Following the successful simulation results, radially symmetric photoconductive antennas were fabricated and tested[5]. The antennas were fabricated on 500-micron thick GaAs substrates. A hyperhemispherical silicon dome was mounted on the substrate opposite the antenna electrodes. The radial antenna presented here possessed an 8 micron diameter inner electrode separated from the outer electrode by 75 microns. A dc bias was applied to the electrodes, and the sample was photoexcited with an 800 nm, 100 fs laser pulse. A 27 cm long, 0.9 mm diameter stainless steel wire waveguide was end-coupled to the apex of the silicon dome. The THz pulse was detected at the end of the waveguide with a standard fiber-coupled photoconductive receiver (Figure 6a). Because the guided mode propagated along the surface of the waveguide, measurements were made at offsets from the axis of the wire. Figure 6b shows the signals detected at the end of the waveguide at +3, 0, and -3 mm horizontal offsets. The signals at +3 and -3 mm were strong, equal in amplitude, but opposite in polarity, demonstrating the radial polarization of the guided mode, consistent with the simulation results. When the waveguide was removed (Figure 6c), a much smaller THz signal was detected at 27 cm away, further demonstrating the increased coupling capability offered by the use of the radial transmitter antenna and the utility of the wire waveguide.

The use of the COMSOL Multiphysics FEM software provided a key aspect to the development of the radially symmetric photoconductive THz antenna and the THz wire waveguide[6]. The modeling of THz wave propagation and phenomena can be quite challenging. To properly simulate wave propagation, the largest mesh element size must be no larger than one-tenth of the radiation's wavelength. To model a wave propagating at a frequency of 100 GHz, the largest mesh element can be no larger than 300 microns; for a wave at 1 THz, this critical element size shrinks to 30 microns. However, a real world model involves feature sizes not only that small but also as large as centimeters. Thus, a small critical mesh-element size results in a model with a huge number of mesh elements. While this requirement makes for a difficult model to develop and solve, with proper knowledge of good FEM modeling techniques and the use of iterative and multigrid solvers, COMSOL Multiphysics can be used to model engineering problems and phenomena associated with THz wave propagation. With a Sun workstation consisting of dual-AMD 64-bit processors with 16 GB of RAM and using the GMRES iterative solver and an SSOR vector preconditioner, a solution for a 3-D electromagnetic wave model consisting of 800,000 mesh elements and 1.1 million degrees of freedom (DOF) was solved in 24 hours. The use of the multigrid solver has been used to solve complex and large 3-D EM models where the refined mesh consists of 1.5 million mesh elements and 4 million DOFs in less than 12 hours. Currently, COMSOL software is being employed to simulate various THz phenomena including the propagation of THz pulses along wire waveguides and the interaction of THz radiation with photonic crystal structures.

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