Generation of Terahertz Vortices Using Metasurface With Circular Slits

We propose a metal device containing circular slits to generate a terahertz (THz) orbital angular momentum beam with numerical simulations. The estimation of the polarization extinction ratio is above 20 dB over the bandwidth ranging from 0.3 to 3 THz, and a mode purity of TC = 1 or -1 is close to 1 over a wide bandwidth range, except for the area near the deteriorated valley. We analyze the OAM spectrum and find that the main noise comes from an OAM mode of TC = -3 or 3. When multiple concentric circular slits are employed, a larger transmittance is obtained without the sacrifice of mode purity. The design of such a device is simple with a size of micrometer order, revealing an option to generate a broadband THz vortex beam.