Laser-processed diffractive optics for terahertz waves

Diffractive optics such as Soret zone plate lenses, multi-level phase-correcting Fresnel lenses, Fibonacci lenses, and antire-flective structures for terahertz (THz) waves were developed for the frequency range of 0.6-4.7 THz by using a direct laser ablation (DLA) technology. The laser-processing of the surface and volume of different metals as well as high-resistivity semiconductors were implemented to demonstrate the applicability and flexibility of the mask-less technology. Main performance of binary-phase profile lenses fabricated either on a silicon wafer or a metal foil was the focusing gain up to 25 dB and the focused beam size down to 1.1 λ. Possibility to increase of the phase quantisation level to eight steps provided higher focusing gain values up to 29 dB without a measurable increase of optical losses. Moreover, the diffractive optics demonstrated the tolerance to misalignment as large as 10 degrees of oblique incidence with a focusing power drop below 10%. We also highlighted some novel approaches of THz imaging with the multi-foci lenses.