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The development of dynamic components for controlling wave fronts in the sub-terahertz region of the electromagnetic spectrum has emerged as a frontier research topic for many applications in sensing and communications. One approach which has attracted much attention involves the use of active metasurfaces, tiled arrays of sub-wavelength elements with properties that can be reconfigured via external actuation. In nearly all cases, these metasurfaces are employed as either transmissive or reflective elements, taking advantage of their strong and tunable interaction with free-space electromagnetic waves. These interactions can be significantly enhanced through the use of surface waves propagating parallel to the metasurface array, although very few studies have exploited this option. Here, we integrate a metasurface into the interior of a parallel-plate waveguide in a configuration explicitly designed to exploit this surface-wave geometry. We show that varying the electrical properties of the active metasurface changes the wave vector of the guided mode, and thereby alters the emission angle of radiation out-coupled through a leaky-wave slot aperture. These results, which are consistent with numerical simulations, represent a new approach to broadband beam steering suitable for the sub-terahertz spectral range. 

Many widely used antennas in terahertz (THz) directional communications (including horn antennas) are not fully compatible with the recently proposed absolute security approach due to the absence of strong frequency-dependent minima in the intrinsic antenna pattern. To this end, we propose to use a multiple-slit aperture to modify these non-suitable radiation patterns in a non-intrusive manner. Based on the principle of diffraction, the multi-slit aperture creates frequency varying minima critical for absolute security. We show that improved security performance, quantified by the size of the secure region in space (termed blind region), can be achieved by employing a wider diffraction aperture with a wider slit opening. We further characterize how the non-uniform wavefront, which is typical in practical transmission and results in varying amplitude and phase at different slit openings, affects the size of the blind region. This diffraction-based scheme is experimentally demonstrated with a horn antenna operating near 200 GHz. We demonstrate that, while the intrinsic horn antenna yields no blind region for angles within 16° from the intended user, the modified antenna configuration produces strong minima sufficient to create blind regions at angles as small as 4° and an expanding blind region with increasing transmission bandwidth, thus validating the security gain with this approach. 

Considerable recent research interest has focused on the possibility of using metasurfaces for manipulation of terahertz wavefronts. For example, metasurfaces allow a beam to be targeted in any desired direction using strategically placed meta-elements. With rapid prototyping techniques, metasurfaces can be fabricated quickly and at a low cost. These techniques also permit the fabrication of metasurfaces on flexible substrates which can be bent easily. This opens the possibility of employing such devices as conformable arrays on non-flat surfaces. To explore this idea, we experimentally and numerically analyze the performance of a terahertz metasurface printed on paper, as a function of its radius of curvature. We observe that when the metasurface is bent, the direction of the refracted beam is minimally impacted and the performance of the metasurface remains very similar to when it is flat. This conclusion will simplify the design and modeling criteria for conformable metasurfaces.