We investigated the propagation of single- and multi-mode terahertz (THz) radiation for various gap sizes a parallel-plate waveguide (PPWG) and polarizations of the incoming THz field. Single TEM (TM_(0)) and TE₁ modes within the 4-THz frequency range propagated through a waveguide with a 103μm plate separation. Multi TE and TM modes were observed for a 360-μm separation. High-order modes have a very high group velocity dispersion near the cutoff frequencies, which causes extensive pulse reshaping and broadening in addition to multimode interference in the time and frequency region. The majority dominant mode is TE₁ (97.96%) and TM_(0) (81.61%) modes because the lowest mode has a small absorption and an incident even field pattern. Because each mode has different transmission and coupling coefficients, absorption, and propagation constant, the theoretical calculation for the total TM or TE modes requires a summation of the components. The theoretical calculation and the measured data fit well in the frequency and time domain.
The high pass filtering effects using TE₁ mode in which blocks off the signal under the cutoff frequency were also investigated. The well filtered signal from TM multi-mode was also observed. And the experiment at (spoof) surface plasmon polaritons(SPP) with PPWG system was performed. The several rectangular and slit holes on the stainless steel sheet with 50μm thickness were perforated. The resonance signals of SPP known as Bragg reflection were observed. Using the period of rectangular holes and slits, the polarity of the detected resonance signal was explained as the phase differences between the main pulse and resonance pulses. The experiment results show that SPPs propagate the upper and under the metal sheet through holes.
The propagation of TM multi-mode terahertz radiation through PPWG with the gap size of 360μm was simulated using the finite-difference time-domain(FDTD) method. The simulation data could explain the difference between the experimental data and the theoretical calculation. Also the experiment of SPP's Bragg reflections by slits was simulated. The simulation shows that how THz wave propagates and reflects in the slits. The results of FDTD simulation fit well with the experimental data and the theoretical calculation.