  Adjoint optimization is an effective method in the inverse design of
nanophotonic devices. In order to ensure the manufacturability, one would like
to have control over the minimal feature sizes. Here we propose utilizing a
level-set method based on b-spline surfaces in order to control the feature
sizes. This approach is first used to design a wavelength demultiplexer. It is
also used to implement a nanophotonic structure for artificial neural
computing. In both cases, we show that the minimal feature sizes can be easily
parameterized and controlled.
