Time domain topology optimization of 3D nanophotonic devices

•Framework for optimization of large scale 3D nanophotonic devices is presented.•3D optimization leads to superior performance compared to 2D optimization.•The optimization procedure is applied for time domain problems. We present an efficient parallel topology optimization framework for design of large scale 3D nanophotonic devices. The code shows excellent scalability and is demonstrated for optimization of broadband frequency splitter, waveguide intersection, photonic crystal-based waveguide and nanowire-based waveguide. The obtained results are compared to simplified 2D studies and we demonstrate that 3D topology optimization may lead to significant performance improvements.