A spiral phase plate prepared via high-resolution 3D printing for THz vortex beam generation

Three-dimensional (3D) printing technologies have recently been used for the fabrication of custom optical components in the terahertz (THz) range, with benefits related with flexible design, fast prototyping, and preparation of on-demand devices. However, standard 3D printing methods have a limited spatial resolution $(\sim 100 \mu \mathrm{m})$, typically allowing the fabrication of components with the necessary precision only for frequencies of a few hundred GHz. The advanced 3D printing method based on two-photon polymerization (TPP) can instead give access to nanoscale resolutions $(< 100$ nm). Here, we employ this technique to fabricate a spiral phase plate (SPP, i.e., a device that can convert a Gaussian beam into a vortex beam with a helical phase) operating at around 1 THz. Using our recently developed scanless THz time-domain imaging (TDI) method, we experimentally retrieve hyperspectral information about both the amplitude and phase of the generated vortex beam, which is found to agree with the results of our numerical simulations.