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Applied Physics Letters

Imaging of microwave intermodulation fields in a superconducting microstrip resonator

Wensheng Hu, Ashfaq S. Thanawalla, B. Johan Feenstra, F. C. Wellstood, Steven M. Anlage

Published Nov 1, 1999• 22 citations
Publisher

Abstract

Nonlinearities give rise to intermodulation distortion in superconducting microwave devices and currently limit their use to low power applications. We have developed a cryogenic imaging technique to spatially resolve intermodulation distortion and used it to image an 8.2 GHz high temperature superconducting Tl2Ba2CaCu2O8 microwave resonator. The images reveal that the fundamental and intermodulation electric fields obey a fixed relation throughout the device. We note that further refinements of intermodulation theory in resonant devices may be required to fully describe the data.

DOI
10.1063/1.125162

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Nonlinearities give rise to intermodulation distortion in superconducting microwave devices and currently limit their use to low power applications. We have developed a cryogenic imaging technique to spatially resolve intermodulation distortion and used it to image an 8.2 GHz high temperature superconducting Tl2Ba2CaCu2O8 microwave resonator. The images reveal that the fundamental and intermodulation electric fields obey a fixed relation throughout the device. We note that further refinements of intermodulation theory in resonant devices may be required to fully describe the data.
We describe the use of a cryogenic near-field scanning microwave microscope to image microwave electric fields from superconducting and normal-metal microstrip resonators. The microscope employs an open-ended coaxial probe and operates from 77 to 300 K in the 0.01–20 GHz frequency range with a spatial resolution of about 200 μm. We describe the operation of the system and present microwave images of Cu and Tl2Ba2CaCu2O8 microstrip resonators, showing standing wave patterns at the fundamental and second harmonic frequencies.
The issue of intermodulation distortion in superconducting devices for rf and microwave applications is discussed. The classical frequency conversion theory for nonlinear two-port devices in the small signal limit is shown to apply to microstrip resonators regardless of their geometry and material. Two tone, third order intermodulation measurements allow more sensitive detection of nonlinearity compared to surface impedance measurements, provided a low noise, spurious free experimental setup is adopted. Measurements carried out on high quality Nb meanderline microstrip resonators show that at very low power level nonlinearity is dominated by intrinsic device properties. Film defects start to play a predominant role above a critical power level that strongly depends on the defect’s nature.
At the resonant frequency, superconducting resonators produce intermodulation distortion (IMD), smaller unwanted signals near the resonant frequency. These signals are a measure of the ratio between fundamental frequencies and distortion products. By inducing external microwave signals, it is possible to analyse the patterns of IMD in several different types of superconducting resonators. These measurements can be used to complement the main peak values like quality factor and frequency shift in order to understand nonlinearities present in the material of the superconductor. Once IMD measurements are taken, they were combined into a 2-dimensional raster image of the resonator chip.
This work presents new experimental results on low-temperature (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 f/sub 0/ and change of the quality factor Q. 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.
Two-dimensional scanning is combined with a method to locally excite intermodulation distortion (IMD) in a cuprate superconducting microwave resonator. The technique provides local maps of second and third order IMD. The two orders originate from distinct processes and require multiple tones in order to both be excited in a narrow passband. This is the first report where multiple orders of IMD are mapped at their points of origin at the operating frequency of the device. Two samples are shown, one pristine sample and one with an engineered defect, revealing a dominance of the defect in the nonlinearity of the device.