On the Influence of Fabrication Tolerances in Terahertz Photoconductive Antennas

We present a study of the radiation pattern of photoconductive antennas (PCA) subject to substrate chip defects, aiming at characterizing fabrication tolerances. We show that the asymmetries observed over the crosspolar radiation patterns may be exploited to numerically estimate the most prominent trends of the substrate chip geometrical imperfections. A figure of merit (FoM) has been defined, characterized, and validated through simulations and experimental measures, proving it is sensitive to both the magnitude and location of the substrate irregularities. The study has been focused on two kinds of substrate defects, the angled dicing and the off-centered antenna gap, and it has considered three planar antenna geometries: dipole, bow-tie and Sierpinski triangle dipole. A numerical antenna factor related to the planar antenna geometry imprinted over the substrate is included in the FoM expression for fair comparison of substrate irregularities among PCAs with different metallic patterns. A simple setup for experimentally obtaining the value of the FoM through collimated beam raster scanning measure of the crosspolar radiation pattern has been proved useful to validate the practical relevance of the FoM. These values asses the substrate chip fabrication quality and help to identify the position and magnitude of the substrate defects that have the greatest impact on the PCA performance.