On-chip broadband power splitters via non-Hermitian subspaces

We propose and demonstrate a broadband 3-dB power splitter based on a non-Hermitian triplet waveguide fabricated on a silicon-on-insulator (SOI) platform. By exploiting mirror symmetry, we show that the triplet can be decoupled into two virtual subspaces: a Hermitian subspace featuring a lossless zero mode and a non-Hermitian subspace supporting lossy modes. The zero mode, with its intensity equally distributed between the outer two waveguides, plays a crucial role in achieving broadband performance, effectively suppressing mode competition and maintaining stable splitting against dimensional errors. Experimentally, the 1-dB operational bandwidth of the splitter is confirmed to exceed 70 nm, ranging from 1480 nm to 1550 nm. Furthermore, this approach can be directly extended to any 1×<i>n</i> power splitter, providing a scalable and robust solution for photonic integration.