Fundamental Electromagnetic Properties of Twisted Periodic Arrays

Linear periodic arrays of elements sequentially rotated about the array's axis display interesting polarization properties. These “twisted” arrays, characterized by both discrete and helical periodicities, exhibit features of periodic gratings and bulk chiral media. For arrays with small periodicities, in which the zeroth diffraction (Floquet) mode dominates, two independent, transverse traveling wave modes are identified. One of these mode types is a guided wave that remains bound to the array, while the other is a leaky (radiating) wave mode. The electric fields characterizing each mode consist of two circularly polarized waves with distinct phase velocities. For larger periodicities, higher diffraction orders of these two modes may radiate. The coupling between different modes may result in stopbands of several types but the broadside stopband typical of leaky wave structures is eliminated. A finite twisted array is simulated to verify the predicted properties, and to show that the two transverse modes may be excited by circularly polarized sources with opposite handedness. Results are demonstrated for arrays of plasmonic particles in the optical regime and arrays of half-wave dipoles in the microwave regime. Twisted arrays can find uses in various microwave and optical applications, such as frequency and polarization sensitive waveguides, couplers, filters, and antennas.