Development of Low Noise THz SIS Mixer Using an Array of Nb/Al-AlN/NbTiN Junctions

We report the development of a low noise and broadband SIS mixer aimed for 1 THz channel of the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver (CASIMIR), designed for the Stratospheric Observatory for Infrared Astronomy, (SOFIA). The mixer uses an array of two 0.24 mum² Nb/Al-AlN/NbTiN SIS junctions with the critical current density of 30-50 kA/cm² . An on-chip double slot planar antenna couples the mixer circuit with the telescope beam. The mixer matching circuit is made with Nb and gold films. The mixer IF circuit is designed to cover 4-8 GHz band. A test receiver with the new mixer has a low noise operation in 0.87-1.12 THz band. The minimum receiver noise measured in our experiment is 353 K (Y = 1.50). The receiver noise corrected for the loss in the LO injection beam splitter is 250 K. The combination of a broad operation band of about 250 GHz with a low receiver noise makes the new mixer a useful element for application at SOFIA.