Coupling terahertz radiation onto a metal wire using a subwavelength coaxial aperture

We demonstrate a simple technique for coupling freely propagating broadband THz radiation to multi-cycle THz pulses on a cylindrical metal wire. This is accomplished by inserting the tapered end of a cylindrical wire into the center of a subwavelength circular aperture fabricated in a freestanding metal film, forming an effective coaxial waveguide. By doing so, we convert the transmission properties of THz pulses through the aperture from an evanescent mode to a propagating mode. By fabricating concentric annular grooves about the aperture, multicycle THz pulses are coupled to the wire. The individual groove geometry, number of grooves, and groove spacing surrounding the subwavelength aperture on the metal film determine the shape of THz pulses launched on the waveguide.