  Microwave resonators made from high-temperature superconducting (HTS) films
are becoming increasingly commercially competitive in the market of modern
communication technologies. However, unlike normal metals, the HTS device
performance suffers from intrinsically nonlinear electrodynamics, resulting in
harmonic generation and intermodulation (IM) distortion at all levels of
circulating power. The dominant sources of IM response are believed to be
inductive and localized in only small regions of the HTS film having the
highest current densities J_RF(x,y). In our previous papers, examining
microwave resonators of simple microstrip geometry, we have demonstrated that
local nonlinearity imaging is possible with the technique of low temperature
laser scanning microscopy (LTLSM) adapted to microwave experiments. Unique
contrast due to IM current density J_IM(x,y) was generated by the IM imaging
method. It was applied to display the regions showing large nonlinear LTLSM
photoresponse (PR) in HTS devices with a few-micrometer spatial resolution.
Direct local correlation between J_IM(x,y) and high J_RF(x,y) at the edges of
the strips, typical defects like twindomain blocks, in-plane rotated grains and
micro-cracks was shown. Here we want to indicate that the geometrical features
in HTS circuit layout for microwave devices of more complex geometry also
radically affect the linear device performance.
