FDTD-based open-source package supporting Python language, a variety of materials (nonlinear, gain, gyrotropic materials etc), GDSII import, Amazon cloud-based computing, adjoint solver etc.
Open source codes developed by Prof. Shanhui Fan's and Prof. Philippe Lalanne's groups, respectively. These codes are based on RCWA method. S4 runs faster, but Reticolo is better documented.
Inverse design of nanophotonic devices
Spins developed by Prof. Jelena Vučković's group.
Lumerical provides examples on waveguide design.
Very fast FDTD- and cloud-based solver for large scale metasurface design and optimization.
I wrote two Reticolo examples on how to perform parameter sweep for a nanopillar and a nanofin, then calculated their polarization conversion efficiency, phase etc.