Inverse design of optical mode converters by topology optimization: tutorial

This tutorial details the use of topology optimization (TopOpt) for the inverse design of electromagnetic mode-converters. First, the design problem under consideration is stated. Second, suitable models for the geometry and physics are formulated and third the TopOpt method is outlined. Then follows three increasingly advanced design examples. In the first, the mode converter is allowed to consist of a non-physically-realizable material distribution, leading to a design exhibiting near perfect conversion from the input mode i to the output mode o in terms of power conversion (P_o,β/P_i,A>0.99) providing a performance benchmark. Then follows two examples demonstrating the imposition of relevant restrictions on the design, first ensuring a physically realizable device blueprint, and second introducing feature-size control and ensuring device connectivity. These examples demonstrate how TopOpt can be used to design device blueprints that only require a minimum of post-processing prior to fabrication, which only incur a minor reduction of performance compared to the initial unconstrained design. A software tool is provided for reproducing the first design example. This tool may be extended to implement the other design examples in the paper, to explore other device configurations or, given sufficient computational resources, to design 3D devices.