Fig. 1 (a) FEM Simulation result of 0.1 THz wave coupling to a wire waveguide using a dual-wire coupling configuration (E_x shown here). The top inset shows a close-up view of the electric field (xz-plane) in the coupling region between the two wires, while the right inset shows the electric field at the end of the wire (xy-plane). Note that the majority of the incident wave scatters into free space, and only 0.4% percent of the incident power is coupled to the guided radial mode. Fig. 1(b) shows the x-component of the electric field along a line 300 microns above the wire and parallel to the wire axis. The inset shows a 0.1 THz sine wave fit to the extracted simulation data. This demonstrates that the simulation is discretized with a mesh density fine enough to resolve the oscillating electric field.

Fig. 2 (a) Photoconductive antenna with radial symmetry (b) FEM simulation of the power emitted by an “ideal” radial antenna in free space (i.e., no substrate) at 0.5 THz. The antenna sits at the center of the sphere in the xy-plane. The emitted field is zero along the z-axis, as expected for a ‘donut’ mode.

Fig. 3. FEM simulation results of the radial antenna in a typical THz configuration. (a) & (b) Plots of the x and y component of the electric field for the idealized radial antenna. (c) & (d) Plots of the x and y component of the electric field for the actual radial antenna.

Fig. 4. FEM simulation model of radial antenna coupling to a wire waveguide (a) Plot of E_x at the end of a waveguide coupled to an ideal radial antenna. (b) Plot of E_y at the end of a waveguide coupled to an ideal radial antenna. (c) Plot of E_x at the end of a waveguide coupled to an actual radial antenna. (d) Plot of E_y at the end of a waveguide coupled to an actual radial antenna.

Fig. 5. Setup for experimental testing of the radially symmetric terahertz emitter antenna. The emitter was pumped with a free-space beam. A 0.9 mm diameter, 27 cm long wire waveguide was end-coupled to the silicon dome of the emitter. THz radiation was detected at the end of the waveguide by a fiber-coupled LT-GaAs detector sensitive to only the horizontal polarization component.

Fig. 6. The wire waveguide end-coupled to the silicon dome, which is mounted on the opposite side of the GaAs substrate from the radial antenna.

Fig. 7. Time-domain measurements of the terahertz pulse detected 27 cm away from the radial emitter. Measurements have been performed with the wire waveguide both present (7a) and absent (7b). All other experimental parameters are unchanged. The terahertz pulse exhibits a polarity reversal when measured at opposite locations at the end of the waveguide (upper and lower curves in (a)), a hallmark of the guided mode’s radial polarization.