Gap-controlled multi-core terahertz waveguides for integration.

Multi-core fibers are pivotal for increasing core density and enabling space-division multiplexing to meet the growing demand for high data transmission rates beyond the limits of single-mode fibers. In the terahertz regime, bundled polymer fibers offer a high-capacity alternative for chip-to-chip links. This work introduces a gap-controlled approach to modulating coupling in solid multi-core terahertz fibers, supported by analytical models and simulations. Experimental validation using 3D-printed cyclic olefin copolymer 1 × 3 fibers demonstrates precise control from -3 dB power splitting to inter-core isolation exceeding -35 dB-sufficient for high-order QAM formats. Rectangular core geometry enables polarization multiplexing, effectively doubling channel capacity, while near-field scanning provides what is believed to be the first spatially resolved visualization of evanescent coupling dynamics. The monolithic design improves integration density by 280% compared to discrete fiber bundles, establishing a scalable framework for high-density terahertz photonic systems.