Guilin Sun
Ansys Employee

Unfortunately, mode expansion monitor only works for waveguides. In your case, you will need to calculate the overlap with a monitor to get the reflection profile and the source fields after simulation. Please refer

https://optics.ansys.com/hc/en-us/articles/360034396834-Understanding-the-mode-overlap-calculation

https://courses.ansys.com/index.php/courses/lumerical-fde-solver-physics/lessons/overlap-and-frequency-sweep-lesson-2/