Imaging of one- and two-dimensional Fiske modes in Josephson tunnel junctions

We report on spatially resolved measurements of dynamic states of Josephson tunnel junctions in an external magnetic field. Using low-temperature scanning electron microscopy we imaged the beam-induced change of the maximum Fiske-step currents and the voltage change of current-biased step states. The excitation of one- and two-dimensional self-resonant cavity modes is clearly visible with a spatial resolution of about 3 μm. We used a 50×50 μ${\mathrm{m}}^2$ Nb/${\mathrm{Al}}_2$ ${\mathrm{O}}_3$/Nb tunnel junction representing a junction of intermediate size. Our experimental results agree well with a qualitative model for the signal generation based on the global change of the junction quality factor Q due to the local electron-beam irradiation. In addition, we present results of a calculation based on the perturbed sine-Gordon equation that takes into account the suppression of the critical current density in the area of the electron beam irradiation.