High-Resolution Binary Zone Plate in Double-Sided Configuration for Terahertz Radiation Focusing

A double-sided phase-reversal binary zone plate is numerically investigated and experimentally characterized at the frequency of 1 THz. The double-sided zone plate has a novel design based on a binary phase-reversal zone plate, in which the total zone thickness is split into two halves located at a distance optimized by means of numerical simulations. The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer. The lenses' characterization at the frequency of 1 THz is performed by means of a terahertz time-domain spectrometer, in terms of both focal length and spatial resolution. The focusing properties of the double-sided zone plate are compared with those of a conventional binary zone plate. The double-sided zone plate features 2.4× smaller focal spot, 1.25× higher focusing efficiency, and an approximately 10λ
₀
longer depth of field than the conventional binary configuration.