Directional Coupling\nto a λ/5000 Nanowaveguide

Silicon-based microdevices are considered promising candidates\nfor consolidating several terahertz technologies into a common and\npractical platform. The practicality stems from the relatively low\nloss, device compactness, ease of fabrication, and wide range of available\npassive and active functionalities. Nevertheless, typical device footprints\nare limited by diffraction to several hundreds of micrometers, which\nhinders emerging nanoscale applications at terahertz frequencies.\nWhile metallic gap modes provide nanoscale terahertz confinement,\nefficiently coupling to them is difficult. Here, we present and experimentally\ndemonstrate a strategy for efficiently interfacing subterahertz radiation\n(λ = 1 mm) to a waveguide formed by a nanogap, etched in a gold\nfilm, that is 200 nm (λ/5000) wide and up to 4.5 mm long. The\ndesign principle relies on phase matching dielectric and nanogap waveguide\nmodes, resulting in efficient directional coupling between them when\nthey are placed side-by-side. Broadband far-field terahertz transmission\nexperiments through the dielectric waveguide reveal a transmission\ndip near the designed wavelength due to resonant coupling. Near-field\nmeasurements on the surface of the gold layer confirm that such a\ndip is accompanied by a transfer of power to the nanogap, with an\nestimated coupling efficiency of ∼10%. Our approach efficiently\ninterfaces millimeter waves with nanoscale waveguides in a tailored\nand controllable manner, with important implications for on-chip nanospectroscopy,\ntelecommunications, and quantum technologies.