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MgB2 hot electron bolometer mixers for THz heterodyne instruments

In this work we present experimental investigation of the MgB2 hot-electron bolometer (HEB) for low noise mixing at terahertz frequencies. A dedicated MgB2 thin film deposition system was designed and constructed based on Hybrid Physical-Chemical Deposition. Films as thin as 15nm have a superconducting transition at 35K, with a critical current density <107 A/cm2 (at 4.2K) in bridges as narrow as 500nm, indicating on good connectivity in the film. The gain bandwidth (GBW) was measured by mixing of two THz sources. The GBW is proportional to the film thickness and it is at least 6GHz for 15nm thick devices. Performance of MgB2 HEBs was compared to performance of one of the NbN HEB mixers made for the Herschel Space Observatory (one of the flight units), for which both the GBW and the Noise Bandwidth (NBW) was measured. MgB2 HEB mixers show a GBW at least a factor of three broader compared to the NbN HEB measured in the same set-up.