Heterodyne receivers combining a NbN HEB mixer with a local oscillator (LO) are the work horse for high resolution ( ≥106 ) spectroscopic observations at supra-terahertz frequencies. We report an MgB2 HEB mixer working at 5.3 THz with 20 K operation temperature based on a previously published paper [Y. Gan et al, Appl. Phys. Lett., 119, 202601 (2021)]. The HEB consists of a 7 nm thick MgB2 submicron-bridge contacted with a spiral antenna. It has a Tc of 38.4 K. By using hot/cold blackbody loads and a Mylar beam splitter all in vacuum, and applying a 5.25 THz FIR gas laser as the LO, we measured a minimal DSB receiver noise temperature of 3960 K. The latter gives a DSB mixer noise temperature of 1470 K. This sensitivity is 28 times better than a room temperature Schottky mixer at 4.7 THz, but about 2.5 times less sensitive than an NbN HEB mixer. The latter must be operated around 4 K. The IF noise bandwidth is about 10 GHz, which is 2.5-3 times larger than an NbN HEB. With further optimization, such MgB2 HEBs are expected to reach a better sensitivity. That the low noise, wide IF bandwidth MgB2 HEB mixers can be operated in a compact, low dissipation 20 K Stirling cooler can significantly reduce the cost and complexity of heterodyne instruments and therefore facilitate new space missions.
Generating multiple local oscillator beams is one challenge to develop large heterodyne receiver arrays (~100 pixels), which allow astronomical instrumentations mapping more area within limited space mission lifetime. Here, We combine a reflective Fourier grating with an unidirectional antenna coupled 3rd-order distributed feedback (DFB) quantum cascade laser (QCL) to generate 81 beams at 3.86 THz. We have measured the beam pattern of the diffracted 81 beams, which agrees well with a simulated result from COMSOL Multiphysics with respect to the angular distribution and power distribution among the 81 beams. The diffraction efficiency of the Fourier grating is derived to be 94±3%, which is very close to the simulated result of 97%. For an array of equal superconducting hot electron bolometer mixers, 64 out of 81 beams can pump the HEB mixers with similar power, resulting in receiver sensitivities within 10%. Such a combination of a Fourier grating and a QCL can create an LO with 100 beams or more, enabling a new generation of large heterodyne arrays for astronomical instrumentation. This paper is essentially a copy of our paper in Optics Express.
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