Topological antennas: Aperture radiators, leaky-wave surfaces, and orbital angular momentum beam generation

摘要

The emerging field of topological photonics has extended our capability to control and manipulate electromagnetic waves. Thus far, however, research has focused on realizing defect-immune waveguides for on-chip communication. Making similar advancement for antennas, which are an integral component and crucial to the performance of wireless devices, could lead to a parallel impact on wireless communication. In particular, the requirement for impedance matching that is fundamental to current antenna technology constitutes a major limitation to transmitting and receiving free-space signals in terms of bandwidth and power efficiency. Here, we show how reciprocal photonic topological insulator (PTI) metasurfaces can be designed to enable a unique radiation mechanism needless of impedance matching. This relies on the fact that counter-propagating spin states are decoupled from each other; hence, this self-matching characteristic spans the entire bandwidth of the PTI's bandgap. Specifically, we present two strategies for out-coupling spin-polarized edge modes: (1) in the form of an end-fire antenna at the abrupt termination of the PTI with free space and (2) in the form of a leaky-wave antenna by engineering the dispersion of the edge mode to lie above the light line. In the first approach, we compare our results to conventional rectangular waveguide apertures and planar Vivaldi antennas, which either have a large profile or flared ends to obtain gradual impedance matching. In the second approach, we demonstrate how our structure can emit orbital angular momentum (OAM) beams with various orbital charge numbers by simply varying the length of the PTI interface when made into a circular loop. The reduced device area and complexity due to removing the need for any matching network and the ability to easily multiplex OAMs of any charge, in addition to the wideband and high efficiency performance, make the proposed topological antennas highly attractive for many applications.