Metasurface tames spreading of orbital angular momentum beams

Researchers have demonstrated a way to stop high-order orbital angular momentum (OAM) beams from spreading as they travel, using a compact transmitarray metasurface that reshapes the radial phase profile of the beam. The work, published in Scientific Reports, addresses a long-standing limitation: the more twist an OAM beam carries, the faster it diverges, which has restricted its usefulness in communication and imaging systems. Beams with higher mode numbers naturally spread more during propagation, so a beam with ℓ = 3 becomes noticeably wider than one with ℓ = 1 after travelling the same distance.

Earlier phase-engineering approaches modified divergence by combining an axicon phase term with a lens phase term and then adjusting the parameters through trial and error until the desired beam shape appeared. The new study takes a different route. The team, led by Akash Paramanik and Venkatachalam Subramanian, exploited an inverse relationship between the prescribed target beam radius at a chosen propagation plane and the radial phase correction needed to achieve it.

The key to the method is a direct relationship between the phase gradient and the trajectory slope of the beam. By specifying the radius a beam should have at a given distance, the designers can compute the radial phase correction that each part of the aperture must apply, rather than searching for it iteratively. This inverse design enables divergence control through phase-only modulation on a metasurface of fixed aperture, without additional optical elements.

To implement the correction, the researchers built a compact metasurface consisting of two dielectric layers and three metallic layers. Operating at 28 GHz, a frequency relevant to millimeter-wave communications, the structure provides full 2π phase coverage and a transmission greater than −3 dB, meaning it passes most of the incident power while imprinting an arbitrary phase pattern. By programming the phase of individual unit cells across the metasurface, the team first generated OAM modes and then applied the radial phase correction to equalise their radii.

Full-wave simulations confirmed the theoretical prediction: the metasurface produced OAM modes with an identical radius. Experimental measurements verified the formation of these equal-radius beams, supporting the simulation results. The authors describe the approach as a general framework for multimodal multiplexing and divergence-controlled beam engineering in millimeter and microwave systems.

Because the correction is computed directly from a target beam radius rather than tuned by hand, it could simplify the design of OAM-based links in which several modes must arrive with matched beam sizes. The findings rest on simulations and bench-scale measurements at 28 GHz, and the study reports performance within that microwave setting rather than in deployed communication networks.

Subject of Research: Divergence control of orbital angular momentum beams using inverse radial phase design on a transmitarray metasurface

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

Article References: Paramanik, A., Rao, M. V., Ukey, A., Venkata, K. C., & Subramanian, V. (2026). Radial phase engineering for divergence control of OAM beams using a transmitarray metasurface. Scientific Reports. https://doi.org/10.1038/s41598-026-76052-4

Image Credits: AI Generated

DOI: 10.1038/s41598-026-76052-4

Keywords: orbital angular momentum, metasurface, transmitarray, beam divergence, radial phase engineering, millimeter-wave, 28 GHz, perfect vortex beams, multiplexing, inverse design, Radial, phase