Topology optimization of photonic crystal structures: a high-bandwidth low-loss T-junction waveguide

A T junction in a photonic crystal waveguide is designed with the topology-optimization method. The gradientbased optimization tool is used to modify the material distribution in the junction area so that the power transmission in the output ports is maximized. To obtain high transmission in a large frequency range, we use an active-set strategy by using a number of target frequencies that are updated repeatedly in the optimization procedure. We apply a continuation method based on artificial damping to avoid undesired local maxima and also introduce artificial damping in a penalization scheme to avoid nondiscrete properties in the design domain.