Phase-sensitive imaging of microwave currents in superconductive circuits

We propose an improved measuring technique for microwave currents in superconductive circuits, demonstrate it experimentally, and validate the experimental data with numerical simulations. Contemporary superconductive electronics widely uses planar circuits with micrometer-scale elements for a variety of applications. With the rise of complexity of a circuit and increase in the number of its components, a simple impedance measurement is often not efficient for the diagnosis of problems, nor for clarifying the physics underlying the circuit response. The established Laser Scanning Microscopy (LSM) technique generates micrometer-scale images of the amplitude of the microwave currents in a planar superconductive circuit, but not the phase of the oscillating currents. Here, we present a more powerful type of LSM imaging containing signal phase information. We employ a fast-optical modulator in order to synchronize the phase of the laser intensity oscillation with the phase of the probing microwave signal. The loss induced in a laser illuminated area strongly depends on the phase difference between the RF probing signal and the laser beam modulation. As a proof of concept, we present the phase-sensitive LSM measurements of the inner mode currents in a planar superconductive resonator and confirm the data with numerical simulation of the resonator's electrodynamics.