  Numerical simulations have become an essential design tool in the field of
photonics, especially for nanophotonics. In particular, 3D
finite-difference-time-domain (FDTD) simulations are popular for their powerful
design capabilities. Increasingly, researchers are developing or using inverse
design tools to improve device footprints and performance. These tools often
make use of 3D FDTD simulations and the adjoint optimization method. In this
work, we implement a commercial inverse design tool with these features for
several plasmonic devices that push the boundaries of the tool. We design a
logic gate with complex design requirements, as well as a y-splitter and
waveguide crossing. With minimal code changes, we implement a design that
incorporates phase encoded inputs in a dielectric-loaded surface plasmon
polariton waveguide. The complexity of the requirements in conjunction with
limitations in the inverse design tool force us to make concessions regarding
the density of encoding and to use on-off keying to encode the outputs. We
compare the performance of the inverse-designed devices to
conventionally-designed devices with the same operational behavior. Finally, we
discuss the limitations of the inverse design tools for realizing complex
device designs and comment on what is possible at present and where
improvements can be made.
