By definition, a material’s refractive index n is the ratio of the speed of an electromagnetic wave in vacuum to its phase velocity in the material. It’s also given by (εμ)1/2, where ε is the material’s electric permittivity and μ the magnetic permeability. So the phase velocity becomes effectively infinite if ε (and thus n) ever approach zero, which can happen when free electrons in materials such as metals and heavily doped semiconductors are driven by the electromagnetic wave to oscillate at their resonance (plasma) frequencies. And for an incident wave of fixed frequency, a huge phase velocity implies a huge wavelength.
In 2006 Mário Silveirinha (now at the University of Lisbon) and the University of Pennsylvania’s Nader Engheta showed theoretically that one can exploit the ultralong wavelength at microwave frequencies to connect two separate waveguides with a long, narrow, two-dimensional channel filled with an...
