason for this is that the HEB mixer gain rolls off as the IF exceeds the mixer's 3 dB gain bandwidth (GBW). The GBW is determined by two consequent processes in the electron energy relaxation: the electron-phonon interaction and the phonon energy relaxation. The second process mainly occurs via acoustic phonon escape into the substrate. The electron-phonon interaction time is usually a function of the temperature. For HEB mixers, the relevant electron temperature is the critical temperature of the superconducting film, T c , to which the electrons are heated by a combination of the LO power and the dc bias current. In NbN thin films, the electron-phonon interaction time is approximately s e-ph % 12 ps at 10 K. 9,10 The phonon escape time is on the order of s esc % 40 ps for NbN films as thin as 3-4 nm. 8
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