Topological inverse design of fabrication-constrained nanophotonic devices via an adaptive projection method

Topology optimization has been widely adopted in the inverse design of nanophotonic devices due to low computation cost, which unfortunately produces intermediate relative permittivity values that fail to meet fabrication constraints. Additionally, the postprocessing required inevitably increases the complexity of the inverse design. In this Letter, we propose an adaptive projection method for topology optimization, in which a two-level hierarchical hyperbolic tangent projection function with linear increment and differentiation is constructed and applied to eliminate inherent defects of conventional topology optimization. Two binarized nanophotonic devices have been designed by our adaptive projection method, among which one ultra-compact dual 90°-bend waveguide reduces the average insertion loss to 20.3% of its similar counterpart and shows an 8.1% reduction for the average crosstalk in the O band, the other ultralow-loss waveguide crossing features an average insertion loss as low as 0.09 dB. With the significant advantages of excellent performance guarantee and fabrication-friendly geometry control fully demonstrated, our inverse design solution shows potential to contribute to nanophotonic devices and integrated chips.