Nanostructured Ultrathin NbN Film as a Terahertz Hot-Electron Bolometer Mixer
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0935-K02-10
Nanostructured Ultrathin NbN Film as a Terahertz Hot-Electron Bolometer Mixer Grigory Gol'tsman, Sergey Maslennikov, Matvey Finkel, Sergey Antipov, Natalia Kaurova, Elisaveta Grishina, Stanislav Polyakov, Yury Vachtomin, Sergey Svechnikov, Konstantin Smirnov, and Boris Voronov Departmen of Physics, Moscow State Pedagogical University (MSPU), 29 Malaya Pirogovskaya str., Moscow, Central Russia, 119992, Russian Federation
ABSTRACT Planar spiral antenna coupled and directly lens coupled NbN HEB mixer structures are studied. An additional MgO buffer layer between the superconducting film and Si substrate is introduced. The buffer layer enables us to increase the gain bandwidth of a HEB mixer due to better acoustic transparency. The gain bandwidth is widened as NbN film thickness decreases and amounts to 5.2 GHz. The noise temperature of antenna coupled mixer is 1300 and 3100 K at 2.5 and 3.8 THz respectively. The structure and composition of NbN films is investigated by Xray diffraction spectroscopy methods. Noise performance degradation at LO frequencies more than 3 THz is due to the use of a planar antenna and signal loss in contacts between the antenna and the sensitive NbN bridge. The mixer is reconfigured for operation at higher frequencies in a manner that receiver's noise temperature is only 2300 K (3 times of quantum limit) at LO frequency of 30 THz. INTRODUCTION Thin superconducting NbN film based hot electron bolometer is the most promising device to be used as a mixer in quantum limited terahertz heterodyne receivers at the frequencies above 1 THz. Its possible application sphere covers short-range high-resolution radar systems, medical and biological imaging and spectroscopy, high-speed inter-satellite communications, ecology and meteorology (remote sensing of the upper atmosphere in the submillimeter waveband for the monitoring of heterogeneous chemical reactions catalyzed by atmospheric trace gases), plasma physics (studying the coherent submillimeter synchrotron radiation from electron storage rings). Low noise NbN HEB based receivers are especially needed for terahertz radioastronomy as the observations in terahertz frequency range yields crucial information about galaxies and stars formation, interstellar matter and origin of chemical elements. Terahertz receivers can also be used to study atmospheres of comets and planets, as well as the cosmic background radiation [1]. Although NbN HEB outperforms other mixer types (SIS and Schottky mixers) at frequencies above 1 THz, its noise temperature is still about 10 times of hf/k at frequencies < 3 THz and amounts to unacceptable values at frequencies near 10 THz. This work is aimed to decrease the noise temperature of NbN HEB mixers at frequencies up to 4 THz and to make it applicable at higher frequencies. Another important task considered in this article is to increase mixer's relaxation rate and consequently its gain bandwidth as it is required by numerous terahertz projects. The article contains following sections: “Standard route of NbN HEB
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