Magnetic-field tailoring of the terahertz polarization profile emitted from a spintronic source

The recent emergence of spintronic terahertz (THz) emitters has provided a low-cost source of high field-strength, broadband THz radiation, exploiting the laser-induced electron spin properties in magnetic multi-layers to produce gap-free emission covering 1-30 THz and electric fields up to 300 kV/cm. An interesting property of spintronic emitters is that they have been shown to generate THz radiation polarized perpendicular to the applied magnetic field and independent of the pump laser polarization. We exploit this magnetic field-dependent emission process to create arbitrary terahertz polarization profiles. We show that by applying a specific magnetic field pattern to the source, it is possible to generate a quadrupole-like THz polarization profile. Measurements of the THz electric field at the focus of the beam revealed a polarity flip in the transverse profile of the quadrupole-like mode with a resulting strong, on-axis longitudinal component of 17.7 kV/cm.