Frequency Following Imaging of Electric Fields from Resonant   Superconducting Devices using a Scanning Near-Field Microwave Microscope
  We have developed a scanning near-field microwave microscope that operates at cryogenic temperatures. Our system uses an open-ended coaxial probe with a 200 mm inner conductor diameter and operates from 77 to 300 K in the 0.01-20 GHz frequency range. In this paper, we present microwave images of the electric field distribution above a Tl2Ba2CaCu2O8 microstrip resonator at 77 K, measured at several heights. In addition, we describe the use of a frequency-following circuit to study the influence of the probe on the resonant frequency of the device. 
Ashfaq S. Thanawalla, B. J. Feenstra, Wensheng Hu, D. E. Steinhauer,   S. K. Dutta, Steven M. Anlage, F. C. Wellstood (Unive...