You can simulate a local cladding anywhere using a large linear waveguide that is thick and consists of whatever material you want to use as cladding. The Depth field in the Linear Waveguide Properties box controls the Y position. The construction of the simulation space is not the issue. There could be a problem with gold, however. That material would only appear as an electrode in an electro-optic simulation in OptiBPM. If the optical field touches the gold, you’ll have to use OptiFDTD for that.