Spectral response of point-contact Josephson junctions

The spectral response of niobium point-contact Josephson junctions with resistance values R between 1 kΩ and 10 Ω has been measured. For R≳100 Ω the response peaks at maxima of d2V̄/dĪ2, whereas the spectral response peaks at frequencies νR≲200 GHz. Decreasing the resistance by burning-in of the contact, it was found that for underdamped contacts νR∝R−1/3, almost independent of the junction geometry. Tapping the contact, i.e., changing the microscopic properties of the flat contact region (diameter, 5–10 μm), changed the absolute value of νR. Maximum resonant response occurred approximately for critical damping at R=80–100 Ω. The observed behavior is interpreted as resonant detection at the plasma frequency. At low resistance values νR disappears, but additional resonances at ν≳150 GHz appear, some of which could be identified as antenna resonances.