NbN Josephson tunnel junctions for terahertz local oscillators

Rugged, high current density NbN1−xCx/MgO/NḃN1−xCx tunnel junctions have been fabricated and tested as voltage tunable Josephson junction terahertz oscillators. The emitted radiation from these junctions is detected on chip by a second junction which is capacitively coupled to the first. For oscillator junctions with a critical current density of Jc∼3.5×104 A/cm2 we find that the junction oscillates with a voltage amplitude of ∼1.5 mV. The detected rf voltage level remains essentially constant from 300 GHz to above 1 THz. The oscillator junction produces 0.5 μW of terahertz radiation of which, due to impedance mismatch, 0.01 μW is coupled into the detector junction.