A near-field scanning SQUID microwave microscope

Abstract

We developed a scanning DC SQUID microscope with novel readout electronics capable of wideband sensing RF magnetic fields from 50 to 200 MHz and simultaneously providing closed-loop response at kHz frequencies. To overcome the 20 MHz bandwidth limitation of traditional closed-loop SQUIDs, a flux-modulated closed loop simultaneously locks the SQUID quasi-static flux and flux-biases the SQUID for amplification of the RF flux up to ${\Phi }_0/4$ in amplitude. Demodulating the SQUID voltage with a double lock-in technique yields a signal representative of both the amplitude and phase of the RF flux. This provides 80 dB of a linear dynamic range with the flux noise density of 4 $\mu {\Phi }_0/\sqrt{Hz}$ at 200 MHz for $YB{a}_{2}C{u}_3{O}_7$ bi-crystal SQUID at 77 K. We describe the electronics performance and present images for RF magnetic field of the travelling wave in a coplanar waveguide, the standing wave in an open-circuited microstrip, and a surface mounted device antenna.

8 pages, 8 figures. Superconductor Science and Technology, 2014

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