A Varactor-Loaded Spoof-Plasmonic Reconfigurable Intelligent Surface Unit Cell for Wideband Phase and Wavefront Control

Reconfigurable intelligent surfaces (RIS) provide a compact and low-loss platform for controlling electromagnetic phase responses. This study investigates spoof plasmonic, varactor-loaded RIS unit cells as an approach for achieving continuous and broadband phase tunability within a simple and scalable architecture. A varactor-diode-loaded spoof surface plasmon polariton (SSPP) unit cell is designed for wideband reconfigurable operation in sub-5.9 GHz wireless sensing and communication systems. The proposed multilayer structure incorporates bias-controlled varactors at regions of strong electromagnetic field confinement within a spoof plasmonic cavity, enabling a continuous 180° phase tuning with reflection loss below − 10 dB over the 2–11.85 GHz frequency range. The unit cell exhibits a compact normalized footprint of 0.20λ₀ × 0.20λ₀ and maintains a |Γ| variation below 0.4 dB across four hybrid bias states, indicating nearly constant reflection magnitude and consistent field confinement under reconfiguration. These characteristics demonstrate broadband phase reconfigurability and support the applicability of the proposed RIS unit cell for electromagnetic wavefront manipulation in wireless sensing and channel control.