Topologically Enabled On-Chip THz Taper-Free Waveguides

Waveguide tapers are seemingly indispensable elements in silicon photonics that usually operate adiabatically to maximize the coupling efficiency between waveguides with different cross-sections, which, however, necessitate large footprints. To date, great efforts have been made to minimize the tapers' footprints while maintaining their efficiency. Contrary to conventional wisdom, here, a concept of taper-free waveguides is proposed in which waveguides with different cross-sections couple with each other efficiently without using tapers, which are enabled by the recently discovered photonic topological phases. This concept is experimentally demonstrated on an all-silicon on-chip terahertz (THz) wave platform. It is further shown that the topologically enabled taper-free waveguides are robust against sharp corners and large-area potential barriers. This work provides a promising taper-free solution for future compact optical/THz photonic integrated circuits.