Imaging local sources of intermodulation in superconducting microwave devices

Abstract

This work presents new experimental results on lowtemperature (LT) characterization of local rf properties of passive superconducting (SC) microwave devices using a novel Laser Scanning Microscope (LSM). In this technique, a modulated laser beam is focused onto and scanned over the surface of a resonant SC device to probe the spatial distribution of rf current. The highly localized photo-induced change of the kinetic inductance of the SC device produces both a shift of the resonant frequency 0 and change of the quality factor . An image of these changes is recorded as the laser spot is scanned over the device. We present the first measurements of spatially resolved intermodulation response in a High Temperature Superconducting (HTS) co-planar waveguide resonator, opening up a new window into the local origins of nonlinearity in the HTS materials.

Measurement of local reactive and resistive photoresponse of a superconducting microwave device

“…A good acoustic match between the YBCO film and LAO substrate of the device at temperatures above 50 K leads to an l that is defined mainly by the properties of LAO, producing a thermal spot with T l T =(k/cρf M ) 1/2 = 4.8 μm with thermal conductivity k = 9 W/m*K, a specific heat c = 0.58 J/g*K, and density ρ = 6.57 g/cm 3 . This estimate of l T is in good agreement with the measured value [9]. The oscillating local heating of the HTS film causes the thermo-induced modulation of transmitted HF power δP OUT (f) ~ δ||S 21 (f)|| 2 , which was detected by a spectrum analyzer and was used as a signal of the LSM PR, creating local contrast of LSM images.…”

Microwave Current Imaging in Passive HTS Components by Low-Temperature Laser Scanning Microscopy (LTLSM)

“…The crack [see Fig. 7] is formed by an area of sharp misorientation of twin domain blocks in LAO [40]. Inhomogeneous distribution of JRF(x,y) here shows a deep penetration of microwave field in the HTS strip.…”

“…Presumably, the more nonlinear parts of the material will contribute a bigger change to the IMD power when they are heated. Based on numerical simulations with the two-fluid model, we can assume to first approximation that the contrast seen by the LSM tuned to the intermodulation frequency is proportional to the local change in intermodulation current density scale, JIMD [40].…”

Imaging of Microscopic Sources of Resistive and Reactive Nonlinearities in Superconducting Microwave Devices

“…The power of the laser P L =10μW was fixed low enough to introduce minimal perturbation on the global microwave properties of the resonator. The intensity of the laser is modulated at a frequency of f M =100 kHz, producing an oscillating thermal probe of about 4 μm in diameter [3,4,6,7]. Transmitted HF signals are amplified and measured by a spectrum analyzer, as shown in Fig.2.…”