Orbital Angular Momentum Generating Thin-Film Conformal Metasurface for Backscattering Control

This paper presents the design, simulation, and experimental verification of a flexible thin-film conformal metasurface for backscattering control via orbital angular momentum (OAM) wave generation in the X-band (9.5-10.5 GHz). The metasurface comprises asymmetric square loop meta-atoms fabricated on an air-equivalent dielectric substrate with subwavelength thickness (lambda/15 at 10 GHz). Analytical modeling define the phase difference requirement (arg(ryy) - arg(rxx) = 180 +- 25 deg for spin-to-orbital angular momentum conversion, enabling OAM mode l = +1 generation upon circularly polarized wave reflection. Full-wave EM simulations and antenna array theory predictions confirmed a characteristic radiation pattern null along the OAM propagation axis. Experimental prototypes were fabricated using adhesive-backed foil on foam-core substrates, demonstrating: 4 dB reflected power reduction for the OAM-generating metasurface compared to a uniform-phase reference, 7 dB reflected waves reduction versus solid metal including for various radii of rounding surfaces. The measured suppression bandwidth reached 1 GHz, though alignment sensitivity due to the narrow null zone was identified as a limitation. Additional radial phase gradients for beam defocusing reduced performance by 1-2 dB due to main lobe distortion. Discrepancies between analytical models and measurements underscored the importance of mutual coupling effects in complex phase distributions. This work confirms thin-film conformal metasurfaces as a viable solution for controllable backscattering.