Radial phase engineering for divergence control of OAM beams using a transmitarray metasurface

Orbital angular momentum (OAM) beams show more divergence with increasing mode number, which limits their use in communication and imaging. Existing phase engineering techniques modify the divergence by adding an axicon and a lens phase term and observing the resultant beam through trial and error. Here, we have used an inverse relationship between the prescribed target beam radius at a chosen propagation plane and the radial phase correction required for it. The radial phase correction is determined by a relationship between the phase gradient and the trajectory slope. This inverse design enables divergence control via phase-only modulation on a fixed-aperture metasurface (MS). To study the efficiency of the phase correction mechanism, a compact two-dielectric three-metallic-layer-based MS is used with 2 \(\pi \) phase coverage and transmission greater than −3 dB at 28 GHz. By modulating the phase of individual unit cells across the MS, OAM modes are generated. Then, the radial phase correction is applied to produce equal-radius OAM beams. Full-wave simulations confirm the theoretical prediction, MSs generate OAM modes with an identical radius. We achieve an equal radius of 4.6 \(\lambda \) for three OAM modes ( \(\ell \) = 1, 2, and 3) at z = 40 \(\lambda \) for 28 GHz. Also, the phase-correction method is used to generate perfect vortex beams for modes 1 and 2 up to a 40 \(\lambda \) distance. Experimental verification is performed to confirm the formation of equal radius beams. The presented approach provides a general framework for multimodal multiplexing and divergence-controlled beam engineering in millimeter and microwave systems.