Characterization of near-field terahertz wavefronts using a 3D-printed Shack-Hartmann lens array

As wireless links move to higher carrier frequencies that lie within the millimeter-wave, sub-terahaertz (THz), and THz regimes, future communications will take place in the radiative near-field of antenna apertures. There is therefore a great need to characterize the engineered wavefronts that will be exploited in these links. To address this gap, in this paper we introduce a method to characterize near-field wavefronts by using an inexpensive and simple-to-produce Shack-Hartmann wavefront sensor (SHWS) for sub-THz and THz spectral regimes. The SHWS is a standard sensor in optics but has seen relatively little use at lower frequencies owing, primarily, to the higher fabrication costs related to the increase in the wavelength and consequently the device size. To tackle this challenge, here we show that it is possible to inexpensively fabricate a functional SHWS by leveraging additive manufacturing through 3D printing. To validate the utility of our device, we design a SHWS for use at 0.2 THz that we then use to characterize three near-field wavefronts that we expect will be used in future wireless networks, namely a Gaussian, an Airy beam, and a tilted focused Gaussian (TFG). We provide both full-wave simulations and experimental results. This effort represents the first attempt to qualitatively study the nature of THz near-field wavefronts - especially the more esoteric Airy and TFG beams - from an experimental perspective.