Efficient generation of a terahertz vector beam with antivortex polarization by using crystal symmetry and the spatial topology of light

In a vector beam (VB), the polarization rotates spatially within the electromagnetic-field distribution. In this paper, we propose a simple method for using a circularly polarized pump pulse that is tightly focused onto a (100)-cut ZnTe crystal to generate a VB with anti-vortex polarization (i.e., with topological charge ℓ = -1) in the terahertz (THz) regime. Relative to normal incidence, we expect the THz output to be enhanced for the oblique incidence of a focused pump pulse. We formed a ring-shaped pump beam using a pair of axicon lenses and focused it onto a (100)-cut ZnTe crystal to achieve oblique-dominant excitation. We then performed two-dimensional polarization imaging based on THz time-domain spectroscopy, and we evaluated the results using cylindrically polarized Laguerre-Gaussian (CPLG) mode decomposition optimized by using the information entropy. We confirmed the successful generation of THz VBs with ℓ = -1, with the output efficiency increasing with focal angle from 6.4° to 17.3°. These results indicate feasibility of using THz VBs for various applications.