Hybrid Multiphase Fresnel Lenses on Silicon Wafers for Terahertz Frequencies

The hybrid multiphase Fresnel lenses (H-MPFLs) were developed on silicon wafer at the selected frequency of 585 GHz. For this reason, the design of a standard MPFL was modified thoroughly in various outer zone areas in order to reduce the complexity and manufacturing time of the diffractive optical elements by employing the direct laser ablation processes. The phase offset was found by precise control over the phase shift of incoming radiation in steps of π/12, revealing its optimal value to be of +π/4 independently on the hybridization order of the lens, the focusing gain of which was found to be up to 10 % higher than that achieved with a standard design MPFL. The numerical modeling data were confirmed by experiments demonstrating the effective THz beam focusing with H-MPFL samples to the diffraction-limited spot size. The proposed development procedure can be further modified by scaling the hybrid lens design to other frequencies with different zone numbers and/or by employing other materials suitable for THz photonics integration on-chip with the semiconductor devices.