Design of Wide Absorption-Band Absorptive Filtering Reconfigurable Intelligent Surfaces

This letter presents a dual-layer absorptive filtering reconfigurable intelligent surface (RIS) with 1-bit reflection phase modulation capabilities in RIS working band and wide absorption bandwidth out of RIS band. The proposed structure integrates a PIN-diode-controlled reflective layer and a resistors-loaded absorptive layer achieving dual functionality. As a 1-bit RIS, it operates from 5.77–6.12 GHz while providing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 10dB reflectivity absorption bands of 2.1–5.18GHz and 7.01–8.13GHz in the PIN diode OFF state, shifting to 2.02–5.09GHz and 6.53–8.17GHz in the ON state. Theoretical analysis establishes the impedance conditions for simultaneous absorption and phase control. Full-wave simulations and measurements confirm stable performance under oblique incidence up to 30<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula>. The proposed design offers a practical solution for implementing planar absorptive-filtering RIS (AFRIS) with wide absorption bandwidth.