Currently, the EigenmodeCoefficient objective function of the adjoint solver can only be specified using an eigenmode/band number (an integer) via its mode member parameter. It would be useful to extend this feature to also support a DiffractedPlanewave object similar to what already exists for the EigenmodeSource (via its eig_band property) and get_eigenmode_coefficients (via its mode proeprty). This would make it easier to use the adjoint solver for the design of diffractive structures.