Non-Hermitian engineering for low-loss terahertz transmission in a lithium niobate-on-silicon platform.

On-chip terahertz (THz) photonics holds great promise for compact, high-speed communication systems that could empower artificial intelligence and cloud-based applications. Lithium niobate (LN), an excellent platform simultaneously enabling THz generation and modulation, unfortunately suffers from severe intrinsic material absorption, restricting the propagation length to the millimeter scale. Here, we propose a grating-assisted, heterogeneously integrated LN-on-silicon (LNOS) waveguide that enables low-loss on-chip THz transmission through non-Hermitian mode engineering. By tailoring the significant material loss contrast between the highly dissipative LN and the low-loss Si, we construct a two-level non-Hermitian framework where the THz wave preferentially evolves along the low-loss eigenstate within the silicon core. This approach suppresses the effective propagation loss to 0.05 mm-1, representing much lower propagation loss compared to an uncoupled LN waveguide. The resulting structure also features a broadband 3 dB operational bandwidth of 0.16 THz, offering a viable route toward long-range, energy-efficient on-chip THz photonics.