Focusing of Terahertz Radiation With Laser-Ablated Antireflective Structures

Numerical simulations and experimental characterization of laser-ablated focusing antireflective and phase-shifting structures for terahertz frequencies are presented. More than 10% shift of reflectance minimum to lower frequencies was predicted by simulations for relatively coarse structures with the period of 100  $\mu$ m in comparison with that of a substantially smaller period and with results of the model used for the design of antireflective surfaces in the terahertz range. Such a shift of the resonance frequency can be employed to optimize the thickness of antireflective layers simultaneously obtaining additional means of more precise control of layer properties due to ablation of larger structures. Nearly 90% transmittance of silicon wafers within 0.5–0.6 THz frequencies was confirmed experimentally. Optical path differences equivalent to a half period at 0.53 THz, suitable for applications in high-efficiency zone plates, were demonstrated with high transmittance simultaneously. Possibilities of delay adjustment up to one wavelength were illustrated by numerical simulations. A focusing binary zone plate for 0.6 THz was produced employing phase-shift differences of the dual-function antireflective layer. Its close to diffraction-limited focusing performance was evaluated, further confirming sufficient uniformity of the structured layer.