Guilin Sun
Ansys Employee

SO this is a VCSEL, please refer this Photonic Crystal VCSEL simulations.

The radiation mode is formed thorugh diffraction with multi-reflections, the same as the regular FP laser. So you can either simulate the whole device with a monitor outside the cavity, or using the mode inside the cavity to approximate it (but the mode is inside the material, not in the air. they are different). You can also use a Gaussian beam approximation. Please consider which method is "better" for you.