Cross-polarized angular emission patterns from lens-coupled terahertz antennas

We report detailed measurements of the cross-polarized radiation from lens-coupled terahertz dipole antennas, of the sort commonly used in terahertz time-domain spectroscopy. We compare two different antenna geometries and measure both polarization components as a function of emission angle. The qualitative features of these patterns are not specific to one particular emitter geometry but appear to be generally characteristic of these lens-coupled systems. These measurements are used to determine the angle-dependent ellipticity of the emitted terahertz beam. We conclude that a low f-number collection system (less than or similar tof/5) unavoidably results in a beam with a small but measurable ellipticity. This has important implications for any measurements in which a purely linearly polarized wave is required. (C) 2001 Optical Society of America.