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 t ime-domain spectroscopy. We compare two different antenna geometries and me asure both polarization components as a function of emission angle. The qua litative features of these patterns are not specific to one particular emit ter geometry but appear to be generally characteristic of these lens-couple d systems. These measurements are used to determine the angle-dependent ell ipticity of the emitted terahertz beam. We conclude that a low f-number col lection system (less than or similar tof/5) unavoidably results in a beam w ith 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.