A more efficient method of coupling the THz radiation to the wire waveguide was required. The crucial problem was the polarization mismatch between the generated THz radiation and the primary mode of the waveguide. Standard photoconductive antennas that produce linearly polarized THz light are poorly suited for coupling to a wire waveguide. A novel photoconductive antenna design was proposed that would generate radially polarized THz radiation (Figure 3a) and was simulated using COMSOL Multiphysics[4]. The electrode structure consists of a circular inner electrode surrounded by a second ring electrode. Laser light is focused onto the center such that a portion of the light is incident on the gap between the two biased electrodes, thus creating charge carriers in the semiconductor. The accelerated carriers create a radial dipole. The antenna design in Figure 3a is an ideal one that is not practical due to the requirement that the electrodes must be biased. Figure 3b is the actual electrode design in which there is a break in the outer electrode to allow for the placement of the feed electrode. Figure 3c plots the simulated power output of the ideal radial antenna in free space. The field produced by the ideal antenna is radially polarized as evidenced by the presence of the cylindrically symmetric “donut” mode propagating along the z-axis. When compared to an analytical model (derived from a superposition of linear dipoles arranged radially), the FEM simulation results were in complete agreement (Figure 3d).

The next stage of modeling simulated experimental configurations. Both the ideal and actual radial antennas were placed on 500-micron thick GaAs substrates with silicon domes placed on the opposite surface. These models were completed using a two-step “multiphysics” approach. First, a dc electrostatics simulation was completed in which a bias is applied to the two electrodes. A charge density with a Gaussian distribution is placed at the center of the antenna so as to mimic the optically generated charge carriers. The model solves for the electrostatic fields and then uses them as the time-varying input fields for the second step that solves for the electromagnetic wave propagation in the frequency domain. Figure 4 shows the simulations results at 100 GHz. Figures 4a and 4b plot the emitted field from the ideal antenna, after the silicon dome, of the x and y components of the electric fields. The emitted field from the idealized antenna is almost perfectly radially polarized as evidenced by the polarity reversal seen in each electric field component. Figures 4c and 4d plot the emitted field from the actual antenna design, which is not perfectly radially polarized. The presence of the feed electrode and the break in the outer electrode clearly affects the results as it breaks the symmetry of the antenna design.

While the simulated output from the actual antenna did not produce a perfectly radially polarized THz beam, the radial nature of the emitted field was enough to warrant simulating its coupling capability. A 0.9 mm diameter wire waveguide was directly end-coupled to the center and exterior of the silicon dome. The model was solved using the same two-step “multiphysics” approach outlined previously in the frequency domain at 100 GHz. The results (Figure 5) show that the field emerging from the silicon dome coupled to the waveguide and that guided propagation took place. The coupling efficiency was found to be greater than 50%, two orders of magnitude greater than the two-wire coupler configuration[5].

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