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 in 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 THz 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 doublesided zone plate features 2.4 times smaller focal spot, 1.25 times higher focusing efficiency, and an approximately 10λ0 longer depth of field than the conventional binary configuration.