  We introduce a solid material that is itself invisible, possessing identical
electromagnetic properties as air (i.e. not a cloak) at a desired frequency.
Such a material could provide improved mechanical stability, electrical
conduction and heat dissipation to a system, without disturbing incident
electromagnetic radiation. One immediate application would be towards perfect
antenna radomes. Unlike cloaks, such a transparent and self-invisible material
has yet to be demonstrated. Previous research has shown that a single sphere or
cylinder coated with plasmonic or dielectric layers can have a dark-state with
considerably suppressed scattering cross-section, due to the destructive
interference between two resonances in one of its scattering channels.
Nevertheless, a massive collection of these objects will have an accumulated
and detectable disturbance to the original field distribution. Here we overcome
this bottleneck by lining up the dark-state frequencies in different channels.
Specifically, we derive analytically, verify numerically and demonstrate
experimentally that deliberately designed corrugated metallic wires can have
record-low scattering amplitudes, achieved by aligning the nodal frequencies of
the first two scattering channels. This enables an arbitrary assembly of these
wires to be omnidirectionally invisible and the effective constitutive
parameters nearly identical to air. Measured transmission spectra at microwave
frequencies reveal indistinguishable results for all the arrangements of the
3D-printed samples studied.
