We introduce a solid material that is itself invisible, possessing identicalelectromagnetic properties as air (i.e. not a cloak) at a desired frequency.Such a material could provide improved mechanical stability, electricalconduction and heat dissipation to a system, without disturbing incidentelectromagnetic radiation. One immediate application would be towards perfectantenna radomes. Unlike cloaks, such a transparent and self-invisible materialhas yet to be demonstrated. Previous research has shown that a single sphere orcylinder coated with plasmonic or dielectric layers can have a dark-state withconsiderably suppressed scattering cross-section, due to the destructiveinterference between two resonances in one of its scattering channels.Nevertheless, a massive collection of these objects will have an accumulatedand detectable disturbance to the original field distribution. Here we overcomethis bottleneck by lining up the dark-state frequencies in different channels.Specifically, we derive analytically, verify numerically and demonstrateexperimentally that deliberately designed corrugated metallic wires can haverecord-low scattering amplitudes, achieved by aligning the nodal frequencies ofthe first two scattering channels. This enables an arbitrary assembly of thesewires to be omnidirectionally invisible and the effective constitutiveparameters nearly identical to air. Measured transmission spectra at microwavefrequencies reveal indistinguishable results for all the arrangements of the3D-printed samples studied.
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