  A laser scanning microscope with a thermal spot size of about 4 $\mu$m is
used to measure a quantity proportional to the rf current density in an
operating superconducting co-planar waveguide microwave resonator. The twinning
of the LaAlO$_3$ substrate produces a meandering of the current at the edges
due to irregularities in the wet etching of the YBa$_2$Cu$_3$O$_{7-\delta}$
film associated with substrate twin domain blocks, and a ($\sim$ 20%)
enhancement of the rf photoresponse at these locations. These irregularities
are candidates for enhanced nonlinear response from the device. The effects of
substrate twinning and the resulting edge features on the superconducting film
are discussed and analyzed.
