Frequency following imaging of electric fields from resonant superconducting devices using a scanning near-field microwave microscope

Abstract

Abstract-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 µ µm 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 Tl 2 Ba 2 CaCu 2 O 8 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.

Imaging of microwave intermodulation fields in a superconducting microstrip resonator

“…1. The superconducting resonator under test is installed in a cryogenic scanning microwave microscope 10,11 and capacitively excited by a center conductor pin. Two microwave synthesizers send continuous wave ͑cw͒ tones at frequencies f 1 and f 2 into the device through isolators, amplifiers, and a combiner.…”

“…To accomplish this, we had to account for perturbations of the fundamental resonant frequency of the device due to the presence of the probe. 11 Consequently, we first scanned the region of interest, recording the resonant frequency of the microstrip as a function of the position of the probe. Then we scanned the same region again, using the resonant frequency data to align the two tones so that they were centered within the passband of the device at each probe location ͑Fig.…”