Observation of Terahertz vector beam generated directly in ZnTe crystal

Earlier, we have reported a theoretical method for the direct generation of terahertz (THz) vector beams in nonlinear crystals. In this method, a cylindrical vector beam is generated by tightly focusing a circularly polarized pump pulse on a symmetric direction of a nonlinear crystal where THz generation is forbidden. This time, we chose the [111] direction as the symmetric direction in the ZnTe crystal and experimentally measured the polarization state of the generated THz beams using a home-built THz spectroscopic polarization imaging system. We succeeded to observe a radially polarized vector beam within entire of the generated spectral region. We consider that such broadband feature of the generated vector beam is due to the topological nature of the focused pump beam, where the wavevectors rotate once about the optical axis.