Anti−Reflection Metamaterials with Phase Gradient Index Generate Surface Waves for Radar Stealth in the Microwave Regime

Although the conversion of propagating waves into surface waves is anticipated to introduce a novel degree of freedom for radar stealth materials, the manipulation of wavelength and traveling direction of surface waves at small phase gradients remains unclear. Here the aforementioned conversion is demonstrated utilizing a metamaterial composed of carbonyl iron powders, where the momentum mismatch is compensated by the material and the phase gradient index of the aligned meta−atoms. Surface waves are generated in the direction of the phase gradient within a phase span of 180°. The metamaterials with 5−level metastructure (5x:1x) generate surface waves with two wavelengths, and get the ξ/k0 value (ratio of the phase gradient index to the wave vectors of propagating wave) of 7.5 and 4.32, respectively. Furthermore, the dual phase gradient metastructure possesses the capability to alter the trajectory of the leaky microwave, converting it into a waveform resembling that of a vortex, while simultaneously preserving the integrity of surface waves. The sample achieves an effective absorption bandwidth of 5.67–7.96 and 20.19–21.10 GHz with an optimal absorption peak of −40.77 dB at 6.87 GHz. Present study develops a novel mechanism to improve the radar stealth properties of microwave absorption materials.