Quadrupole radiation from terahertz dipole antennas

We report what is to our knowledge the first detailed investigation of the polarization state of radiation from lens-coupled terahertz dipole antennas . The radiation exhibits a weak but measurable component that is polarized orthogonally to the orientation of the emitter dipole. The angular radiatio n pattern of this cross-polarized emission reveals that it is quadrupolar, rather than dipolar, in nature. One can understand this result by taking in to account the photocurrent flowing in the strip lines that feed the dipole antenna. A Fresnel-Kirchhoff scalar diffraction calculation is used for ca lculating the frequency-dependent angular distribution of the radiation pat tern, providing satisfactory agreement with the measurements. (C) 2000 Opti cal Society of America OCIS codes: 320.7080, 260.5430, 260.5150.