Computation and Analysis of Terahertz Wire Grid Polarizer Self-Resonance Using Transmission Line Theory

A terahertz metal wire-grid polarizer on a low-loss dielectric (crystalline quartz) substrate was simulated using High Frequency Structure Simulator (HFSS) and modeled with equivalent-circuit theory. The transmittance of the polarizer was calculated for electromagnetic radiation at normal incidence from 100 to 1000 GHz for both s-polarization (perpendicular to the grid) and p-polarization (parallel to the grid). The phase difference in S21 between the HFSS input and output ports was calculated and plotted versus frequency and versus fill-factors of 0.3, 0.5, 0.7, 0.9, and 0.95 for both polarizations. Analysis of the S-polarized S21 shows that the phase-angle differences are all negative. This is interpreted as the phase of the current in the wire grid leading the phase of the voltage across the gaps, and thus the electric field across the gaps. This behavior was seen for all fill-factors. Therefore, a capacitive effect is exhibited by the wire-grid polarizer for S-polarization. Analysis of the P-polarization results show the phase angle differences are positive for fill-factors less than or equal to 0.9 and negative at the fill-factor of 0.95. Thus, the wire-grid current lags the gap voltage for P-polarization at fill-factors ≤ 0.90 (an inductive effect) while wire current leads the gap voltage at fill-factor = 0.95 (a capacitive effect). This behavior is the spatial analog of a parallel LC circuit.