Orbital angular momentum generation using a bi-layered complementary metasurface with a high conversion efficiency

Abstract

A bi-layered complementary metasurface composed of subwavelength scatterers are proposed for orbital angular momentum (OAM) generation at microwave frequencies. It converts a circularly polarized plane wave to a transmitted helical wave carrying the OAM of desired orders with a conversion efficiency of 75%. Our design has low profile and is easy to fabricate, considering the unit cell size, substrate thickness and number of printed-circuit-board layers. By mathematical derivation, we conclude that if the co-transmission coefficients of a scatterer in the Carteisian basis satisfying T xx = −T yy while the cross-transmission coefficients are zero, an OAM of arbitrary orders can be introduced in the transmitted waves by rotating the scatterers. In the design process, we use an equivalent circuit model to describe the behaviour of the proposed scatterer. The fundamental resonance of the scatterer is considered and examined. Simulation results based on circuit model and full-wave simulation show a good agreement with each other. For the whole metasurface simulation, usually, it is very time consuming due to the complicated fine structure and large simulation domain. Therefore, according to equivalence principle, we replace the complementary scatterers by equivalent magnetic dipole sources. We calculate the electromagnetic response from the equivalent sources with the help of the magnetic dyadic Green's function. The theoretical model largely reduces the simulation time and facilitates a fast and simple engineering realization. Finally, full-wave simulations are done to validate the proposed model. Two metasurfaces that can generate OAM of order 2 and 4 based on the proposed model are simulated. Both the near- and far-field patterns are explored and characteristics of OAM are confirmed.