Terahertz focusing properties of polymeric zone plates characterized by a modified knife-edge technique

The focusing properties of a series of polymeric zone-plate lenses are investigated around a target frequency of 1 THz. Their characterization is performed by means of terahertz (THz) time-domain spectroscopy, employing a modified knife-edge technique that compensates for asymmetries of the impinging THz beam shape of typical photoconductive antenna-based THz sources. The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer. Three different zone plates are studied, a conventional binary zone plate, a conventional four-level zone plate, and a recently introduced double-sided zone plate consisting of the stack of two phase-reversal binary zone plates, which is simpler to fabricate and less sensitive to mechanical damage than multilevel zone plates. Experimental results, coupled with finite element simulations, demonstrate that the double-sided zone plate features a resolution increased by about 3λ with respect to the binary zone plate and comparable with that of the four-level zone plate. The double-sided zone plate has 40% lower focusing efficiency and approximately 7λ shorter depth of field compared to its four-level counterpart. Nevertheless, it outperforms conventional binary zone plates by 25% in power focusing efficiency and features a 10λ longer depth of field.