Direct generation of a terahertz vector beam from a ZnTe crystal excited by a focused circular polarized pulse.

A vector beam is a type of topological beam in which the polarization direction of light rotates around a singularity on the wavefront. This paper proposes a method to generate a vector beam by tightly focusing a pump beam in the crystalline direction such that the second-order nonlinear optical effect is forbidden. The directional dependence of the effective nonlinearity in zincblende crystals, such as ZnTe, was analytically investigated. Two types of nonlinear polarization singularities were found in [111] and [100] directions. Their polarization topological charge ℓ was +1 and -1, respectively. To experimentally demonstrate the proposed method, a (111) cut ZnTe crystal was selected as the nonlinear crystal. The polarization state of the generated terahertz (THz) beams was measured with a custom-built THz spectroscopic polarization imaging system. Radially polarized distributions were observed within the entire generated spectral region. Such a broadband feature of the generated vector beam is likely due to the topological nature of the focused pump beam, where the wavevectors are winding once about the optical axis. This simple method for generating THz vector beams will accelerate its applications.